Stratospheric Flight: Aeronautics at the Limit

Stratospheric Flight Aeronautics at the Limit AndraÂs SoÂbester Stratospheric Flight Aeronautics at the Limit Publi...

Author: Andras Sóbester

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Stratospheric Flight Aeronautics at the Limit

AndraÂs SoÂbester

Stratospheric Flight Aeronautics at the Limit

Published in association with

Praxis Publishing Chichester, UK

Dr. AndraÂs SoÂbester University of Southampton School of Engineering Sciences Southampton U.K.

SPRINGER±PRAXIS BOOKS IN POPULAR SCIENCE SUBJECT ADVISORY EDITOR: Stephen Webb, B.Sc., Ph.D.

ISBN 978-1-4419-9457-8 e-ISBN 978-1-4419-9458-5 DOI 10.1007/978-1-4419-9458-5 Springer New York Dordrecht Heidelberg London Library of Congress Control Number: 2011931715 # Springer ScienceBusiness Media, LLC 2011 All rights reserved. This work may not be translated or copied in whole or in part without the written permission of the publisher (Springer ScienceBusiness Media, LLC, 233 Spring Street, New York, NY 10013, USA) except for brief excerpts in connection with reviews or scholarly analysis. Use in connection with any form of information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed is forbidden. The use in this publication of trade names, trademarks, service marks, and similar terms, even if they are not identi®ed as such, is not to be taken as an expression of opinion as to whether or not they are subject to proprietary rights. Cover design: Jim Wilkie Project copy editor: Dr. Stephen Webb Author-generated LaTex, processed by EDV-Beratung Herweg, Germany Printed on acid-free paper Springer is part of Springer ScienceBusiness Media (www.springer.com)

To my parents

Contents

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . XI Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . XIII About the author . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . XV Prologue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . XVII Millimeters of mercury . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . XXIII I.

In a hostile environment

1.

A sense of not belonging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 In the ‘death zone’ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Bagfuls of O2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 From home to near space . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4 Nitrogen bubbles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5 A head for heights . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.6 Cabin altitude – an uneasy compromise . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.7 Health matters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3 3 6 7 13 15 16 19

2.

Comfort zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Adrift . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Micro-climate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Cabin pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 The ﬂick of a switch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5 Frosted windows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6 Broken arrow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7 The captain’s spectacles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.8 40 000 feet – a line in the sand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.9 The bag may not inﬂate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.10 Disregarded protests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.11 A bottle on the rampage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

27 27 29 31 33 35 38 41 42 46 48 49

VIII

Contents

II. New heights of ﬂight 3.

A tale of two Comets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 A national symbol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Tu quoque? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 A different era . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 The Hall inquiry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 The W¨ohler curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6 No Highway . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7 Consternation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.8 Epilogue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

55 56 60 61 62 64 65 67 73

4.

Higher . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 The ‘410’ club . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Into thin air . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Cofﬁn corner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 The trans-Atlantic race . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5 The Swede who could see air . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

75 75 81 85 87 90

5.

Faster . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Making waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Supersonic men . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Of triumphs of technology and white elephants . . . . . . . . . . . . . . . . . . . . . 5.4 Burning through the skies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

95 95 99 101 104

III. ‘Above the weather’ 6.

Deep freeze . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 A cold morning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Ten hours earlier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3 Data mining . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4 Melting evidence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

113 113 114 117 118

7.

Rivers of air . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1 Meteorology as a weapon of war . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2 A half a kilogram stratospheric aircraft . . . . . . . . . . . . . . . . . . . . . . . . . . . .

123 123 125

8.

Rough ride . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1 The natural state of things . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2 ‘Just so you know...’ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3 Serendipity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.4 Billows . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5 But for a chime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.6 A monstrous cat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.7 An artiﬁcial ‘force of nature’ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.8 A strong recommendation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

131 132 135 138 142 146 149 152 155

Contents

9.

A gray area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1 Descent into Anchorage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2 A modern menace . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.3 Too much of a bad thing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4 The ash forecast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.5 Clear of cloud . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

IX

159 159 160 162 164 165

IV. Where next? 10. Higher still . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.1 96 863 feet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2 Manhigh . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.3 The highest step in the world . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.4 Above alien worlds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

169 169 173 176 182

Epilogue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

187

V.

Appendices

11. Unit conversions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

193

12. Temperature proﬁles around the globe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

199

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

207

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

213

Preface

Flying long distances in a short time is a common experience for many people today. During the ﬂight some will be sitting more comfortably, and with better entertainment options than if they were in their car. The experience has become so commonplace that afterward passengers are more likely to talk about how they got on at the departure airport than about the ﬂight itself. To appreciate the technical problems that had to be overcome to make this happen we only need to consider what is outside the passenger’s window – the stratosphere. The stratosphere is what the author deals with brilliantly in this book. It offers so much for all of us from aeronautical students to non-technical airline passengers, from engineers to medical doctors and lifetime workers in the industry. Many books, especially technical ones, make big demands on the reader. This one does not and is so readable that it frequently drew me along page after page. I have put in many hours looking out of airliner windows, even more in aircraft design ofﬁces and years test ﬂying but this book fascinated me and taught me a lot. John Farley, Chichester, January 2011

Acknowledgements

At ﬁrst I was slightly reluctant to ask John Farley, test pilot and living legend of modern British aviation, to read the ﬁrst draft of this book, as, even in his retirement, his time and his views are commodities in high demand. Nevertheless, John offered his help without hesitation and, after reading the manuscript, patiently talked me through his observations, helping to improve the text with precisely the kinds of subtle technical insights that I had always sought his company for. He even agreed to write the short endorsement that graces the front of the book, a gesture that made me feel as if Salvador Dal´ı had just offered to say a few words about my ﬁrst elementary school art class drawing. I owe a great deal of gratitude to other reviewers too: Prof. Andy Keane, Prof. James Scanlan and Dr. David Toal of the University of Southampton and Dr. Raj Nangia of Nangia Associates also contributed their suggestions for improving the manuscript. I must also acknowledge the role of the publisher in making Stratospheric Flight happen. In particular, I am grateful for Clive Horwood’s support – his (and his staff’s) prompt attention to any concerns I had along the way made for a considerably more relaxing writing process. Praxis also delighted me by asking their subject advisory editor Dr. Stephen Webb to check the manuscript – the book owes much to his patient, meticulous and competent copy editing. And ﬁnally, a ‘last, but deﬁnitely not least’ paragraph. Writing a book can be a dispiritingly solitary activity – fortunately, not the case for this author. I had the beneﬁt of the support, inspiration, encouragement, criticism and the most helpful nit-picking of four people, who were also willing to put a great deal of time and effort into making the following pages read much better than my original manuscript did. Most importantly though, Dr. Elizabeth Hart, Dr. Steven Johnston, Rosalind Mizen and Neil O’Brien, writing Stratospheric Flight would have been a much less rewarding experience without you.

About the author

Dr. Andr´as S´obester is an aerospace engineering lecturer at the University of Southampton in the Faculty of Engineering and the Environment. He holds a Research Fellowship funded by the Royal Academy of Engineering and the UK Engineering and Physical Sciences Research council. The remit of the Fellowship is to investigate technologies for the reduction of the environmental impact of the next generation of passenger airliners. In particular, his research is aimed at the design of airframe surfaces for minimizing the noise footprint of aircraft, while maintaining their performance. His other research areas include high altitude ﬂight (including lighter-than-air systems), the multi-disciplinary design optimization of aircraft, the mathematical description of geometries used in aerospace engineering, as well as machine learning technologies underpinning the use of high ﬁdelity computer simulations in aircraft design.

Prologue

The coldest place most of us are ever likely to visit is only seven or eight miles from home: seven or eight miles high, that is. A state-of-the-art high speed ski-lift would convey its passengers to this altitude in about two increasingly uncomfortable hours. On the way up the temperature would drop roughly two degrees Celsius (3.6 ◦ F) with every thousand feet of ascent, before reaching a minimum somewhere between −90 ◦ C (−130 ◦ F) and −50 ◦ C (−58 ◦ F) (depending on the latitude of the lift) and stabilizing there or starting to rise again straightaway. This turning point would indicate that we are entering the stratosphere. With the possible exception of a handful of polar explorers, no one has a real grasp of just how cold even the milder end of that spectrum would feel, but it is worth considering that exposure to a (positively balmy by comparison) −40 ◦ C (−40 ◦ F) would freeze Jet A, the type of jet fuel most commonly used in the US. Even at its warmest, any skin exposed in the stratosphere would be frostbitten in less than ﬁve minutes, even in the complete absence of wind. Of course, wind is rarely absent in the stratosphere. In fact, wind speeds in excess of 150 miles per hour are fairly common near its lower boundary, though these blasts carry much less kinetic energy than those at ground level (half of that speed would already be classiﬁed as hurricane force 12 according to the standard Beaufort scale of wind speed measured ten meters above ground level). This is because the barometric air pressure in the stratosphere is considerably lower than that measured at sea level and so is the density of the air. Seven miles high the ambient pressure is only about a ﬁfth of the approximately one thousand millibars most of us are used to on the ground (see Figure 0.1). In fact, a 50 % drop in ambient pressure, which is what you would experience as the lift ascended through 16 000 feet (three miles), is probably the worst that most humans can cope with for prolonged amounts of time. There is an International Standard Atmosphere (ISA) model, which represents a simpliﬁed image of the variation of temperature, pressure, etc. with altitude in Earth’s atmosphere (Figure 0.1 is based on the ISA). The ISA models the troposphere (the layer of the atmosphere that we live in) as featuring a steady temperature lapse rate of 1.98 ◦ C (3.56 ◦ F) per 1000 feet, up to a clearly deﬁned turning point at 36 089 feet (the start of

Fig. 0.1 The International Standard Atmosphere model.

XVIII Prologue

Prologue

XIX

the stratosphere), where it has the temperature settling at −56.5 ◦ C (−69.7 ◦ F) for the next 35 000 feet or so, before starting a slow, steady rise through the upper layers of the stratosphere. While the ISA is handy for ‘back of the envelope’ aircraft performance calculations, it paints a deceptively clear-cut image. In reality, the tropospheric lapse rate sometimes begins to diminish several thousands of feet below the stratosphere, through a layer known as the tropopause. The temperature may then settle at a constant value through the lower stratosphere, as per the ISA, but, equally, it may also start rising again immediately. To further complicate matters, the altitude of the transition layer, as well as the ‘turnaround’ temperature vary enormously with the weather, the time of the year and, most signiﬁcantly, with latitude. Near the poles, the stratosphere may begin as low as at 25 000 feet, with the temperature never dropping below −50 ◦ C (−58 ◦ F). The tropopause and the stratosphere occur at much higher altitudes as we move towards the Equator – here the turnaround region might be as high as 60 000 feet, where temperatures can drop as low as −90 ◦ C (−130 ◦ F) – see Figure 0.2. So why have explorers been aiming to get into this hostile layer of Earth’s atmosphere ever since the dawn of ﬂight? Beyond scientiﬁc curiosity and the unquestionably reasonable expectation that the views from up there were going to be arresting, many pioneers were also driven by a sheer determination akin to the much quoted ‘because it is there’ reasoning that famously drove George Mallory up the north-east ridge of Everest in the 1920s. The most important reason, however, is that it is the only layer of the atmosphere where high speed, long distance ﬂight is economical. The stratosphere is a density ‘sweet spot’, where air breathing engines can still operate, yet the drag opposing the progress of an aircraft is low enough to keep fuel burn down. This attraction comes at a price though: the inconvenience of the thin and spectacularly chilly air is merely the cover story in a lengthy catalogue of ways in which the stratosphere can catch out even the well-prepared traveler. Naturally, the failures of early explorers have signposted many of these dangers, but, as regular news headlines and the series of vignettes that punctuate this book illustrate, the learning curve has not leveled off, it has merely become shallower. We shall embark on a journey through some of the more remarkable episodes of the recent history of this learning experience, aiming to gain insights into the present and the future of humankind’s stratospheric endeavors. We shall revisit some of the disasters and close calls, as well as some of the triumphs that have shaped our collective experience of stratospheric ﬂight, reﬂecting each time on the physical, physiological and psychological phenomena, the careful consideration of which drives the design and operation of stratospheric aircraft. High altitude ﬂight is a practical proposition only when a delicate balance is achieved between a number of conﬂicting demands placed on the machinery and on the humans operating it; the following pages will highlight some of these ﬁnely poised trade-offs and the perils of any imbalances affecting them. Let us begin this journey in the late hours of 27 May 1931, when an aluminium ball, about two meters in diameter, fell from the night sky and bounced to a halt on an Alpine glacier. Inside were two tired, but elated men, each wearing, somewhat incongruously even for that era, a wicker basket on his head.

XX

Prologue

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Fig. 0.2 Stratospheric temperatures above eight locations, averaged over the period 1999–2009. The width of the grey band either side of the (white) average temperature curves represents variability. Consider the differences between stations in terms of their latitudes: at mid-latitudes the International Standard Atmosphere model (heavy black lines) approximates the average temperatures closely, whereas the stratosphere is considerably colder in the tropics (a 10-year average close to a staggering −90o C (−130o F) near the Equator) and, somewhat counterintuitively, warmer in the polar regions. The variability of the temperatures at various altitudes also diminishes as we get closer to the Equator; here the constant temperature layer of the stratosphere practically disappears.

An Inverted Submarine Swiss scientist Auguste Piccard knew that if a balloon ﬂight to the stratosphere was to succeed one needed to construct an airtight gondola with its own, pressurised, artiﬁcial environment inside. This had become abundantly clear to him when American Army Captain Hawthorne C. Gray, the ﬁrst man to ascend into the stratosphere, died in his open basket

Prologue

XXI

balloon in 1927 (after running out of oxygen and having to parachute to safety near the end of his previous ﬂight). It was also clear to Piccard that the technology required to build a pressurised gondola was, in fact, available at the time, albeit in a slightly different guise. World War I had seen the advent of the submarine and he reasoned that for a high altitude gondola he practically needed an inverted submarine – instead of the hydrostatic pressure of water acting on a hull containing low pressure air, the relatively high pressure of near sea-level cabin air was to act on the inside of the gondola when it was in the low pressure air of the stratosphere. An oxygen supply was also required, as well as some means of ‘scrubbing’ the air inside the gondola of the carbon dioxide breathed out by its occupants. This technology was available too in the shape of the so-called Dr¨ager apparatus, commonly used at the time in mines, as well as on submarines. The gondola had to be light, yet strong enough to resist the pressure from the inside, which gives rise to a stress in exactly the same way as the air inside an inﬂated football stretches its skin (as we shall see later, the exact prediction of the values of this stress in pressurized cabin structures was to become one of the greatest challenges of stratospheric aircraft design). For structural reasons the best shape for such a pressure vessel is a sphere, so Piccard designed a spherical gondola, big enough to accommodate him and an assistant. It was made of 3.5 millimeter thick aluminium sheet, with, in the designer’s own words, “eight round portholes of a convenient diameter” of 3.5 inches (8cm) [112]. The overall diameter of the vessel was seven feet (just over two meters). Next time you despair at the thought of spending hours in an economy class airliner cabin, spare a thought for Piccard and his assistant Paul Kipfer as, just before dawn on 27 May 1931, they climbed through a small, circular hatch into their two meter aluminium ball featuring ‘convenient’ windows, each the size of an orange. As a safety measure they donned the wicker baskets mentioned earlier, which, through the pillows lining them, were meant to protect the head of the wearer in case of an impact1 . Hovering above them at their launch site in Augsburg, Germany, was the gigantic CH113, a hydrogen balloon with a total capacity of over 14 thousand cubic meters (about ﬁve and a half Olympic-size swimming pools), attached to the gondola with two dozen or so thick ropes. At the time CH-113 was easily the largest balloon ever built, though this was not immediately obvious to the crowd gathered at the launch point, as it was only about a third full of hydrogen, allowing for the expansion caused by the ambient pressure dropping by about three quarters during the ascent to the stratosphere. And what an ascent it turned out to be. Less than 25 minutes after the tethers were cut Piccard and Kipfer were basking in the early morning sunshine at an incredible 50 000 feet above the surface. They had climbed into the stratosphere faster than a modern-day jet airliner and, in spite of a massive scare during the climb – an air leak had to be stopped hastily with some makeshift plugging – both the balloon and the gondola appeared to be working as intended. Well, mostly. One exception was the temperature control system. This was remarkably clever and simple, but it was proving somewhat inadequate: the inside air temperature was slowly creeping past the 40 ◦ C (104 ◦ F) mark. Here is how it worked. Piccard had one hemisphere of the gondola painted black, the other white. The plan was 1

See the August 1931 issue of Popular Mechanics magazine [101] for a photograph of Piccard with the basket – it will make your day.

XXII

Prologue

to turn the white side towards the sun during the day to reﬂect some of its heat and then to gradually show more and more of the heat-absorbing black side to the setting sun during the afternoon. The limitations of this system meant that the two balloonists had to bear desert-like heat only 3.5 millimeters of aluminium sheeting away from the ocean of −60 ◦ C (−76 ◦ F) air surrounding them. Then, a second scare, this time a much more serious one. The ﬂight plan called for a gradual release of some of the hydrogen from the balloon, so that they could begin their slow descent that would take them back down to Earth, with touchdown expected about six hours after liftoff. However, as they tried to open the release valve they realised that it had jammed. This was a somewhat unusual emergency, as there was little they could do other than wait. They had to wait for the night, which would, they hoped, reduce the temperature (and thus increase the density) of the hydrogen sufﬁciently for the balloon to begin to sink. But was it going to sink fast enough to let them down before the warmth of daybreak would bring another unintended climb? Equally worryingly, where were they going to come down? The Mediterranean, perhaps? A mountain top in the Alps? As you will know from the introduction, they did come down. At around eight o’clock in the evening they eventually dropped out of the stratosphere [112] and, eleven hours later than planned and given up for dead by their families (who no doubt remembered Hawthorne Gray’s tragic end), they ﬁnally touched down on a glacier near what later turned out to be Obergurgl, Austria. As far as alpine accommodation goes, a 3.5 millimeter thick aluminium sphere may not be most people’s dream, but Piccard and Kipfer had little choice. After what one can only assume to have been an uncomfortably cramped and chilly night, the following morning they made their way to safety – and into the headlines. The instruments recovered from the gondola had recorded a maximum altitude of 51 775 feet, which was not only the highest any human had ever been at that time, but it conﬁrmed that theirs had been the ﬁrst successfully completed ﬂight into the stratosphere. One could, of course, argue about the deﬁnition of ‘successful’ – after all, they landed where the winds happened to take them, eleven hours later than planned, having at times experienced frightening emergencies and oppressive heat. Much more importantly though, it seemed that the basic idea of the ‘inverted submarine’ as a basis for a stratospheric aircraft was fundamentally sound; and indeed, the decades that followed have proven the worth of this design. Piccard’s ﬂight was to become the starting point of the long learning process mentioned earlier. Many pilots feel that the vast majority of the time they spend airborne is routine, even dull. It is, however, punctuated by short spells of the exhilarating joy of ﬂight that made them choose the profession in the ﬁrst place and, occasionally, by moments of sheer terror that make them wish they had not. This is true of ﬂying in the stratosphere too – it is an environment that has given us the freedom of routine and affordable hops across and between continents, splendid views of Earth from several miles above and spectacular and, as we are about to see, often mystifying breakdowns of the fragile technological balancing act that enables us to ﬂy up there.

Millimeters of mercury

It could be reasonably argued that, of all human endeavours, aviation operates with the least consistent set of units of measurement. This is, of course, partly due to the prevalence of imperial units in the US and metric (SI) units elsewhere, but there is much more to its bewildering variety. The relatively short history of ﬂight has introduced numerous supplementary twists into the mathematical fabric of aeronautics, mostly as a result of the indiscriminate and wholesale import of the systems of units of older, related ﬁelds – chieﬂy navigation and meteorology. In fact, it would seem that no aeronautical unit has ever become obsolete but the scientiﬁc and geographical growth of the idea of ﬂight has certainly added plenty of new ones. It is apt then, perhaps, to begin a book on a speciﬁc area of aeronautics – high altitude ﬂight – with a feast of its units. I cannot think of a better way of doing this than considering the example of the METAR, the standardized format for the publication of airﬁeld area weather observations. Here is the METAR for my local airport, Southampton / Eastleigh (ICAO code: EGHI), United Kingdom, for the day I am writing this: visibility

date and time

EGHI 061620Z 01007KT 5000 BKN015 location

wind

cloud base

temperature/dewpoint

06/05

Q1018 . pressure

The ﬁrst two groups of symbols simply denote the location and date and time of issue (the 6th of the month at 1620 Zulu time – properly known as Coordinated Universal Time, or ‘UTC’). The third group describes the wind – interestingly for an ofﬁcially ‘metric country’, in non-metric units: ten degrees at seven knots, that is, seven nautical miles per hour (the metric unit ‘meters per second’ is used in METARs in some countries in mainland Europe). Next up is visibility, which is expressed in metric units: 5000 meters on this slightly hazy winter afternoon. We have imperial units for the state of the sky: the cloud cover is BKN (shorthand for ‘broken’, that is between ﬁve and seven eighths coverage), with the cloudbase at 1500 feet. After all this, it is not even surprising that the temperature and the dew point are expressed in metric units again (six and ﬁve degrees Celsius respectively), as is the sea level barometric pressure, 1018 hPa (though aviators

XXIV

Millimeters of mercury

often read it as ‘millibar’, which is exactly the same thing, but it is, strictly speaking, not an SI unit). In contrast with today’s unremarkable Southampton afternoon, the East Coast of the US is experiencing its ‘bleakest midwinter’ in decades. Here is today’s METAR for Washington Ronald Reagan National Airport: KDCA 061552Z 01015KT 1/2SM R01/2800V3500FT SN BR BKN002 OVC007 M02/M03 A2964 RMK AO2 SLP037 DRSN ALQDS P0001 T10171033 To the initiated this translates roughly as “you will stay at home if you have any common sense at all”, but, more to the point, it is further evidence of the complexities of unit of measurement usage. Note, for example, that while the wind speed is, as in the UK, measured in nautical miles per hour (15, at the moment), visibility is expressed in statute miles (a mere half a mile at the moment). Moreover, the runway visual range (the next block of symbols), which, though subtly different from visibility, refers to the same physical quantity, is measured in feet! (varying between 2800 and 3500 feet along runway 01). I will only cherry-pick one more item: the sea level barometric pressure (beginning with an ‘A’ denoting ‘altimeter setting’) is expressed in inches of mercury (29.64). And there is yet more variety! For instance, my retired pilot father treasures an old altimeter rescued decades ago from the wreckage of a Soviet Bloc aircraft. This indicates the altitude in meters and the pressure setting in millimeters of mercury (a unit familiar to most people through its almost universal use in the measurement of blood pressure). The units used in this book reﬂect some of this diversity (by ‘some’, I basically mean no millimeters of mercury), in an attempt to offer ﬁgures that feel reasonably comfortable on both sides of the Atlantic. All of the foregoing is meant, in fact, as an elaborate apology to those who may, in spite of my best efforts, still not always ﬁnd their favorite units in the text – I hope that such readers will ﬁnd the conversion charts included in the Appendices useful.

Part I

In a hostile environment

1. A sense of not belonging

“...I gently topped out, 200 ft short of 88,000 ft. From there, Singapore looked tiny and I convinced myself that I could see from the very southern tip of Vietnam over my left shoulder, past the Borneo coast in my 11 o’clock, to the western coast of Sumatra on my right-hand side. The sky was pitch black above me and all of a sudden I realised that I did not belong here.” Lightning pilot Flt Lt Dave Roome cited in [28]

1.1 In the ‘death zone’ The human species has evolved at relatively low altitudes and most of us have stayed there. In fact, around a third of the Earth’s population lives on coastal plains, within 100 vertical meters (328 feet) of sea level, in spite of familiar dangers, like ﬂoods, tsunamis, increased risk of malaria, etc. Climb to an elevation of a mere 500 meters (1640 feet) and three quarters of the global population will be beneath you. The ﬁgure rises to over 90% by the time you have reached 1500 meters (just below 5000 feet) [37]. We are poorly adapted to high altitudes – we function less and less well as the ambient air pressure decreases and the partial pressure of oxygen in our lungs decreases with it. As we go higher, this hypobaric hypoxia1 gives us increasingly severe acute mountain sickness (with symptoms like dizziness, nausea, headaches, etc.), blood clotting, dehydration, pulmonary edema, cerebral edema, lack of judgement, ataxia (severe impairment of muscle coordination), sleep disturbances, weight loss and so on. The speed of the ascent is a critical factor, but sometimes even when we ascend very slowly, in stages, allowing our body ample time to acclimatize gradually, these conditions will still strike sooner or later. 1

Simply meaning oxygen deﬁciency associated with low ambient pressure.

A. Sóbester, Stratospheric Flight: Aeronautics at the Limit, Springer Praxis Books, DOI 10.1007/978-1-4419-9458-5_1, © Springer Science+Business Media, LLC 2011

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1. A sense of not belonging

It is perhaps this very hostility of high altitudes that has always fascinated mankind and has drawn us ever higher, long before any form of human ﬂight was even thought possible. In the autumn of 1995 a team of scientists led by American anthropologist Johan Reinhard was climbing in the northwestern Argentinean Andes when, upon reaching the summit of Mount Llullaillaco, they stumbled upon one of the most remarkable ﬁnds in the history of archeology: the frozen bodies of three Inca children, along with more than a hundred objects, including textiles, gold and silver statues, pottery and feathered headdresses [104]. They had most probably been sacriﬁced to the mountain gods around the middle of the 15th century. It was the altitude of their icy graves that made the ﬁnd unique: a (quite literally) dizzying 20 700 feet. Incidentally, this ﬁgure is close to what had been seen until well into the twentieth century as the ultimate limit a human could climb to without breathing supplemental oxygen. Reporting to the Royal Geographical Society on the Everest Expedition of 1936, expedition doctor Charles Warren noted that animal experiments had shown that “it was useless to attempt to acclimatize to altitudes greater than 21 000 feet”, an observation that the expedition had appeared to have conﬁrmed. He concluded that “physiologists may have been right after all when they put the greatest altitude at which man can live and thrive at this same level” and attempting to climb any higher without bottled oxygen is thus probably futile2 . This remained the scientiﬁc orthodoxy for a long time to come, until a rather spectacular event forced a revision of the theory. On 3 June 1950 two French climbers, Maurice Herzog and Louis Lachenal, reached the summit of Annapurna – at 26 545 feet, the 10th highest peak in the world – and they did it without using bottled oxygen. To this day climbers look upon this achievement with great reverence: Annapurna is seen as one of the most difﬁcult and dangerous mountains amongst the so-called eight-thousanders (the 14 peaks higher than 8000 meters, see Table 1.1) and almost certainly the harshest climbing challenge in the Himalayas [127]. In Table 1.1 Earth’s highest peaks: the fourteen ‘8000-ers’. Without proper acclimatization a climber would only be conscious for about two minutes on any of these summits.

2

Mountain peak

Range

Height [ft]

Height [m]

Everest K2 Kangchenjunga Lhotse Makalu Cho Oyu Dhaulagiri Manaslu Nanga Parbat Annapurna Gasherbrum I Broad Peak Gasherbrum II Shishapangma

Himalayas Karakoram Himalayas Himalayas Himalayas Himalayas Himalayas Himalayas Himalayas Himalayas Karakoram Karakoram Karakoram Himalayas

29 029 28 251 28 169 27 940 27 766 26 906 26 795 26 781 26 660 26 545 26 470 26 400 26 361 26 289

8848 8611 8586 8516 8463 8201 8167 8163 8125 8091 8068 8047 8035 8013

Warren’s notes are included in the appendices of the report [111] read in front of the Society on 2 November 1936.

1.1 In the ‘death zone’

5

spite of this much celebrated success, Everest, almost two and a half thousand feet higher than Annapurna, was still seen at the time as physiologically impossible to reach without ‘artiﬁcial aids’. Indeed, this remained the prevailing opinion for the next quarter of a century and Edmund Hillary and Tenzing Norgay in 1953, as well as all subsequent expeditions in the decades that followed used bottled oxygen to reach the ‘top of the world’. In 1978, however, further re-thinking was prompted by Tyrolean Reinhold Messner and Austrian Peter Habeler, when they climbed Everest without supplemental oxygen. In fact, Messner, who is now widely regarded as probably the greatest mountaineer of all time, would go on to climb all 14 eight-thousanders in the same ‘pure’ style.

TIBETAN PLATEAU °

° ° °

°

°

Fig. 1.1 It is impossible to walk up into the stratosphere. The world’s highest mountains are in the subtropical regions of the globe, where the stratosphere is extremely cold (between about −80 ◦ and −65 ◦ Celsius, as shown by the grey band either side of the white mean temperature curve) and the characteristic turn in the temperature lapse that marks its beginning is extremely high – about twice as high as the summit of Everest. The heavy black line is the ISA temperature curve.

These are, of course, the feats of highly trained, highly gifted and patiently acclimatized mountaineers. Most people would be unconscious in less than two minutes if suddenly deposited, without any acclimatization, on the summit of an eight-thousander. In fact, only a handful of climbers have since managed to match Messner’s sequence of ascents on the 14 highest peaks without supplemental oxygen. The last thousand feet of Everest are well into the ‘death zone’, as are the summits of the other eight-thousanders. Here further acclimatization is indeed impossible and even the ﬁttest climbers retreat after summitting, as rapidly as the conditions allow it. ‘Climb high, sleep low’ is the high altitude survival mantra most climbers live by, referring to the observation that restful sleep is impossi-

6

1. A sense of not belonging

ble in the death zone – the body merely deteriorates further with every passing hour3 . So what is the current thinking on the vertical limits of human endurance? According to Southampton University Hospitals Trust consultant Dr. Mike Grocott, leader of a recent scientiﬁc expedition to Everest, the planet’s highest peak is probably very close to the limit of human survivability without supplemental oxygen – in fact, he estimates the ultimate survivability boundary to be at about 9000 meters [52]. The key to having any chance of getting as far as that – and here is the pivotal difference between coping with altitude in mountaineering and in ﬂight – is acclimatization. Climbing into the death zone without supplemental oxygen is only possible after ascending through the zone in which acclimatization is readily possible without gaining more than about a thousand feet a day and taking rest days every three days or so. Incidentally, the exact physiology of acclimatization is poorly understood, especially because it is not a single phenomenon – it is an ensemble of adjustments, which favor increased oxygen delivery to the cells and more efﬁcient oxygen use4 . All this is, of course, merely of academic interest to ﬂyers, whose ascents to high tropospheric or stratospheric altitudes rarely take much longer than half an hour, during which no discernible acclimatization will occur. Although this was already known in the late 19th century, the precise physiological impact of fast ascents into the thinner layers of the atmosphere was only understood after much subsequent sacriﬁce.

1.2 Bagfuls of O2 The early days of scientiﬁc ballooning and thus also of the new science of ﬂight physiology belonged to France. Sorbonne professor Paul Bert is credited with the ﬁrst systematic studies of what happens to humans ascending to high altitudes. For these experiments, conducted in the 1870s, he needed a low pressure chamber – he designed and built the world’s ﬁrst – and some willing subjects. The latter came in the shape of two colleagues, Joseph Croc´e-Spinelli and Th´eodore Sivel, whose ﬁrst major contribution was to help Bert demonstrate the feasibility of breathing an oxygen and nitrogen mix from a bag through a rubber tube when exposed to the low pressure environment of the chamber at ﬁrst and the upper troposphere later, in the course of a number of balloon ﬂights to altitudes up to 24 000 feet. On 15 April 1875 Spinelli and Sivel decided to push the hazy boundaries of the effectiveness of the bagged oxygen system a little further, in the pursuit of an altitude record held at the time by a British crew. To maximize their endurance they were planning to use their oxygen supply only intermittently, apparently against Bert’s advice. The vehicle was the balloon Le Z´enith and on this occasion they were joined by Gaston Tissandier, a well-known balloonist of the era. They lifted off from Paris just before mid-day. Upon passing 23 500 feet, Tissandier noted that the two scientists were looking ‘sleepy’ and he, 3

A physiologically remarkable case is that of Sherpa Babu Chiri, whom one might call the highest overnighter in history after having once spent 21 hours on the highest point on Earth. 4 The interested reader might ﬁnd further details on this, as well as practical acclimatization advice in the authoritative High Altitude Medicine Handbook [102].

1.3 From home to near space

7

himself, was feeling drowsy. As for the rest of the ﬂight, we can only reconstruct little of what happened, based on the patchy recollections of a Tissandier drifting in and out of consciousness: he later remembered vignettes of being unable at times to use his oxygen bag, as were the other two, yet not being particularly concerned about this. In fact, he even recalled one of his companions throwing equipment overboard to enable further ascent, while he and the other scientist were already barely conscious [57]. The balloon eventually landed 155 miles southwest of Paris after having reached, on the evidence of the primitive on-board barograph, a peak altitude of 28 000 feet. By then, however, only Tissandier was alive. The deaths of Spinelli and Sivel were made all the more poignant by their sheer needlessness: the three oxygen bags were found to be nearly full [107]. The unused oxygen, as well enthusiastic continuation of the ascent by the balloonists who have abandoned their oxygen tubes, were typical hallmarks of the insidious way in which hypoxia took its ﬁrst aviator victims and still occasionally kills the unwary today. Its onset is gradual and, most importantly, pleasant. Loss of judgement is amongst the ﬁrst symptoms and this makes the threat of hypoxia exceptionally difﬁcult to design out of any high altitude aircraft. Spinelli and Sivel had been aware of the importance of using their oxygen above certain altitudes and they will have no doubt learnt in Bert’s chamber how quickly the time of useful consciousness diminishes with altitude. Yet, as hypoxia began to set in, they clearly did not care about any of this. Thus, the moment they decided that they would ration their oxygen supplies during the ﬂight by only using them intermittently, they were doomed. The fatal error lay in their belief that, after short periods of breathing ambient air, they would reconnect their oxygen bags when required. And this is the lesson that air forces around the world hammer into every novice pilot by putting them into hypobaric chambers (modern versions of Bert’s basic design) and taking them past this ‘judgement barrier’: you do not realize when you require more oxygen – by the time you do, you are in a euphoric state in which your oxygen-deprived brain will perceive everything to be going very well. Few have managed to describe this state more eloquently than former Conservative member of the British Parliament, Michael Portillo, who experienced hypoxia in a hypobaric chamber as part of his search for a humane way of executing a death sentence. “It’s like getting drunk really” he recalled after his hypoxic ‘trip’. “You experience pleasure, lightheadedness and an extraordinary amount of self-conﬁdence.” He was given a number of simple tests of judgement, which he failed, but was convinced in his state of euphoria that he had performed brilliantly. Portillo went on to conclude that hypoxia could be the most humane form of capital punishment [120].

1.3 From home to near space Evolution has optimized the human respiratory system for operation at an ambient pressure of around 1 atm (one atmosphere = 1013.2 mbar). This is the sea level sum of the partial pressures of oxygen (around 0.2 atm), nitrogen (just under 0.8 atm) and very small amounts of water vapor, carbon dioxide, argon and other gases. These numbers fall proportionally as we ascend through the various layers of the atmosphere. The total ambient pressure of

8

1. A sense of not belonging

Table 1.2 Time of useful consciousness in individuals who had been breathing pure oxygen before having their masks disconnected. These times would be considerably shorter in a pilot who had been breathing air (data from [99]). Altitude [feet]

22 000 25 000 28 000 30 000 35 000 40 000 65 000

Time of useful consciousness After breathing oxygen After breathing air Minimum observed Average 5–10 minutes 2–3 minutes 60–90 seconds 45–75 seconds 30–45 seconds 18–30 seconds 12 seconds

75 seconds 50 seconds 36 seconds 17 seconds 10 seconds

110 seconds 75 seconds 60 seconds 33 seconds 18 seconds

the air halves by an altitude of 18 000 feet, drops to a quarter by 34 000 feet and to just over 1% of the sea level value by 100 000 feet, though the composition remains around one ﬁfth oxygen and four ﬁfths nitrogen throughout. So what happens to the air inhaled in normal, sea level conditions? As it enters the trachea it warms up to body temperature and becomes saturated with water vapor. The latter has a constant pressure of about 0.06 atm, so the sum of the partial pressures of the inspired gases here drops to 0.94 atm (again, maintaining their respective proportions). Next the moist, warm air enters the pulmonary alveoli (the small cavities in the lungs where the transfer of gases in and out of the blood takes place), where the partial pressure of oxygen drops further to around 0.13 atm. The CO2 released from the blood (partial pressure 0.05 atm), nitrogen (partial pressure 0.75 atm) and the water vapors (still at 0.06 atm) make up the rest of the 1 atm [25]. The oxygen is then transported by the blood to the tissues in solution and in chemical combination with haemoglobin. At sea level air pressure arterial haemoglobin is about 98% saturated with oxygen. This is a very important number in terms of the efﬁciency of the blood supply of the tissues and its sensitivity to variations in alveolar oxygen pressure therefore determines our tolerance of altitude changes. Interestingly, if the partial pressure of the oxygen inside the alveoli drops from 0.13 atm to 0.1 atm, roughly corresponding to an ascent to 5000 feet, the haemoglobin saturation only drops by about 3–4%. A climb to an altitude of 10 000 feet causes a drop of the alveolar oxygen pressure to about 0.08 atm – once again, the oxygen saturation only falls another four 4% or so. At around this point, however, the saturation begins to drop more sharply – for example, a further 0.04 atm reduction in the partial pressure of oxygen will lead to a drop of the oxygen saturation in the haemoglobin of over 40%5 . 10 000 feet is therefore seen as the altitude above which hypoxia becomes a factor in the safety of unpressurized ﬂight without supplemental oxygen; crews of unpressurized aircraft (whether unpressurized by design or by accident) must always breathe supplemental oxygen beyond the 10 000 feet mark. At this point the human body enters a compensatory stage if supplemental oxygen is not provided. A number of physiological adjustments occur that should compensate for the 5

See, for example, [64] for the complete oxygen–haemoglobin dissociation curves.

1.3 From home to near space

9

effects of the reduction of the oxygen supply. The respiratory rate increases (technically as a result of the increase in the partial pressure of CO2 , rather than a direct result of the reduction of the O2 pressure – the direct effect of oxygen deprivation on the respiratory center is actually slightly depressive), as does the pulse rate and the systolic blood pressure. In spite of these adjustments, however, a number of potentially dangerous symptoms begin to appear at this stage: fatigue, irritability, headache and decrease in judgement and mental alertness. Such compensations must, of course, not be confused with the much slower process of acclimatization (taking days or weeks), nor are they necessarily relevant to the case of a sudden failure of the oxygen equipment or sudden decompression in a pressurized aircraft (with potentially more severe symptoms) – rather they would occur if no antihypoxia measures were taken or if these measures were to fail gradually and therefore unnoticed. An altitude of about 15 000 feet marks the start of the next, more serious stage of hypoxia: the disturbance stage. The US Naval Flight Surgeon’s Manual [64] provides a stark list of the symptoms an aviator may expect upon passing 15 000 feet without pressurization or supplemental oxygen: headache, fatigue, somnolence, lassitude, ‘air-hunger’, impaired visual acuity, loss of touch and sense of pain, slow thinking, faulty judgement, poor memory, euphoria, elation (or, in some cases, moroseness, pugnaciousness), gross overconﬁdence, loss of muscular coordination, hyperventilation... In other words, the physiological responses seen at the compensatory stage are now completely inadequate6 . Increasing amounts of supplemental oxygen are required upon further ascent if safe mission performance is to be maintained. Due to uncertainties resulting from differences between individuals and the inherent inaccuracy of oxygen pressure regulators it is hard to pin down a boundary beyond which 100% oxygen must be delivered to an aviator’s mask, but it is estimated that breathing 100% O2 at around 34 000 feet will result in roughly the same alveolar oxygen pressure as when breathing air at sea level and this pressure will drop to its minimum acceptable value (around 0.08 atm) at around 40 000 feet. Therefore, beyond this altitude the pressure at which the 100% O2 is delivered to the mask must be increased to greater than the ambient pressure. This positive pressure breathing essentially forces oxygen into the lungs, distending the chest. This has the rather unpleasant effect of reversing the instinctive cycle of breathing: inspiration is now aided by the oxygen regulator and expiration necessitates active effort. For most aviators this requires training and, on top of an already considerable workload, it can get tiring relatively quickly. Another awkward side effect is that speaking becomes difﬁcult when exhaling requires effort. The altitude freedom gained by positive pressure breathing is relatively meagre too: beyond about 45 000 feet the pressure that would have to be fed to the mask to maintain the necessary O2 partial pressure in the alveoli might just be sufﬁcient to cause injury to the lungs. The inevitable can therefore only be postponed in this way by about 5000 feet. This is not a great deal, but a positive pressure breathing system is still a very useful emergency device in case of a depressurization of the cabin and/or a high altitude ejection. It cannot be relied upon for routine ﬂying though, so it is common practice to design high altitude ﬁghter jets to ensure a maximum cabin altitude that is comfortably 6

This list of symptoms experienced above 15 000 feet illustrates the extraordinary power of acclimatization: a climber heading towards Everest will not have even reached base camp at this point...

10

1. A sense of not belonging

below the positive pressure breathing limit. In fact, even on air superiority ﬁghters capable of ﬂying at 60–65 000 feet, the cabin pressure is usually below the 100% oxygen requirement limit too. A typical cabin altitude is around 15–25 000 feet when ﬂying near the maximum operating altitude and it often stays as low as 8000 feet (as we shall see, this is almost passenger airliner comfort!) at ﬂight altitudes of up to 20–25 000 feet. Incidentally, an effective aid to relieving the fatigue caused by the effort of exhaling at higher cabin altitudes is the partial pressure suit (see Figure 1.2 for an early example) or partial pressure jerkin, which, through cross-stitches and inﬂatable tubes, can compress the chest to make up for the reduction in ambient pressure. The operating principle is the same as that of the g-suit, which exerts pressure on the legs and the abdomen during high positive acceleration manoeuvres, reducing the amount of blood draining away from the aviator’s brain and thus reducing the probability of the often fatal ‘G-LOC’, or G-Induced Loss of Consciousness.

Fig. 1.2 An early partial pressure suit worn in 1958 by NASA test pilot Joe Walker. Image courtesy of NASA.

To spare a thought at this point for designers of airliners, their task has an added dimension beyond what military or research aircraft engineers have to contend with. They have to design spacious, comfortable cabins to be occupied by passengers and crew who cannot be expected to be on oxygen as a matter of routine (and cannot be issued with pressure suits along with their boarding passes). Taking a ‘shirt sleeve’ environment into the stratosphere at costs that an airline passenger is readily willing to part with is a technological

1.3 From home to near space

11

challenge richly deserving of deeper analysis, so we shall return to it later – for now, let us resume our imaginary ascent through the stratosphere. At around 50 000 feet we enter what is sometimes termed the space equivalent zone. Here, at least as far as hypoxia is concerned, one might as well be in outer space. Even when pressure breathing 100% O2 the arterial saturation of oxygen drops below 60% of the normal, sea level value, roughly the same as when breathing air at 25 000 feet [64]. This, as we have seen, is well into the critical range in terms of oxygen transport. For possible exposure to the atmosphere at altitudes above 50 000 feet the aviator must therefore wear a full pressure suit. Like the suits worn by astronauts, these have an inﬂatable bladder layer inside, which is pressed against the wearer’s body by a restraint layer. As the ambient pressure drops, the restraint layer prevents the bladder from expanding to compensate and this results in an increased pressure on the body of the wearer. The bladder is generally pressurized to values considerably lower than the sea level ambient pressure, mainly because a ‘softer’ suit allows for greater mobility. The space equivalent zone is a rather sparsely populated layer of the stratosphere, with only a handful of aircraft types ever having been capable of routine operations beyond 50 000 feet. A notable example is the Lockheed SR-71 Blackbird (more on which later), with a maximum operating altitude well in excess of 80 000 feet. Remarkably, the SR71 (and other members of its family) could, in theory, provide its crews with a ‘shirt sleeve’ environment – or ‘shirt sleeve plus oxygen mask’, to be precise – being capable of maintaining a constant cabin altitude of 26 000 feet, which it reached after the aircraft passed about 30 000 feet in the climb [89]7 . Nonetheless, as illustrated by the photograph of NASA test pilot Don Mallick (Figure 1.3), the results of a potential pressurization failure at the Blackbird’s typical operating altitudes (70–85 000 feet) were seen as dramatic enough to warrant the wearing of a full pressure suit on every mission above 50 000 feet. The ﬁnal physiological milestone of this theoretical unprotected ascent is related to the water contained within our bodies. As with other ﬂuids, water will vaporize rapidly (boil) at speciﬁc combinations of ambient pressure and temperature values. For example, at sea level ambient pressure (1013 mbar) boiling will occur at 100 ◦ C, on the summit of Everest (314 mbar) at around 70 ◦ C and so on. Clearly, a point of interest along this scale is the altitude (and corresponding pressure) at which water will boil at body temperature, 37 ◦ C. As it turns out, this occurs at around 63 mbar – the atmospheric pressure drops to this value at an altitude of about 63 000 feet – this is known as the Armstrong Point (named after Major General Harry George Armstrong, a highly decorated US Armed Forces ofﬁcer and doctor with a rich track record in aerospace medicine). Of course, with well over half of our body weight made up of water, the prospect of exposure to a pressure where that water might boil would not ﬁll anyone with joyful anticipation, but the Armstrong Point is best viewed as a theoretical datum, rather than a practical ‘point of no return’. This is mainly because, in spite of the popular image of 7

More impressively still, the Blackbird also had a ‘10 000 feet’ cabin pressurization schedule. With this selected, the cabin pressure stayed at 10 000 feet up to 26 500 feet ﬂight altitude, corresponding to a pressure differential of 5 psi between the cabin pressure and the ambient pressure, after which it rose at a rate that would maintain this differential, reaching a cabin altitude of 26 000 feet at a ﬂight altitude of 90 000 feet. This schedule, however, did not cool the cockpit during descents from high altitude as well as the ‘normal’ one [89].

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1. A sense of not belonging

Fig. 1.3 NASA test pilot Don Mallick, in full pressure suit, stands in front of the YF-12A Blackbird. Image courtesy of NASA.

blood boiling and bodies exploding, only superﬁcial bodily ﬂuids would be immediately affected – for instance, the saliva in the mouth of the victim would begin to ﬁzz. Considerably lower pressures would be required for the water contained in tissues (not to mention the blood) to begin to vaporize. In very low ambient pressure experiments conducted on unprotected human hands (belonging, one would assume, to staggeringly brave volunteers) the vaporization of tissue water – as indicated by swelling – never occurred below an equivalent altitude of 80 000 feet. In several experiments up to ten minutes elapsed before any swelling was visible, by which time the equivalent altitude in the pressure chamber was as high as 110 000 feet [64]. Incidentally, the history of humankind’s presence in the stratosphere has recorded the hair-raising story of an intrepid individual who inadvertently acquired ﬁrst hand (sorry!) experience of exposing a hand to such pressures (Chapter 10 will provide a detailed account of the event) . To sum up then, for the purposes of stratospheric ﬂight, ebullism (to give the vaporisation of bodily ﬂuids its technical name), is much less of a concern than hypoxia. As we are about to see, it is also a less likely prospect than decompression sickness, the source of much discomfort even at pressures considerably closer to what most of us are used to.

1.4 Nitrogen bubbles

13

1.4 Nitrogen bubbles One of the most remarkable aircraft ever to have graced the more forbidding and rarely visited heights of the stratosphere is the Lockheed Skunk Works U-2 ‘Dragon Lady’. We shall dedicate part of a later chapter to this unique 18 tonne Cold War ‘eye in the sky’; for the purposes of the present discussion of decompression sickness it is sufﬁcient to note that it has single- and two seat versions, a classiﬁed service ceiling known to be in excess of 70 000 feet and an endurance of around half a day. Such eye-watering performance requires a special breed of ﬂight crew. These are pilots whose ‘ofﬁce’ comes with a view of the blackness of space above and a horizon with a pronounced curvature underneath. Populating these mesmerizing borderlands of ﬂight comes at the cost of some unique risks though. The cockpit of the U-2 is pressurized, but only to a maximum equivalent altitude of 35 000 feet, so the pilot must constantly be on positive pressure 100% oxygen to prevent hypoxia. Additionally, in case the cabin pressurization system fails and the cabin altitude rises above 50 000 feet, at least a partial pressure suit must also be worn, including a full helmet. All this makes a 12-hour mission a less than luxurious proposition. These inherent discomforts, as well as ‘old man problems’ were ﬁrst blamed by a 47year old U-2 pilot on a long endurance sortie over enemy territory in 2006 when, two and

Fig. 1.4 A pilot climbing into the cockpit of a Dryden Flight Research Center ER-2, a research version of the U-2. Image courtesy of NASA.

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1. A sense of not belonging

a half hours into the ﬂight, he began feeling various aches and pains. First his knees, then his ankles gave him trouble, which he tried to alleviate by adjusting his rudder pedals, then by increasing the pressure in his suit. Over the next two hours he was feeling progressively worse, with confusion, headache, fatigue and poor concentration adding to his symptoms. Eating, drinking and adjusting his oxygen supply did not help and four hours later he reported his problems to ground control. They immediately instructed him to return to base. This was to be a nightmare trip. He was sick in his helmet several times and his mental functions (including hearing and vision) deteriorated to the extent that he was soon unable to communicate via the radio. His arrival at base must have been an unnerving sight as he attempted to land on the wrong runway three times (once coming within feet of what was probably going to be a fatal impact) before ﬁnally managing a safe touchdown. To the physician arriving ﬁrst at the aircraft the symptoms of the pilot added up to a clear picture: he was obviously suffering from dysbarism – that is, his condition was caused by changes in the atmospheric pressure. More speciﬁcally, he diagnosed severe decompression sickness with neurological symptoms and incipient cardiovascular collapse, probably brought on by the pressure changing from that experienced on the airﬁeld before the ﬂight to the minimum ambient pressure in the cabin of the U-2 in cruise, equivalent, on that ﬂight, to an altitude of 28 000 feet (no technical problems were found upon inspection of the environmental systems of the Lockheed) [68]. Decompression sickness (DCS) is a collective term referring to the effects of the formation of bubbles from gases (mainly nitrogen) dissolved in body tissues, when this is the result of a reduction in the environmental pressure. Its best known manifestation is the bends, an occupational hazard that has always blighted divers, who, upon rising to the surface with undue haste, can suffer from severe abdominal cramps. DCS can strike aviators when exposed to altitudes above 18 000 feet or so and the symptoms can range from the bends to, as the (admittedly extreme and unusual) case of the U-2 pilot illustrates, serious neurological problems. In fact, while seemingly recovering within a few months, he was eventually found to have suffered permanent brain damage severe enough to prevent him from ever ﬂying again [68]. The dissolved gases coming out of a solution in tissues as a result of a drop in the ambient pressure have two harmful effects. The more obvious is the simple mechanical impact of the bubbles distorting tissues and obstructing vessels. The second effect is damage caused to the inner lining of the blood vessels and a release of pain mediating substances. The U-2 incident is also an illustration of the progressive nature of DCS: the initial symptoms may be relatively trivial, but further expansion of the bubbles, as well as delayed formation of bubbles elsewhere in the body may have a life-threatening impact [64]. There is little one can do to prevent DCS, though pre-breathing 100% oxygen at rest for at least an hour before ﬂight (and then switching seamlessly, without breathing any air, to the aircraft’s oxygen supply) is thought to be effective in reducing the probability of occurrence. This is a ‘denitrogenation’ process, whereby nitrogen is ofﬂoaded from the tissues into the blood. If DCS does occur, re-compression should be ensured as soon as possible, ﬁrst by descending to a low altitude, then on the ground by treatment in high pressure chambers. If possible, the patient should be breathing 100% oxygen during this hyperbaric treatment.

1.5 A head for heights

15

While DCS is at best a nuisance and at worst a serious threat to the life of air crews, it is not especially frequent. A NASA study examined records of their ﬂights in Northrop T-38 Talon high altitude training aircraft (designed to maintain a maximum cabin altitude of around 22 000 feet at the maximum operating altitude of 50 000 feet). Based on analysis of 20 years’ worth of ﬂying data, altitude chamber data and mathematical models of bubble growth they estimated that exposure to 20 000 feet for an hour and a quarter puts an inactive crew at a DCS risk of under 1% (physical activity makes DCS more likely). They also found, however, that the probability of developing DCS could be above 30% for even relatively brief exposures to altitudes above 40 000 feet [108]. Beyond the hypoxia considerations discussed earlier, this is another good reason for wearing a full pressure suit when ﬂying at very high altitudes – the pilot of, say, a U-2, could otherwise suffer very serious DCS in case of a cabin depressurization at 70 000 feet. Hypoxia and DCS are, clearly, the pivotal physiological drivers of high altitude manned aircraft design. However, relatively abrupt changes in ambient pressure, of the magnitude to be expected by passengers and crew of high altitude aircraft, can have a range of other unpleasant effects on our emphatically earthbound species, some of which even airline passengers are painfully familiar with.

1.5 A head for heights It could be argued that few parts of the human anatomy are as poorly equipped for variations in ambient pressure as the middle ear – indeed, it can be at the root of severe discomfort even on routine airline ﬂights. Its basic ‘ﬂaw’ lies in its slightly deﬁcient pressure equalization mechanism known as the Eustachian tube. The middle ear is essentially an air-ﬁlled housing for the ossicles that transmit the vibrations of the eardrum to the cochlea in the inner ear. The thin membrane of the eardrum itself is the divider between the outerand the middle ear and it can be the source of excruciating pain when there is a difference between the pressure inside the middle ear and the ambient pressure of the outer ear. The Eustachian tube connects the inner ear with the pharynx (the part of the throat behind the nasal cavity and the mouth) and it thus offers a pressure equalization path, but it is normally closed, except when yawning or chewing. When climbing, as the ambient pressure becomes progressively lower than the pressure in the inner ear, the tube will open – this is usually signalled by an ‘ear popping’ sound – and thus prevent eardrum trauma. On the way down, however, the process is much more problematic. When the ambient pressure increases and the pressure inside the middle ear remains constant or increases at a much lower rate – that is, when descending – the Eustachian tube opens far less readily and, at higher rates of descent, the ﬂyer will be painfully aware of this. The eardrum is ‘sucked in’ towards the lower pressure middle ear, resulting in pain and a dulling of hearing, as the taut eardrum cannot vibrate as normal. In extreme cases middle ear barotrauma may result, when the stretching is intense enough to cause tissue damage. The defensive strategy is therefore to attempt to open the Eustachian tube when descending. If yawning or chewing does not do the trick, the Valsalva manoeuvre is usually worth a try. Named after 17th century Italian anatomist Antonio Maria Valsalva, this in-

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1. A sense of not belonging

volves forcibly exhaling against closed airways, that is, while keeping the mouth shut and the nose pinched. This should inﬂate the Eustachian tube and thus enable pressure equalisation8 . If you are one of the estimated 9% of people [139] who suffer some level of barotitis media (middle ear inﬂammation caused by pressure changes), the Valsalva manoeuvre may be a useful trick to master. Barodontalgia is the collective term for another family of unpleasant side effects of high altitude ﬂight: those involving toothache. The common perception (widely held among air crew too) is that this is caused by ‘trapped air’ remaining inside the tooth after a dental procedure. In fact, barodontalgia is far more complex and more mysterious than that and the subject is rather controversial in medical circles. For instance, there is a school of thought espousing that the majority of barodontalgia is simply referred pain from barotitis media or some other form of facial barotrauma, while the authors of several studies argue that only 7–9% of cases of barodontalgia have non-dental causes. This uncertainty has much to do with the difﬁculties in reproducing occurrences of barodontalgia on the ground, even when pressure chambers are available [139]. In any case, pulpitis (the inﬂammation of the pulp part of the tooth) is thought to be the main culprit for genuinely tooth-related barodontalgia, though the exact mechanism is unclear.

1.6 Cabin altitude – an uneasy compromise The greatest difﬁculties of the somewhat fraught beginnings of the era of stratospheric passenger travel – as we shall see in the second part of this book – had much to do with having to pressurize the cabin to create a comfortable cocoon for the passengers in an environment in which they would otherwise expire with disturbing rapidity. While the error margins of structural design and the tolerances of airframe manufacture have narrowed considerably since the early days of high altitude commercial ﬂight, the basic trade-off of pressure cabin design remains the same: structural weight versus passenger comfort. As our brief foray into high altitude physiology indicated, increasing passenger comfort means decreasing cabin altitude – that is, increasing cabin pressure. This, in turn, increases the cyclic loads on the cabin structure, which, to satisfy fatigue resistance criteria, thus has to be made stronger. For a given material and a given design philosophy, stronger means heavier. Ergo, the price of comfort is measured in additional airframe weight and increases in weight have a sharply detrimental impact on almost every aspect of performance – most painfully on fuel burn. Of course, ﬂight altitude also has a serious impact on fuel burn and it determines the pressure differential that needs to be maintained. This, in turn, affects structural weight – and thus completes this rather complicated loop of ﬁnely balanced trade-offs. Once again, economical stratospheric ﬂight is aviation at the limit: here this limit is whatever is acceptable for both the passengers’ lungs and their pockets. Naturally, given a choice, most of us would like to travel in sea level conditions. However, the weight penalty of maintaining a cabin altitude of zero at a ﬂight altitude of, say, 40 000 feet, turns out to be prohibitive, so some kind of compromise is inevitable. But 8

Incidentally, the technique is used by physicians to detect chest sounds speciﬁc to certain heart conditions [138].

1.6 Cabin altitude – an uneasy compromise

17

where do we draw the line? As any weight saving is worth having in aircraft design, the question actually amounts to deciding what is the highest acceptable cabin altitude that passengers can cope with without signiﬁcant discomfort. We have seen that above altitudes of around 10 000 feet the oxygen saturation of haemoglobin plunges quite abruptly, so this ﬁgure would seem to be a sensible upper boundary on the allowable normal, cruising cabin altitude. At the lower end, there is some evidence (though by no means overwhelming) that the comfort beneﬁts of lowering the cabin altitude below 4000 feet are negligible [59]. The optimum is therefore probably between these two ﬁgures, but as to where exactly it is, there is no clear consensus, particularly because the exact physiological differences between adopting higher or lower values within this range are relatively poorly understood. The regulatory authorities on both sides of the Atlantic have drawn one line in the sand: no passenger aircraft will receive its airworthiness certiﬁcate unless it is “equipped to provide a cabin pressure altitude of not more than 8000 ft at the maximum operating altitude of the aeroplane under normal operating conditions.”9 There is, however, no similar regulation requiring the airlines to actually operate the aircraft at a maximum cabin altitude of 8000 feet – it seems, however, that most of them do. A 1989 study [48] recorded the average maximum cabin altitude of 123 commercial airline ﬂights as 5500 feet, with another report from around the same time estimating the average at 5673 feet (with a standard deviation of 2019 feet and a median value of 6214 feet) [40]. More recently, a Boeing study conducted over one week in 2000 indicates that 90% of the cumulative time commercial aircraft spent ﬂying over the continental United States was with cabin altitudes not exceeding 7000 feet [87] (as relatively few airliners in service in 2000 had been designed to offer passengers anything better than the minimum requirement of 8000 feet at maximum operating altitude, this ﬁgure is simply a result of most ﬂights cruising well below the maximum operating altitude of the aircraft). What little evidence we have for the physiological reaction of passengers to being exposed to these typical cabin altitudes paints a somewhat uncertain picture. A recent set of trials funded by the Boeing Company [88] monitored subjects exposed to cabin altitudes of 650, 4000, 6000, 7000 and 8000 ft over simulated 20-hour ‘ﬂights’. The analysis revealed symptoms of acute mountain sickness in 7.4% of the participants, but, interestingly, there was no signiﬁcant difference across the ﬁve cabin altitude groups! What did change, however, was the occurrence of various types of discomfort reported by the subjects: malaise, muscular discomfort, exertion, fatigue and cold stress were signiﬁcantly more frequent at 7000 and 8000 feet than at the other three altitudes combined. This latest study thus joins a long line of earlier experiments, stretching back to the earliest days of pressurized passenger ﬂight, which contain little in the way of black and white results that might allow us to pinpoint a ‘correct’ cabin altitude. The Aerospace Medical Association has recently published a ‘Position paper’, which, upon reviewing all relevant reports to date, concludes that “we did not ﬁnd sufﬁcient scientiﬁc data to recommend a change in the cabin altitude of transport category aircraft.” They also recommend that further research should be funded

9

As stated in Part 25 of the Federal Aviation Regulations and the Joint Aviation Requirements in the US and in Europe respectively.

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1. A sense of not belonging

“to evaluate the safety, performance and comfort of occupants at altitudes between 5000 and 10 000 feet [5].” As an aside, there is a category of frequent ﬂyers, for whom higher cabin altitudes are actually desirable: performance athletes ﬂying to competitions held at high altitude venues have a chance to begin their acclimatization on the ﬂight. Especially on long haul ﬂights this may allow them to leave home a little later, though, once again, the complexities and uncertainties of high altitude physiology make quantifying this ‘head-start’ quite difﬁcult [53]. The structural design side of the cabin pressure question is a little easier to pin down – airframes behave much more predictably than humans, though, as we shall see in Chapter 3, this knowledge came at a great cost in the earliest days of stratospheric ﬂight. Today, thanks to sophisticated numerical computer models, the approximate relationship between cabin pressure and stress in the frames and the skin of the fuselage is relatively easy to establish. In turn, the stress values allow engineers to estimate the all-important fatigue life of the airframe. To gain an insight into the impact of cabin pressure increases on fatigue life, consider the example of the Boeing 737 family. Most models in the series have a maximum cabin altitude of 8000 feet, as stipulated by the airworthiness standards. Depending on maximum certiﬁed altitude, this means a maximum differential pressure (or pressure cycle amplitude) of between 7.5 psi and 9.1 psi, which gives the 737 a safe service life of 75 000 airframe cycles (as we shall see in a later chapter, in the early days of high altitude commercial ﬂight 10 000 cycles was seen as a reasonable service life to design for!). The BBJ, or Boeing Business Jet, which is, essentially, a luxury version of the 737-700, aims to improve the well-being of its passengers by dropping the maximum cabin altitude to 6500 feet. The trade-off: it is only certiﬁed for 65 000 cycles [17]. This is not a problem for operators of the BBJ, as they hardly ever ﬂy the punishing schedules of eight sectors a day, 365 days a year, which many short haul airlines buy their 737s for, but it serves to make the point that a 1500 feet cut in cabin altitude reduces the service life by 20%. The BBJ is not alone in the world of business aviation in offering an extra bit of ‘altitude comfort’. The Bombardier Global Express XRS can drop its cabin altitude to as low as 4500 feet when cruising at an altitude of 45 000 feet and to 6000 feet at its rather impressive service ceiling of 51 000 feet. While the peak cabin altitude (corresponding to the maximum operating altitude) is the headline ﬁgure that is usually quoted, designers of some VIP jets go to great lengths to keep the altitude low throughout the operating cycle of the aircraft. For instance, the Dassault Falcon 7X, which also boasts a 6000 feet cabin altitude at 51 000 feet, additionally features the option of remaining at a cabin altitude of only 1000 feet up to a ﬂight altitude of 27 000 feet [59]. Advances in structural design and materials (including radical developments, like the move towards composite fuselages) mean that this sort of comfort will soon be routinely available to the rest of us too – the cabin of the Boeing 787 Dreamliner is pressurized to 6000 feet at its service ceiling of 43 000 feet. Incidentally, all this puts into perspective the engineering wizardry that maintained the passengers of Concorde at a cabin altitude of only 6000 feet at its staggering service ceiling of 60 000 feet – all this with 1960s technology. Of course, cynics might argue that proﬁtability took a serious hit somewhere amongst Concorde’s impressive performance metrics, once again serving as a reminder of the fact that the boundaries of safety, performance, comfort and economy can be very close to each other in stratospheric ﬂight.

1.7 Health matters

19

If one was to summarize the foregoing, one might conclude that the human passenger is extremely fussy and rather fragile and the environment where most commercial ﬂying happens is extremely inhospitable. A lot is asked therefore of aircraft design engineers who have to develop cabin pressurization and air conditioning systems with these two immovable constraints in mind – we shall dedicate the next chapter to this design challenge. Cabin environment systems may appear unglamorous, but, as we shall see, the consequences of their (thankfully very rare) failures can bring disaster in sinister and insidious ways. Before we broach this rather complex subject though, we shall conclude this chapter by asking the question: do we (passengers and/or air crew) risk our health by ﬂying at stratospheric altitudes?

1.7 Health matters The pilot quoted at the outset of this chapter recalls ﬂying the English Electric Lightning, a supersonic ﬁghter of the Cold War, to the ragged edge of its performance, climbing to nearly 90 000 feet. We also touched upon the case of a U-2 pilot suffering permanent neurological damage on a very high altitude mission in an aircraft that, based on the available evidence, is assumed to have worked as designed. What these two pilots have in common is the rather extraordinary nature of the environments they ﬁnd themselves in on a regular basis, poised on the boundary of what mankind knows and is capable of doing in the stratosphere. With that in mind, neither the reader, nor the pilots themselves will be surprised to learn that the cost of being at this spearhead of aviation technology may be measured in increased health risks (for the purposes of this discussion let us not consider the possibility of accidental injury as a health risk). After all, their working environment is often cramped, uncomfortable, subject to extreme pressure variations, relatively intense cosmic radiation and potentially still unknown perils; they accept the resulting risks as inevitable consequences of the way in which they have chosen to serve. What about the rest of us though, more or less frequent ‘shirt sleeve’ visitors to more hospitable altitudes? Do airline passengers, ﬂight crew and cabin crew risk their short- or long term health taking to the skies – especially if they do so fairly regularly (as the latter two categories inevitably do)? A highly controversial aspect of airline passenger health, which has received much media exposure in recent years and has led to much litigation with the airlines is the suspected link between air travel – especially economy class air travel – and blood clotting in the deep veins of the legs (deep vein thrombosis or DVT). This is an especially worrying phenomenon, as there is a risk of the blood clot traveling to the lungs and causing a fatal pulmonary embolism, though the vast majority of DVT occurrences are not only non-fatal, but also entirely symptomless. We shall not pursue this complex subject too far here as travelers’ DVT is almost certainly not unique to long haul, high altitude ﬂight – indeed, it is not even unique to ﬂight (there is some evidence that bus travel, for instance, has comparable risks), so it is outside the scope of our stratospheric musings (though it must be pointed out that a link with hypoxic conditions cannot be ruled out altogether). The strength of feeling on the subject is reﬂected by the availability of mountains of related

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research reports – wading through these is facilitated, perhaps, by recent meta-reviews10 . Here we shall limit ourselves to highlighting one such review, produced under the aegis of the Cochrane Collaboration (a worldwide group of volunteers maintaining The Cochrane Library, a database of systematic reviews of clinical trials seeking to establish the effectiveness of medical interventions). In this, Clarke et al. [35] considered the results of ten randomized trials involving 2856 subjects, all designed to determine whether the wearing of compression stockings is an effective DVT prevention measure. They found a substantial reduction in the incidence of symptomless DVT and leg oedema in those wearing the stockings. Unsurprisingly, none of the subjects died or suffered pulmonary embolus or symptomatic DVT – a trial shedding any light on the probability of potentially fatal (or at least symptomatic) outcomes would need to involve a very large number of people, so for medical researchers conducting observational studies the probability of symptomless DVT has to do, for the time being, as a surrogate measure of the risk. Beyond the wearing of compression stockings, getting up from one’s seat and walking around the cabin is also recommended by some, though it might be worth comparing the reduction thus achieved in the (already rather remote) chances of developing DVT with the increased (though also fairly remote) risk of getting hurt as a result of a sudden turbulence encounter (more on this peril later). This is not to mention the inconvenience caused to cabin crew trying to do their job and to aisle seat passengers trying to sleep – in a typical economy class cabin every ‘DVT walker’ will be leaving a trail of annoyed and/or awoken passengers in their wake at a rate of two for every 32 inches of aisle distance covered. Perhaps more research is warranted into the comparative effectiveness of walking versus simply doing in-seat leg exercises? Another much-discussed potential high altitude occupational hazard is the inevitable increased exposure to cosmic rays originating in the Sun and in many other sources in the visible Universe. Of these, ionizing radiation is of the greatest signiﬁcance – through its ability to dislodge electrons from atoms and molecules (including DNA) it can cause cancer when the exposure exceeds a certain dose. What is less certain, as with many of the other physiological aspects of stratospheric ﬂight, is the dose threshold above which it becomes a danger to air crews’ health. Radiation dose, or, more precisely, the biological equivalent dose is measured in Sievert (Sv), which has dimensions of energy per unit weight (1 Sievert = 1 Joule / kilogram). The Sv measures the biological effects of radiation – it only accounts for the proportion of radiation energy that has actually had an effect on the body. So what are typical dose values received by air crews? At ground level the Earth’s magnetic ﬁeld and its atmosphere protect us from much of the ionising radiation, the intensity of which therefore changes with latitude – considerably higher at the poles than at the Equator – and with altitude – the higher we go, the worse the exposure. This was reﬂected remarkably sharply by a study by Langner et al. [76], who looked at the radiation exposure of ﬂight crew across most of the 20th century. They found values ﬂuctuating between around 0.1–0.25 μSv/hour (that is, 0.1–0.25 × 10−6 Sv/hour) up to the 1950s – then, as the de Havilland Comet 1 (to which a separate chapter will be dedicated later) launched the era of high altitude jet travel, exposure values rose with 10

See, for instance, that of Adi et al. [2].

1.7 Health matters

21

Mean dose rate ( μSv / hour )

3 2.5 2 1.5 1 0.5 0 1920

1930

1940

1950 1960 1970 Calendar year

1980

1990

2000

Fig. 1.5 Mean radiation exposure of ﬂight crew – an increase of an order of magnitude marks the advent of high altitude jet travel in the mid- to late 50s (data based on Langner et al. [76]).

stunning rapidity. By the mid-1970s an increase of an order of magnitude was recorded! (see Figure 1.5) Unsurprisingly, long-haul ﬂights expose air crews (and passengers, for that matter) to higher doses, around 4–6 μSv/hour, while short-haul ﬂights record, on average, 1–3 μSv/hour (Concorde, ﬂying at much greater altitudes than other commercial trafﬁc, used to stand out slightly with 12–15 μSv/hour). Given the average number of hours ﬂown by air crew, a pilot’s annual tally adds up to 2–4 mSv and 1–2 mSv for long-haul and shorthaul respectively [11]. It is hard to offer a tangible point of reference for these numbers, but according to a study published in the British Journal of Radiology, the ionizing radiation exposure from a typical chest X-ray is 0.04 mSv, while from an abdominal Computed Tomography (CT) scan it is likely to be as high as 10mSv [130]. In other words, working as a pilot or a ﬂight attendant would expose you to a radiation dose equivalent to having approximately one chest X-ray a week or an abdominal CT scan once every ﬁve years (Table 1.3 contains more ﬁgures to help put air crew exposure into perspective). This does not sound particularly worrying and nor do the corresponding air crew mortality statistics. Of course, such observational analysis is notoriously fraught with difﬁculties, not least because it is extremely hard to isolate the effect of any particular variable (such as occupation) from mortality statistics. In particular, it is difﬁcult to account for the effect of air crews not being representative of the population at large – they are generally what the statistics literature refers to as ‘strong, healthy workers’ and they have a considerably higher than average socio-economic status (especially cockpit crews). Nevertheless, such ﬁgures are all we have to go on, so they deserve a mention. A study published in 2009 by the UK Ofﬁce for National Statistics [36] analysed UK death certiﬁcates issued between 1991 and 2000. The authors calculated (attempting to account for some of the effects mentioned above) proportional mortality ratios (PMRs) for a range of combinations of occupations and medical conditions, highlighting, for each condition, the list of occupations who suffered from them in statistically signiﬁcantly higher

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Table 1.3 A comparison of effective ionising radiation doses from natural and artiﬁcial sources (data from [124, 11, 130]). Nature of exposure

Typical effective dose (mSv)

Cosmic background radiation at sea level (annual average) Cosmic background radiation at high elevations (annual average) Cosmic background radiation (global annual average) Antero-posterior abdominal X-ray Antero-posterior thoracic spine X-ray Lateral chest X-ray Posterio-anterior chest X-ray Barium meal examination Computed Tomography (CT) scan of head Abdominal Computed Tomography (CT) scan Long-haul aircrew annual mean exposure Short-haul aircrew annual mean exposure Maximum annual occupationally exposed limit Maximum limit not to be exceeded in any one year Average exposure of a Chernobyl accident recovery worker

0.3 1 0.39 0.7 0.4 0.04 0.02 3 2 10 2–4 1–2 20 (5 yr average) 50 150

percentages than the overall population. Female air crew do not appear in any of the lists and male air crew (‘ﬂight deck ofﬁcers’ in the report’s standardized terminology) only feature in the list corresponding to melanoma of the skin (a malignant tumor of the melaninproducing cells located in the bottom layer of the skin’s epidermis), with a PMR of 2.45. This means that 2.45 times more air crew died of this condition than would have been expected from the population average. It is worth noting that, as there were only 12 air crew deaths recorded in the UK during the decade analyzed by the study11 , the error margin of this estimate is quite large (the 95% conﬁdence interval is 1.27 to 4.29!). By comparison, the same ﬁgure for ‘persons involved in sport’ is 2.34 (sun exposure?), for female architects and surveyors 2.91, for compositors 2.96... Similar recent studies were conducted across Europe, focusing speciﬁcally on air crews [76] and, while the ﬁndings contained therein are also difﬁcult to interpret due to the high uncertainty margins, they seem to point broadly in the same direction: there is no substantially increased risk of cancer due to ionizing radiation (though the preliminary results of a follow-up study, published in 2010, seem to indicate a raised cancer risk amongst crew employed for more than 30 years, compared to those who worked for less than 10 years [140]). Ultimately, however, the conﬁdence bounds on the numbers generated by such studies will always reﬂect the (thankfully!) very limited available data. A statistical model with numerous intractable circumstances (varying air crew lifestyles and histories) confounding the effect of the main variable (radiation exposure) on cancer rates is already

11

The word ‘only’ here is used in a simple, mathematical, comparative sense – as in, ‘only’ 12 deaths compared to, say, 907 lorry drivers having died in road accidents during the same period (according to the same study).

1.7 Health matters

23

likely to have large error margins – such a model based, additionally, on a mere handful of positive cases will rarely be suited to underpin any ﬁrm inferences. Let us now brieﬂy touch upon a highly controversial health issue related to ﬂying on pressurized aircraft, which has caused much angst amongst cabin- and ﬂight crew in recent years: cabin air contamination. More speciﬁcally, so-called fume events, that is, instances when harmful volatile compounds are thought to have entered the environmental control systems of aircraft, have provided reason for many airline personnel to fear for their health. There is much anecdotal evidence that seems to point towards this being a genuine cause for concern. As an example, here is an anonymous report by a pilot, as published by CHIRP, the Conﬁdential Human factors Incident Reporting Programme: “I’ve been an Airline Pilot for several years, and from quite early on I found that I was often feeling fatigued, and I assumed it was due to the unusual work schedule that I wasn’t used to before. As time went on, I found that my levels of fatigue were increasing, and I was beginning to feel that my short-term memory was getting worse, it was getting more difﬁcult to concentrate, and generally I was really not enjoying the job anymore. It wasn’t until about a year before I stopped ﬂying, that I began to realize this was probably not normal, and a few months later began to experience neurological problems including tremors, muscle twitches, speech problems, light-headedness and worsening fatigue and cognitive problems. I had heard that bleed air12 could get contaminated by engine oil that contains TCP, an organophosphate and neurotoxin, and I suspected that breathing day-today background levels was causing my health problems. I had an opportunity to change aircraft types, and I hoped after the change to the new type, my problems would disappear. As it happens they worsened, to the point I had to stop ﬂying due to concerns for my health, and the safety of the aircraft [33].” Such reports, as well as a few incidents of air crew experiencing the smell of fumes in ﬂight in the cabin and in the cockpit and suddenly feeling unwell to the point of becoming partially incapacitated13 , caused sufﬁcient alarm that the UK Government ordered a series of investigations into the issue, including one by the Committee on Toxicity of Chemicals in Food, Consumer Products and the Environment (COT). The issue turns out to be extremely complicated. As the CHIRP reporter quoted above speculated, fumes from engine oil leaking into the bleed air system and thus ultimately into the cabin air supply, are the most likely cause of the fume incidents. According to a Civil Aviation Authority report14 there are over 40 different chemicals contained in oil breakdown products and many have no published toxicity data, so “it is not possible to be certain whether any of these products contribute to, or are the sole cause of the recorded incidents.” According to the COT report, ‘only’ approximately 1 in 2000 ﬂights are estimated to be affected by fume events, which makes obtaining contaminated cabin air samples a difﬁcult and expensive process – nonetheless, such a study, commissioned by the UK Department 12

On most pressurized aircraft powered by gas turbine engines the primary source of cabin air is one of the early compressor stages of the engine – a valve is placed here through which the required amount of air is bled into the air conditioning system – more on this in the next chapter. 13 See, for instance, Air Accident Investigation Branch Aircraft Accident Report 1/2004, Report on the incident to BAe 146, G-JEAK during the descent into Birmingham Airport on 5 November 2000. 14 CAA Paper 2004/04.

24

1. A sense of not belonging

for Transport and carried out by Cranﬁeld University has been completed, with the results expected in the near future. We shall conclude our foray into the health aspects of ﬂying on the airlines by considering some of the circumstances that might make a passenger or crew member temporarily unﬁt to ﬂy. This is a rather tricky subject, as few passengers will be keen to forfeit an expensive ﬂight or even an entire holiday – most people would require a very compelling case for missing, for instance, the connecting ﬂight to the beginning of their Caribbean cruise or their end-of-holiday Sunday afternoon return ﬂight. An equally convincing argument would have to be made for a pilot or a ﬂight attendant to miss a shift when they might feel ﬁt to report for duty. Speaking of warm holiday destinations, let us ﬁrst consider an easily avoidable pitfall: scuba diving and ﬂying on the same day. Earlier we introduced decompression sickness as a hazard traditionally associated with diving, or, rather, rapid resurfacing after diving. Even relatively shallow dives can dissolve a great deal of nitrogen in a diver’s body and, after longer exposures, a slow ascent to the surface may be necessary to allow time for the gradual release of this nitrogen in the tissues in the form of bubbles. The absence of decompression sickness symptoms (typically the bends) upon the diver’s careful return to the surface is, however, not necessarily evidence of the dissolved nitrogen levels having completely returned to normal and this may become painfully obvious if the ascent is continued from sea level – for instance by boarding an aircraft. Even the relatively small pressure changes associated with pressurized aircraft could have unpleasant consequences. Establishing rules for determining a safe pre-ﬂight surface interval after a dive is not easy, mainly because it depends on the diving schedule (depth under water versus time) and the pressurization schedule experienced during the ﬂight (cabin altitude versus time). Correspondingly, different studies have looked at different combinations of these variables over the years. To pick a recent report relevant to recreational divers wishing to take commercial airline ﬂights, let us consider the ﬁndings of a recent series of experiments conducted at the Center for Hyperbaric Medicine and Environmental Physiology at the Duke University Medical Center. The 495 subjects of the trial completed a total of 802 simulated dives, followed after various pre-ﬂight surface intervals by 4-hour simulated airline ﬂights (in a pressure chamber) at a cabin altitude of 8000 feet. The diving schedules were chosen to be near the single and repetitive non-decompression limits, that is, they were at combinations of depth (40, 60 and 100 feet) and exposure time that, according to US Navy guidelines, were near the boundary of what was safe to perform without decompression stops during the ascent. Technicians watched the subjects during the ‘ﬂights’ for signs of decompression sickness and any subsequent symptoms were also recorded via phone interviews the following day. A total of 40 cases of decompression sickness were recorded, eight of which resolved spontaneously, three resolved during the descent and 29 cases required hyperbaric treatment. From these observations estimates of pre-ﬂight surface intervals for low decompression sickness risk were determined as 11–12 hours after single dives and 17 hours after multiple dives [125]. Of course, this being a fairly limited study with simulated, rather than actual ﬂights, only nine recreational dive proﬁles and fewer than a thousand dives, the two numbers given above should be treated with caution. Nevertheless, they may provide a basic guideline to help recreational divers plan their return ﬂights from a holiday.

1.7 Health matters

25

Let us now consider a possible reason for postponing a ﬂight that may be much harder to avoid: inﬂammations of the passageways of the ears, perhaps as a result of a severe upper respiratory tract infection or otitis media (middle ear inﬂammation). We have already touched upon the critical role played by the Eustachian tube in bringing back the pressure in the middle ear to the ambient pressure when the latter changes and we have seen how even a healthy Eustachian tube can cause discomfort on descent. If an inﬂammation causes severe blockage in the Eustachian tube, such that it cannot be cleared before descent, the result of ﬂying could be middle ear bleeding or even eardrum rupture. Chewing, yawning, the Valsalva manoeuvre and conscious attempts at ‘popping’ the Eustachian tube might clear some blockages, depending on the severity of the infection – what about small children though or adults who cannot do any of the above effectively? Pre-ﬂight treatment with antibiotics, anti-inﬂammatory drugs and decongestants can help, but more severe cases may still present the passenger or even the physician with a tricky judgement call as to whether it is safe to ﬂy or not. If ﬂying cannot be postponed (perhaps for an unrelated medical reason), myringotomy (a pressure relieving incision in the eardrum) might be considered [6, 26]. To end this short foray into the physiology of ﬂight, we might conclude that humans make fussy stratospheric ﬂyers. This presents aeronautical engineers with rather momentous technical challenges and with relatively narrow error margins – just how narrow, is the subject of the next chapter.

2. Comfort zone

“The sky can be a hostile place, deceptive and dangerous [...] Flight takes us to places where we’re alien, unﬁt to live.” William Garvey and David Fisher Introduction to [51]

2.1 Adrift Shortly after 08:00 in the morning of the 14 August 2005 two Hellenic Air Force F-16 Fighting Falcon jets were headed towards the airspace above the Greek capital. Their sortie had an unusual destination: a Boeing 737-300 airliner, which, operating as Helios Airways Flight 522, was in a holding area 34 000 feet above Athens airport, just commencing its sixth lap along the racetrack-shaped pattern. The reason for the interception: the crew of the aircraft had, by then, not been answering any radio calls for over two hours. Helios 522 had left Larnaca, Cyprus, at 06:07, bound for Athens1 . Five minutes later the air trafﬁc control centre at Nicosia cleared it to its cruising level of 34 000 feet (FL340) – the crew acknowledged this and continued the climb, passing 10 000 feet, heading towards the next waypoint of the ﬂight, the Greek island of Rhodes. A further three minutes later, shortly after passing 12 000 feet in the climb, the captain made a radio call to the Helios Airways operations centre with a technical question. He wanted to know where the equipment bay cooling fan circuit breakers were, as the cooling fan lights on the control panel were off. He also mentioned that the ‘Take-off Conﬁguration’ warning horn was sounding. The dispatcher and the ground engineer who answered the call found all this somewhat bafﬂing. The cooling fan lights were supposed to be off – this simply meant that they were working properly – how could the captain not know this? Stranger still was the comment regarding the take-off conﬁguration warning. This is a horn designed to alert the crew that the aircraft is incorrectly set up for take-off – when it is on the ground. If the system detects 1

The timings, altitudes and other details in this account are based on the ﬁndings of the Greek authorities, presented in [7].

A. Sóbester, Stratospheric Flight: Aeronautics at the Limit, Springer Praxis Books, DOI 10.1007/978-1-4419-9458-5_2, © Springer Science+Business Media, LLC 2011

27

28

2. Comfort zone

a sudden opening of the throttles, it interprets this as a the start of the take-off roll, it checks the conﬁguration of the aircraft and, if any discrepancies are found (say, the ﬂaps are not deployed) the horn sounds, prompting the crew to abandon the take-off attempt and go through their pre-departure checks once again. But why would this alert sound in the air? After a brief conversation that shed no light on any of this, the ground engineer attempted to call Helios 522 once more to seek further clariﬁcations, but no response came. In fact, the next two hours would go by without them or any of the en-route air trafﬁc control centres managing to contact Flight 522, which, following this last radio contact, climbed to its cruising altitude and proceeded towards its destination. Temporary losses of radio communication are not unheard of, but, by the time the aircraft reached Athens and began its silent laps around the holding pattern, there was sufﬁcient concern to prompt the Air Force to scramble the ﬁghter jets to investigate. The two F-16s joined the 737 on its high altitude circuits at 08:23. As the pilots lined up in close formation with the airliner, an unexpected and rather sinister sight greeted them. The ﬁrst ofﬁcer (or whoever was in the right-hand seat in the cockpit) was slumped over the controls, motionless. The captain’s seat was empty. The passenger cabin was dark, but two motionless silhouettes could be seen against the light through one of the windows. They were both wearing oxygen masks and a few other oxygen masks could be seen through some of the other windows. Then, at 08:49, on the tenth lap of the holding pattern, one of the Fighting Falcon pilots suddenly spotted movement in the cockpit. A man in a light blue shirt and a dark vest entered and sat down in the captain’s seat. Seemingly unaware of the presence of the ﬁghter jets he put on a pair of headphones and leant forward, putting his hands on the panel. The aircraft continued along the holding pattern for a few more seconds... then the ﬁnal, precipitous act of the drama began. The 737 ran out of fuel and, with the engines ﬂamed out, it began to descend rapidly. As the aircraft sank through 7000 feet the occupant of the captain’s seat appeared to ﬁnally notice the F-16s, but his only reaction was to point downwards... The airliner slammed into a hillside, 20 miles northwest of Athens airport – the time was 09:03. All 115 passengers and six crew perished in the accident, though fortunately (to the extent to which the word could be used in this context), as the post-mortems would reveal, it is unlikely that any of them were conscious at the moment of the impact. They had clearly been in a very low pressure, hypoxic environment for a long time – but how, when and why did the cabin pressure of the Boeing drop to incapacitating levels? And who was the mysterious man in the cockpit in the ﬁnal minutes of the ﬂight – and, perhaps even more intriguingly, how did he stay conscious, when everyone else was incapacitated? It took the Greek accident investigators over a year to piece together a probable timeline of the events of 14 August 2005 and we shall review their ﬁndings shortly. First, however, we shall take a closer look at one of the greatest engineering challenges of high altitude passenger transport: how to maintain comfortable pressure and temperature levels inside the passenger cabin of an aircraft that can take off from a warm, sea level airport and, within less than half an hour, cruise in the freezing, hypoxic, inhospitable stratosphere.

2.2 Micro-climate

29

2.2 Micro-climate From the perspective of a passenger, a high altitude ﬂight aboard a modern jet airliner is generally an environmental non-event. The variations in the composition, pressure and temperature of the air we breath in the course of a ﬂight are largely unnoticeable, as are the differences between these conditions and what we are used to in everyday circumstances, say, inside an ofﬁce building. In fact, the airborne period is probably the most comfortable part of the average long distance journey. And yet, we seldom give a second thought to the technological wizardry that keeps us equally comfortable while taxiing on a hot runway, climbing swiftly through rapidly changing external conditions or cruising for hours in the chill of the stratosphere. We are often (understandably) impressed by the sheer thrust of gas turbine engines capable of accelerating tens or hundreds of tons of airplane down a runway, pressing us into our seats, or by the expediency of having breakfast in London and lunch in New York on the same day2 , but the complexity and the demands placed on the unglamorous air conditioning system usually fade into the seldom visited ‘stuff taken for granted’ pigeon-hole of our minds, like the soundtrack of an action movie. Of course, replace the soundtrack of the ﬁlm with a badly written or performed one, or take it away altogether and the audience will suddenly realise its importance – similarly, any glitch in the cabin pressurization and air conditioning system would make us at least very uncomfortable or, as we have just seen with the case of the Helios ghost ﬂight, the subject of news headlines. For a glimpse into the workings of the type of environmental control system most high altitude airliners are equipped with, let us consider the path of a small parcel of air3 as it travels through the system (and the passengers’ lungs). Most of the air that the passengers (and crew) breathe enters through the engine. As the air enters the core of the engine and it begins a multi-stage compression process, its temperature rises rapidly. As it is compressed to around twice the sea level ambient pressure, progressing along the axial ﬂow turbomachinery, the temperature of the parcel of air rises to between 150◦ C (302◦ F) and 250◦ C (482◦ F), at around which point it leaves the compressor through a bleed air port (this is a relatively early compressor stage – most of the rest of the air will be compressed much further); and straightaway there is a design challenge here. An engine idling on a taxiway will raise the pressure to the required level more slowly (over more stages) then during a screaming, high performance climb, when the air might be compressed to over 30 times the sea level pressure, far more than what is required for air conditioning. The entry temperature might vary wildly too – it could be a torrid 40◦ C (104◦ F) on the ground and −40◦ C (−40◦ F) only few minutes later, during the climb. Thus, generally, two or three such bleed ports are required at different points along the compressor, but even so, excess bleed air pressure and heat may have to be vented in certain ﬂight conditions (this occasional waste of energy is one of the reasons behind some manufacturers considering the use of electrically powered compressors to produce cabin air – the Boeing 787 Dreamliner is the ﬁrst to feature such a system).

2 3

...or, indeed, another breakfast in New York when Concorde was ﬂying – those were the days! A Langrangian viewpoint, in physicists’ jargon!

30

2. Comfort zone

It is worth mentioning here in passing that a serendipitous effect of the heat resulting from the compression is that the bleed air thus produced for air conditioning purposes is sterile! Even more important, perhaps, than the removal of any microorganisms is the reduction of the concentration of ozone – this is achieved through passing the air through a set of catalytic converters, before it can ﬁnally enter the air conditioning packs. The role of the air conditioning packs is as simple as it is challenging. They have to produce about 300 litres of air per passenger per minute of exactly the correct temperature (as set by the ﬂight crew or, on some aircraft, by the cabin crew) and do this by only using the air passing by the aircraft as a coolant. This is then mixed with an equal amount of ﬁltered, re-circulated air, providing a total of about 10 litres of ‘new’ air per passenger per second. In terms of the volume of the average aircraft cabin this means that the entire cabin air is exchanged with outside air about 10–15 times an hour – by comparison, the same ﬁgure for a typical building is about 1 to 2.5 and most buildings have far less effective ﬁlters than airliners. Incidentally, the reasons behind recycling some of the cabin air have to do with the desire to use as little bleed air as possible and thus with fuel economy. In a paper presented at an Aerospace Medical Association meeting in 1995 Hunt et al. [62] estimated that using only fresh, outside air on the Boeing 767 ﬂeet alone would have consumed 14 super-tankers worth of additional fuel in just over a decade of operations. There is a persistent fear amongst air travellers that they are breathing in each others germs while on-board: the “I must have caught this cold on the plane” syndrome. This, generally, has more to do with collective conﬁrmation bias than with the germ content of cabin air: chances are that unless the person sitting next to you or behind you sneezed or coughed on you, your cold has nothing to do with the ﬂight. Most airliners are ﬁtted with high efﬁciency particulate air ﬁlters, which ensure operating room levels of microbe removal in the re-cycled half of the air (the other half is, as we have seen, already sterile). Moreover, the airﬂow patterns around airliner cabins are generally designed not to have an ‘along the isle’ component, so the spread of any airborne pathogens is generally limited in any case to the row where the infected person is sitting. While listening to an airline passenger complaining about the ﬂu they may have picked on their last ﬂight (which they did not4 ), spare a thought for the ﬂight attendants; they are exposed to the sneezing of many more potential germ carriers than any passenger – every single working day. To this and other workplace perils affecting cabin and ﬂight crews we shall return in the next chapter. The cabin air is evacuated through ﬂoor-level grilles into the cargo bay, where the ambient pressure is the same as in the cabin (the temperature is generally lower in the cargo hold). These outﬂow grilles have an important safety role too: they ensure that in case of sudden depressurization in the cargo hold (say, as a result of the accidental opening or loss of a cargo door) the cabin ﬂoor does not collapse as a result of the sudden pressure differential. As a slight digression here, it is worth noting that the landing gear wells of passenger airliners are never pressurized and, while initially the tires, warmed up during the take-off run, might provide some heat, they are extremely cold places too – as many a stowaway has found over the years. In fact, surviving a prolonged stratospheric ﬂight in the landing 4

See [79] for a recent review of the epidemiology literature on the subject.

2.3 Cabin pressure

31

gear well of an airliner is nearly impossible and it is hard to tell whether ignorance or sheer desperation (perhaps most often a combination of the two) drives some to accept the extreme odds against them when they sneak into the wheel well of an aircraft. Perhaps the most remarkable such journey ever recorded is that of a 17-year old Cuban stowaway, Armando Socarras Ramirez, who in 1969 climbed into the wheel well of an Iberia DC-8 departing from Havana on an overnight ﬂight to Madrid. An unsafe landing gear warning light illuminated in the cockpit shortly after departure, which, terrifyingly for Ramirez, led to the ﬂight crew attempting to clear the fault by lowering the landing gear and raising it again. The operation did silence the warning and, remarkably, did not drop the stowaway to certain death. Upon arrival at Madrid airport staff had a day to remember, as the boy’s body dropped out of the jet’s gear well... and, astonishingly, he was alive! It is thought that three fortunate circumstances contributed to his almost unprecedented survival. First, the ﬂight cruised at a comparatively low altitude of 29 000 feet (about the same height as the summit of Everest, where, as we have seen, survival without supplemental oxygen is possible, albeit only after thorough acclimatization). Second, the heat from the tires and from the hydraulic lines passing through the wheel well kept his surroundings at a slightly higher then the ambient temperature. Third, as a result of the deep hypoxia and hypothermia he may have entered a so-called poikilothermic state, where the thermoregulatory center of the brain ceases to function and the body enters a state reminiscent of hibernation, where its oxygen requirements are greatly reduced [126, 131]. Sadly though, stories such as that of Ramirez are rare and the vast majority of wheel well stowaways – mostly children, as few aircraft have landing gear wells sufﬁciently spacious for an adult – perish during their stratospheric journey. The survival statistics are grim and they are likely not to even include the majority of cases – most failed attempts probably go unrecorded, as the stowaways fall to their deaths anonymously and tragically unnoticed, over water or remote locations.

2.3 Cabin pressure As the previous chapter has shown, the human body is a sensitive barometer. The chief function of the environmental control system of an aircraft is therefore to keep pressure changes within the limits that the fussy ‘self-loading freight’ (to use the somewhat unﬂattering synonym for ‘passenger’ from the airline industry thesaurus) will accept uncomplainingly. This is a substantial challenge, which the hardware described in the previous section can only meet with the aid of some very fast, robust and precise control mechanisms, which must be able to respond to a variety of changes in the operating conditions. The airﬂow around the aircraft and into the engines might change quite rapidly, as can the temperatures. The aircraft might have to interrupt its climb and return rapidly to the departure airport or perhaps divert to another airport at a considerably different elevation. In all such cases the cabin pressurization schedule must re-adjust ‘on the ﬂy’ to ensure not only that the cabin altitude does not rise above the safe limits discussed in the previous chapter, but also that the rate of pressure change is below what might cause discomfort: the equivalent altitude in the cabin must not increase faster than about 500 feet/minute and must not de-

32

2. Comfort zone

crease at a rate greater than about 300 feet/minute at any point during the ﬂight (as we have seen in the previous chapter, the Eustachian tube opens less readily when the pressure in the middle ear is lower than the ambient pressure, as opposed to when it is greater – hence the difference between acceptable cabin rates of descent and climb). Another potential cause of discomfort is the so-called take-off bump. As the aircraft begins its take-off run, it would seem sensible to to keep the air conditioning system outﬂow valves open to maintain the airﬁeld-level pressure inside the cabin. As the aircraft rotates and begins to climb, the valves would have to close immediately to allow the system to begin to pressurize the aircraft, as the rate of climb is likely to be greater than the 500 feet/minute or so most passengers would be comfortable with. This would, however, lead to a sudden, uncomfortable momentary surge (‘bump’) in the cabin pressure in the ﬁrst seconds of the climb – to eliminate this, most aircraft are pre-pressurized on the ground. As the throttle levers are advanced to begin the take-off run, the cabin pressurization system gently reduces the cabin altitude by about 150 feet – the cabin pressure then starts falling gradually as the aircraft leaves the ground (Figure 2.1). A similar phenomenon can occur on landing too, which is why some aircraft feature a pressurization schedule with a slight overpressure on landing too. As this rather cursory review of a typical cabin pressurization system illustrates, maintaining a safe and comfortable cabin altitude in all possible ﬂight conditions and operational circumstances is quite a sophisticated task. While the environmental control system

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Fig. 2.1 A typical cabin pressurization schedule – note that in this case the arrival airport is at a higher elevation – the pressurization system has to be ‘told’ about this before the descent to allow it to compute the appropriate cabin rate of descent.

2.4 The ﬂick of a switch

33

of most aircraft has a manual mode to be used in case of certain failures and test procedures, the complex sequence of adjustments that controls the cabin altitude must be left to the automatic controllers in normal operations – and this is our segue back to the chilling story of Helios Flight 522.

2.4 The ﬂick of a switch The Flight Data Recorder and the Cockpit Voice Recorder of the Helios Boeing were recovered in good order from the hillside where Flight 522 ended, but so insidious can be some of the effects of hypoxia that they can even go undetected by these electronic ‘ﬂies on the wall’. Compounding this, as the Cockpit Voice Recorder runs on a 30 minute loop, it only ever covers the last half hour of each accident ﬂight. The soundtrack on this occasion was little more than 30 minutes of eerie silence. It was only when investigators analyzed the Flight Data Recorder together with the reports of the F-16 pilots, the maintenance records of the jet, the autopsy reports, as well as the wreckage itself, that a coherent story began to emerge. The following timeline is based on the ofﬁcial ﬁndings of the Greek authorities, assisted by Boeing [7], but some of the gaps left by the available evidence had to be ﬁlled with conjecture – it is now likely that these unwitnessed details will never be revealed with absolute certainty. Air accidents seldom result from a single cause and Helios 522 was no exception. In fact the chain of critical events, made up, in this case, of at least ﬁve separate links (highlighted in the following), began a day before the accident. 13 August 2005, 21:00 The Helios Airways 737 departs London Heathrow for Larnaca, Cyprus. En route the cabin crew hear a series of “hard bangs” from the back of the aircraft. Upon investigation they ﬁnd the seal around the aft service door frozen. They record the alarming anomaly in the service log. 14 August 2005, 03:15 The local maintenance agent in Larnaca carries out a visual inspection of the door and runs a full pressurization test on the aircraft. For this he needs to set the cabin pressurization mode selector switch to MANUAL. A green advisory light with the word ‘MANUAL’ is illuminated on the pressurization control panel. Upon completing the test, he concludes that no abnormal noises can be heard and releases the aircraft for ﬂight. LINK ONE: He does not return the selector switch back to AUTO (for automatic) – nor does the rather vaguely worded section of the maintenance manual speciﬁcally instruct him to do so. 06:00 Helios 522 has ﬁnished boarding. The ﬂight crew conduct their pre-departure drills. LINK TWO: We will never know exactly why, but they fail to notice the incorrect setting on the pressurization panel, in spite of the advisory light. The mode selector switch remains on ‘MANUAL’. 06:07 Helios 522 is airborne. The ﬂight crew perform their ‘After Take-off’ checklist. In the absence of the sound recording for this portion of the ﬂight it is unclear how, but once

34

2. Comfort zone

more they miss the incorrect setting of the pressurization system – LINK THREE. The 737 is now climbing towards its cruising altitude of 34 000 feet, practically unpressurized. 06:12 According to the Flight Data Recorder, as the aircraft passes 12 010 feet, the cabin altitude reaches 10 000 feet – hypoxia territory. Here lies the Boeing’s most aggressive line of defence: the strident cabin altitude horn sounds. LINK FOUR: Puzzlingly, the experienced ﬂight crew misinterpret it as the take-off conﬁguration warning horn – as we have seen, this is a bizarre conclusion: the take-off conﬁguration horn only ever sounds on the ground. What causes an experienced crew to react in this way? Stress? The unusual situation (few pilots are ever likely to hear a cabin altitude warning)? 06:14 Aircraft altitude 18 000 feet. Cabin altitude 14 000 feet. As designed, the passenger oxygen masks deploy. The captain has a radio conversation with the company’s technical center about the warning horn. He also makes a rather confusing comment about the equipment cooling system. As we now know, at this point the cooling fans in the equipment bay can no longer operate effectively in the thin air – the pressure sensor triggers a Master Caution alarm on the control panel. The crew do not appear to respond in any way to the deployment of the oxygen masks (for example, by initiating an emergency descent to a safe altitude) and the Master Caution alarm is also allowed to sound for nearly a minute – LINK FIVE. Are the ﬂight crew beginning to be affected by hypoxia by now? The ground engineer does not get a response to his subsequent queries; nor do any of the several air trafﬁc control stations attempting to contact the Helios ﬂight in the next two hours. The cabin altitude is now passing 15 000 feet – this, as we have seen, is the limit beyond which the body cannot compensate for the low oxygen pressure. 06:23 The aircraft levels off at 34 000 feet, the cruising altitude programmed into the computer. The cabin altitude for the two-hour cruise that follows is estimated to be around 24 000 feet. What happens in the cabin during these two hours is largely unknown, though it appears that the cabin crew used up the content of three of the four available portable oxygen masks. 07:37 After a high altitude missed approach procedure over Athens airport, Flight 522 enters the holding pattern. 08:48 Signs of life. As we know from the noises recorded by the Cockpit Voice Recorder and the report of the one of the F-16 pilots, a man enters the cockpit and takes the captain’s seat. The investigation has determined his identity as a member of the cabin crew, who also happened to hold a Commercial Pilot’s License. He is probably severely starved of oxygen and, worse, the aircraft is starved of fuel and begins to descend as its engines ﬂame out in quick succession. 08:54 The Cockpit Voice Recorder captures the ﬂight attendant’s very weak voice as he makes a Mayday call. He does not press the microphone key though, so the call is never transmitted. Nor would it make any difference. At 09:03 the 737, with its 121 unconscious occupants, glides into a hillside near the village of Grammatiko.

2.5 Frosted windows

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Much of what happened on-board the Helios 737 remains a mystery. The F-16 pilots noted that the captain’s seat was vacant until it was taken by the ﬂight attendant – where was the captain? Had he lost consciousness while attempting to assess the situation in the cabin? Did the cabin crew attempt to contact the cockpit after the deployment of the oxygen masks? After all, this was a highly unusual event; even more unusual would have been the continuation of the cruise after the deployment of the masks – they would have expected an immediate descent to a safe altitude (10 000 feet, the conventional hypoxia limit). We also know little about the crucial minutes of the climb, when the cabin altitude warning horn went off. Was it misunderstood by both pilots or did the captain over-rule any concerns the ﬁrst ofﬁcer may have raised? The investigation did reveal concerns the ﬁrst ofﬁcer had repeatedly raised about the attitude of the captain – in fact, he was looking for another job at the time. To date, the Helios accident remains the cause of the greatest loss of life resulting purely from depressurization (that is, not from any associated major structural failure) and its eerie and ambiguous circumstances will have made it all the more difﬁcult for passengers’ families to come to terms with. It is also extremely difﬁcult for the manufacturers and the regulatory authorities to put forward safety actions after an accident where ﬁve lines of defence have been bypassed en route to disaster; nevertheless, the Greek authorities did recommend the addition of three further links to the safety chain. First, the member of the cabin crew nearest to the ﬂight deck should get in touch with the ﬂight crew if, after a depressurization made evident by the deployment of the passenger oxygen masks, the aircraft does not begin to descend. Second, they recommended that a visual or oral warning is added to augment the horn in case the cabin altitude rises above 10 000 ft. Finally, they suggested that airline crews receive mandatory hypoxia training, as do military air crews. Of course, the extreme rarity of such incidents and the high cost of some of these measures make the implementation of these recommendations a difﬁcult decision.

2.5 Frosted windows The Tour Championship used to mark the end of golf’s PGA Tour season each year, with the 1999 edition of the prestigious event taking place at the end of October at the Champions’ Golf Club in Houston, Texas. In spite of the stellar line-up of talent present that year – Tiger Woods, Justin Leonard, Ernie Els and several other big names were competing for the winner’s prize of 900 000 dollars – the ’99 Tour Championship will always be remembered for the name of one competitor who never made it to Houston. Payne Stewart, winner in June that year of the US Open and member of the winning Ryder Cup team in September, had died in a plane crash three days earlier, on his way to Texas. As a lone piper played “Going Home” marching through the morning mist at a sombre commemorative ceremony on the ﬁrst tee of the Houston course, two thousand miles to the north in Aberdeen, South Dakota, National Transportation Safety Board ofﬁcials were sifting through the wreckage of the small Learjet 35, in which Stewart had lost his life. This was the start of what would turn out to be a remarkably challenging investigation into a very mysterious accident.

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Three days earlier... The privately chartered aircraft, operated by Sunjet Aviation, landed at Orlando International Airport shortly after eight o’clock on the morning of 25 October 1999. The four passengers, including Stewart, boarded half an hour later and the small Learjet departed for Texas at 09:20 on a routing via Cross City, Florida5 . After a series of routine radio calls, at 09:27 the Air Route Trafﬁc Control Center at Jacksonville, Florida cleared the Learjet to its planned cruising level FL390 (approximately 39 000 feet). The ﬁrst ofﬁcer acknowledged the clearance: “Three-nine-zero, Bravo Alpha”6 .

Fig. 2.2 The Learjet 35 that was to ﬂy Payne Stewart to the Tour Championship. Image courtesy of the NTSB.

The Jacksonville controller then instructed the Sunjet ﬂight to contact another controller, but rather alarmingly, even after several repeated calls, no answer came. Even more worryingly, the Learjet reached its designated cruising altitude – and shot straight past it. At the same time, it began to deviate from its correct course, heading north-west, instead of turning onto a westerly heading towards northern Texas, as it was reaching the top of the Florida panhandle. This was beginning to look like a serious emergency and an F-16 from Eglin Air Force Base was dispatched to investigate. Upon intercepting the Learjet, the Air Force pilot reported that the windows were dark, so he could not see the passengers. Nor could he see into the cockpit, because condensation or a thin layer of ice was covering the windshield. This was to be the story of the next four hours. F-16s from various air bases intercepted the small jet in several rounds, reporting no change in the situation, other than further climb: at one point the Learjet reached a staggering 48 900 feet. This was nearly a mile 5 6

As per NTSB report DCA00MA005, which much of the rest of this account is also based on. The registration of the aircraft was N47BA.

2.5 Frosted windows

37

higher than its maximum operating altitude! Eventually, after an almost full south–north crossing of the continental United States, the aircraft ran out of fuel over South Dakota and, to the consternation of the accompanying F-16 pilots, it spiralled into the ground, its ghostly ﬂight ending in an unsurvivable crash.

Fig. 2.3 The planned route (green) of Payne Stewart’s jet and the actual route (red) the Learjet ﬂew. Image courtesy of the NTSB.

The last ﬂight of Payne’s Learjet bore the clear hallmarks of cabin depressurisation: the sudden loss of radio contact, the departure from the normal ﬂight path, the excessive climb, far too deep into the stratosphere, and, perhaps most tellingly, the frosted over windshields (indicating a relatively sudden drop in cabin temperature, below the dewpoint of the cabin air), all indicated a failure of the cabin air system. But how did this happen in an aircraft equipped with all the lines of defence that we saw in the section describing the crash of Helios 522? Alas, unlike the 737, the small business jet did not carry a Flight Data Recorder (nor was it mandated to) and, not unusually for accidents of this kind, the Cockpit Voice Recorder, running on a 30 minute loop, did not record a single word. It did, however, contain a telling piece of evidence. In addition to the noise of the engines, the constant, sharp bleeping of the cabin altitude warning horn could be heard on the tape, conﬁrming the suspicion of depressurization, as well as the fact that it was doing its duty, as designed. In fact, the tape captures the moment as the out of control aircraft plummets through the skies over South Dakota, when the horn falls silent – at the moment when the cabin altitude of the aircraft was estimated to have descended back through 10 000 feet. While not providing an ironclad bill of health for the warning system,

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this was a very strong indication that the barometric sensor controlling the horn was able to detect its correct trigger value. In spite of a thorough examination of the wreckage, the maintenance records of the aircraft, the track records of the two pilots, the radio conversations, as well as computer simulations of the pressurization system, the deﬁnitive cause of the loss of cabin pressure remains a mystery. In fact, it is unclear even whether the cabin altitude fell sharply (as a result of an explosive decompression through a crack small enough to be missed by several Air Force pilots during the four-hour ghost ﬂight) or, as in the case of the Helios ﬂight several years later, gradually. Whichever scenario is the true one, it is also unclear what prevented the crew and/or the passengers from using the oxygen equipment (the main oxygen supply valve was found in the open position), or, more to the point, from using it effectively. Like the eerie discovery of the empty Mary Celeste ﬂoating in the Atlantic in ﬁne weather over a century earlier, such ﬂights with tragic ends unseen by surviving witnesses and unrecorded by electronic surveillance, discomﬁt and intrigue in equal measure. They offer little by way of take-home lessons though: for those we must turn to the notable near misses of the modern story of humans taking their ‘shirt sleeve’, comfortable, near-sea level environments with them into the stratosphere.

2.6 Broken arrow After the Hungarian Revolution of 1956 and the continued wrangling between the Western powers and the Soviet Union over the fate of East Germany throughout the 1950s, the early 1960s promised unsettling times. And, indeed, before 1961 was out, the world had watched with angst the failed invasion at the Bay of Pigs, the start of the erection of the Berlin wall and the build-up to the Cuban missile crisis. It was against this inauspicious backdrop that on 13 March 1961 two B-52F heavy bombers departed Mather Air Force Base, near Rancho Cordova, California. They were heading out on a high altitude, long endurance training mission over the Paciﬁc, an air navigation exercise, which, with two rounds of aerial refueling, was to take them about 24 hours to complete. One of the aircraft, ﬂight Doe 11, was carrying two Mark 39 TN thermonuclear weapons, each with a possible yield between three and four megatons7 . Things began to go wrong on board Doe 11 a mere 20 minutes after departure. The air entering the cockpit through the air conditioning vents was far too hot and all efforts by the crew to close the vents failed. Six hours and an in-ﬂight refuel later the temperature problem and the resulting discomfort in the cockpit still persisted, in spite of several troubleshooting attempts. The guidance from the base was for the crew to continue the mission, but “if it gets intolerable, of course, bring it home” [98]. A persistent problem with one of the engines and another refueling rendezvous kept the crew busy for the next hours, though by this time the heat was getting so hard to bear that

7

Details from [45] and the transcription of the Department of Defence report by Oskins and Maggelet [98].

2.6 Broken arrow

39

Fig. 2.4 The B-52 Stratofortress: still going strong and still looking menacing half a century after the ‘Broken Arrow’ incident described here. The B-52 is expected to remain in service until at least 2040 – a staggering 90 years after the ﬁrst ﬂight of the prototype.

Fig. 2.5 High altitude refueling: a KC-135 Stratotanker crew member maneuvers the fuel boom into the receptacle of a B-52. Image courtesy of the US Air Force.

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they decided to don their oxygen masks and depressurize the cabin brieﬂy to allow it to cool down. The heat returned though as soon as the aircraft was re-pressurized. By the 14 hour mark, the temperature was beginning to take its toll not only on the crew, but on the aircraft itself too. The outside panel of one of the cockpit windows shattered, followed by the glass on several instruments. According to subsequent estimates the temperature in the upper compartment of the B-52’s cockpit may have reached 70◦ C (158◦ F). At this point the pilot decided that they could take no more of the punishing conditions and, at an altitude of 33 500 feet, they depressurized the aircraft once more. With the temperatures thus reduced, the crew, who had been in the air for 14 extremely difﬁcult hours, had cause for fresh concern: the sudden increase in cabin altitude was causing them abdominal cramps and the radar operator was suffering from the bends. After about 20 minutes it became apparent that they could no longer maintain their altitude in these conditions – the decision was made to descend to 12 000 feet. At this new altitude conditions were much better and the crew could even remove their oxygen masks. The mission now had to be re-planned though: Doe 11 (ﬂying alone by this time, the other B-52 involved in the exercise carried on at the normal altitude) was now in much denser air and was therefore burning through its fuel at a drastically increased rate. Heading back to base now, the pilot’s calculations indicated that they would arrive over Mather Air Force Base with around 20 000 pounds remaining in the tanks. They were now 17 hours into the ﬂight and the low altitude was causing a further problem: they were under overcast skies and thus unable to use their celestial navigation system to check their position estimates. 21 hours into the ﬂight, after having obtained a navigational ﬁx through a break in the clouds, the estimated time of arrival at Mather had to be pushed back to 22 hours and 30 minutes after departure. The estimated fuel remaining was now looking “a little light” too at 14 000 pounds, so the crew requested a tanker aircraft to be placed on standby. It is hard to imagine how yet more could go wrong that day, but more did go wrong. The B-52 was now in turbulence and the crew had to make several course changes to avoid the worst of the weather ahead. More alarmingly still, one of the fuel gauges appeared to be stuck – as the co-pilot suddenly noticed, it had been reading the same amount for the last hour and a half or so. With uncertainty over the fuel situation now mounting, the Control Room at Mather informed them that a tanker was on its way to meet them. Doe 11, however, never made it to the rendezvous point. 22 hours and 40 minutes after take-off the fuel ran out and the eight thunderous turbojets of the B-52 fell silent. There was only one thing left to do for the exhausted crew: turn the aircraft toward a clear area and bail out – this they managed to do, with the pilot leaving the aircraft last at an altitude of about 7000 feet. The huge, menacing glider, which the B-52 had turned into, struck the ground at a speed of about 200 knots near Yuba County Airport, California. It is not clear what caused the air conditioning system to malfunction early on in the ﬂight, but it is clear that the most important system of the aircraft did work ﬂawlessly: the bombs, in spite of sustaining considerable damage in the crash, did not detonate – their safety devices performed as designed. The Yuba County crash was unique in terms of the immense risk factors that the B-52 carried in its bomb bay. It was, however, far from unique as a classic case of an air crew becoming so focused on troubleshooting a problem that they fail to notice another, slowly

2.7 The captain’s spectacles

41

and quietly developing crisis, a potentially much more serious one: in this case that of a chain of uncertainties combining into insufﬁcient knowledge of the fuel situation. Of course, evaluating pilots’ actions in the cold light of day is the armchair critic’s privilege; managing a complex machine for almost an entire day in heat that can break glass dials on the instrument panel is an altogether different challenge. It is a strange irony of the history of the Cold War that just over a year later Soviet submariners found themselves heading towards Cuba in similarly oppressive heat on board their ill-ventilated vessels, designed for battles to be waged in the cold waters of the North Sea; and the decisions they had to make carried perhaps even more weight than those of the B-52 crew. The trade-offs involved in the engineering of submarine cabins, designed to operate in very high pressure environments is, however, a subject we shall leave to another book.

2.7 The captain’s spectacles At around 21:00 on 13 August 1998, a Peach Air Boeing 737-200, cruising on autopilot at 35 000 feet on a ﬂight from Dubrovnik, Croatia, was nearing the expected start of its descent into London Gatwick Airport. The captain had just returned from the passenger cabin with a guest in tow, a female passenger who wanted to see the ﬂight deck (pre-9/11 times!). As he took his seat they received clearance by air trafﬁc control to descend to 28 000 feet. The ﬁrst ofﬁcer started his descent drill, but almost as soon as he pulled back the throttle levers, he began to feel pressure in his ears. A glance at the overhead pressurization panel revealed the reason: the cabin rate of climb indicator was at the top of its scale – clearly, the cabin was depressurizing. This demanded swift action. The ﬁrst ofﬁcer donned his oxygen mask, while the captain sent the ﬂight deck visitor back to her seat in the cabin and, as the cabin altitude indicator was now reading a rather alarming 20 000 feet, he too attempted to put on his oxygen mask. A seemingly small, but, as it turned out, critical error thwarted him: the mask became entangled with his spectacles, which then fell to the ﬂoor. Attempting to retrieve them, the captain leaned forward... and remained there, apparently unconscious. Meanwhile, the woman who had just been in the cockpit found herself in a misty cabin, where the other 114 passengers were already wearing their oxygen masks – she quickly regained her seat and put her mask on too. The cabin crew had also just deployed their oxygen masks. In the ﬂight deck the ﬁrst ofﬁcer had little time to attempt to raise the captain, as the aircraft, with the throttles retarded and the altitude still set to 35 000 feet, was now slowing dangerously. He quickly lowered the nose to regain airspeed and descend to a safe altitude, though this process was complicated by a slightly confused radio exchange with the Maastricht air trafﬁc controller who was handling the ﬂight at the time. The ﬁrst ofﬁcer also knew he needed the help of his colleague in handling this developing emergency and he used the cabin call chime to seek the help of the cabin crew. The lead ﬂight attendant rushed in to oblige, leaving her dangling oxygen mask behind and not pausing to pick up one of the portable oxygen bottles. Her eagerness to help had severe consequences: she

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only had time to take in the drama unfolding in the cockpit before intensifying it further by collapsing in the doorway. The sight of two fallen crew members now prompted the ﬁrst ofﬁcer once more to try and ﬁt the captain’s oxygen mask and this time he succeeded, with the immediate beneﬁt that the captain regained consciousness – bizarrely not even realizing that he had ever lost it. With both ﬂight crew now in action and a further cabin crew member assisting the unconscious ﬂight attendant, the emergency was gradually brought under control and the 737 landed at Gatwick without further drama. We shall not delve here into the causes of the fatigue failure that was subsequently found to have caused the high altitude depressurization of this ‘middle-aged’ airliner – sufﬁce to say that a corner of the aft cargo door was found to have had three cracks, which had been growing unnoticed for 17 years, before leading to the rapid tearing that depressurized the aircraft on this, its 35 385th ﬂight8 . The more startling take-home lesson of this incident is just how little time crews might have to react to a sudden loss of cabin pressure. Clearly, even at a comparatively low 35 000 feet there is relatively little room for hesitation or delay – perhaps there are clues in the Peach Air case in what may have gone wrong on Payne Stewart’s last ﬂight? Clearly, it is technologically possible to take a comfortable environment to high altitudes, but, once again, we see that stratospheric passenger ﬂight approaches the limits of what is economical, feasible and safe at the same time.

2.8 40 000 feet – a line in the sand We saw in the previous chapter that the ‘time of useful consciousness’ is a relatively ambiguous concept – moreover, different sources often quote different numbers. For instance, the Peach Air Safety and Emergency Procedures Manual quoted 3–4 minutes for a change from the normal high ﬂight altitude cabin pressure equivalent to 8000 feet to 25 000 feet (closest to the estimated maximum cabin altitude reached on the ﬂight described in the previous section). It is interesting to compare this to NASA’s Bioastronautics Data Book [99], which, for a change from sea level to 25 000 feet quotes 2–3 minutes after breathing pure oxygen (obviously not the case here, the crew had been breathing cabin air) and as low as 75 seconds after breathing air. Incidentally, the same source, already quoted in the previous chapter, suggests that at 40 000 feet the time of useful consciousness may be as little as 10 seconds and an average of 18 seconds. As far as ﬂight crews are concerned, this means that when climbing above 40 000 feet there may simply not be sufﬁcient time to don oxygen masks in case of a rapid decompression of the aircraft. Of course, one might argue that any event dramatic enough to fully depressurize an aircraft so suddenly that the pilots do not have time to reach for their masks, is likely to have involved structural damage severe enough to preclude a safe landing anyway. Moreover, the probability of such an event is extremely remote, in particular on aircraft with tail-mounted engines (an uncontained engine turbine disk burst is one of 8

For further details see Air Accident Investigation Branch Report EW/C98/8/6, published in their 6/99 Bulletin.

2.8 40 000 feet – a line in the sand

43

the few events that might cause such an abrupt loss of cabin pressure and on tail-mounted engines the shrapnel would merely pierce the unpressurized tail section of the fuselage). Nevertheless, regulations usually include additional safeguards above 40–41 000 feet. In particular, some standard operating procedures require the wearing of the oxygen mask at all times by at least one of the pilots, when ﬂying at very high altitudes (especially when a pilot is alone in the cockpit). This solution, however, while being somewhat uncomfortable for the ﬂight crew, is not always feasible either. For example, the passengers of business jets can usually look into the ﬂight deck and they might be alarmed by the sight of the masked crew. As a compromise solution some business jet pilots simply place the oxygen masks on their laps, thus potentially shaving a few seconds off their response time, should cabin pressure be suddenly lost. Incidentally, the airways above 41 000 feet are actually dominated by business jets. While few airliners are certiﬁed for these altitudes, some higher end business jets have rather impressive service ceilings. For example, the Dassault Falcon 7X, the Gulfstream V, and the Bombardier Global Express can cruise at a spectacularly lofty 51 000 feet – over two and a half miles above the bulk of the commercial trafﬁc. Being in a sparsely populated layer of the stratosphere gives these aircraft more ﬂexibility: air trafﬁc control can often issue them with more direct routings. So why are airliners limited to lower altitudes, where fuel burn is higher and congestion is more likely? The answer can be found in a regulatory battleground known as FAR 25.841. This is the ‘Pressurized cabins’ section of the document issued by the US Federal Aviation Administration (FAA) that deﬁnes certiﬁcation criteria for transport aircraft, whose substantial number of Amendments – ‘25-87’, issued in 1996, being the latest – reﬂects the intensity of the debate surrounding the ‘40 000 feet’ issue. As it stands, the document states that aircraft must be designed so that the cabin altitude will not exceed 25 000 feet for more than 2 minutes, nor will it exceed 40 000 feet for any time, should any systems failure occur that is not extremely improbable. We mentioned earlier that the standard worst-case scenario is the bursting of an engine rotor, which might propel parts of the turbo-machinery (especially turbine blades) towards the pressure cabin with sufﬁcient force to puncture it. In practice this means that aircraft with tail-mounted engines (such as most business jets) are considered safer than large transport aircraft (usually featuring wing-mounted engines), which therefore cannot be certiﬁed as per the amended FAR 25 to operate above 40 000 feet. In the eyes of many, this is a rather expensive restriction, stiﬂing the efﬁcient use of the stratosphere. The wrangle revolves around the comparatively little data available on the severity of the dangers of exposing passengers to very high cabin altitudes for short periods, in case of a depressurization. The aircraft manufacturers, keen to obtain exemptions from Amendment 25-87, argue that the history of aviation is yet to record a single passenger death caused by hypoxia. Are the regulatory authorities thus preventing the industry from enjoying the economic beneﬁts of going deeper into the stratosphere, in spite of a lack of evidence that relaxing the altitude constraints may signiﬁcantly increase the risk of serious harm to victims of depressurization incidents? Prior to 1996 (when Amendment 25-87 was issued) there was no regulatory ‘line in the sand’ at 40 000 feet and several large transports were permitted to operate at higher altitudes – a signiﬁcant late example being the Boeing 777, certiﬁed in 1995 to a maxi-

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mum operating altitude of 43 100 feet. A special representative of the pre-1996 era was, of course, Concorde, with its cruising altitude of 60 000 feet. Its manufacturers (BAC and A´erospatiale) went to great lengths to demonstrate that its unique design would reduce the impact of any high altitude depressurizations. Their argument was that, as a result of the very strong cabin structure (designed to withstand the heavy cyclic pressurization loads caused by maintaining a cabin altitude of only 6000 feet at 60 000 feet) and the very small windows, the cabin altitude would never exceed 15 000 feet after a reasonably probable (1 in 100 000 ﬂight hours) failure and it would take a remote (1 in 10 000 000 hours) failure, such as the loss of a window, for the cabin altitude to reach 25 000 feet. They showed that even extremely remote double failures (loss of a window and failure of a discharge valve) would not cause the cabin altitude to rise above 30 000 feet [60] – it is interesting thus to speculate whether Concorde’s designers would actually have had a good case for certiﬁcation even in the post-1996 era! In practice, the introduction of Amendment 25-87 did not stiﬂe commercial ﬂight at the higher stratospheric levels. In 2004 Brazilian manufacturer Embraer were granted a 1000 feet leeway, the ﬁrst of their regional E-jets, the E-170 being certiﬁed to a maximum operating altitude of 41 000 feet9 .

Fig. 2.6 The Embraer E-170 (seen here drawing condensation streaks out of the humid air with its ﬂaps, as it climbs out after a go-around) was the ﬁrst airliner to be granted exemption from Amendment 25-87, receiving FAA certiﬁcation to a maximum operating altitude of 41 000 feet.

In December 2004 Airbus petitioned for exemption from the amended 25.841 for the gigantic A380, which they had designed for a maximum operating altitude of 43 000 feet. Their case centered around the ability of the aircraft to descend very quickly in case of a 43 000 feet decompression. Airbus argued that in case of a total loss of all hydraulic power and a signiﬁcant damage of the pressurized fuselage the aircraft will still be able to deploy at least two pairs of spoilers in order to descend quickly to a safe altitude, as the 9

US Department of Transport Exemption No. 8160.

2.8 40 000 feet – a line in the sand

45

hydraulically actuated spoilers have an electrical back-up mechanism. The FAA received a string of letters regarding Airbus’s petition, including an unequivocally supportive one from Boeing (!) and one from the Airline Pilots’ Association (ALPA) urging the FAA not to grant the exemption. ALPA’s letter picked up on several of Airbus’s claims made in the petition, including the statement that a complete depressurization (from 8000 feet or lower cabin altitude to 40 000 feet or higher) may occur in as little as 17–20 seconds. That may be sufﬁcient, ALPA argued, to cause some occupants to experience decompression sickness regardless of the resultant cabin altitude or time of exposure, which can result in permanent physiological harm that cannot be treated onboard an aircraft, and can be fatal. They also sought to remind the FAA of the results of a 1967 study by Nicholson and Ernsting [91], a set of aggressive decompression trials conducted on baboons. One of their decompression proﬁles, which simulated a 6-inch diameter defect at an altitude of 42 500 feet, resulted in an exposure time of 1 minute and 30 seconds to a cabin altitude greater than 40 000 feet – this, Nicholson and Ernsting noted, caused their subject permanent brain damage10 . Eventually the FAA granted Airbus a partial exemption in 200611 , relaxing the rule to allow the cabin altitude to exceed 40 000 feet for one minute (but not to exceed 43 000 feet for any duration) after decompression from any uncontained engine failure. Further, the exemption will allow the A380 to exceed 25 000 feet of cabin altitude for more than two minutes (but not more than three minutes), allowing time for the airplane to descend from an altitude of 43 000 feet to 25 000 feet. At the time of writing, it appears that the Boeing 787 Dreamliner is also on course to be cleared to operate at altitudes up to 43 000 feet, allowing it to better fulﬁl the potential of its aerodynamically efﬁcient airframe and its very low fuel burn engines. As for how much exposure to very high cabin altitudes is likely to cause lasting physiological harm, the jury is still out. It appears that more research is needed, as the available body of evidence is insufﬁcient for ﬁrm decision making. Much of the current debate is informed by studies that are limited in scope – perhaps most pressingly, future studies ought to consider the effects of hypoxia and decompression sickness on those with compromised health or old age (ALPA’s letter objecting to the Airbus exemption points out that the few human studies available all involved young, healthy subjects). In the meantime the FAA have formulated an interim policy [58] on the high altitude decompression issue, which simply states that no exposure is acceptable to altitudes above 45 000 feet, the cabin altitude must not rise above 40 000 feet for more than one minute, above 25 000 feet for more than three minutes and above 10 000 feet for more than six minutes. Beyond all the arguments about the possible long term harm victims of high altitude decompression incidents might come to, the design measures that could be considered to reduce the likelihood of such events and a possible future harmonization of the relevant regulatory requirements (there are signiﬁcant differences here between European and US rules), it is ﬁnally worth taking the statistical long view on the 40 000 feet issue. 10

US Department of Transportation / FAA Docket 20139 contains Airbus’s full petition, making the case for exemption in much greater detail. The same docket also contains the rejoinders by various companies, institutions and private individuals, including those from Boeing and ALPA. 11 US Department of Transport Exemption No. 8695.

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2. Comfort zone

A study ran under the auspices of the FAA’s Aviation Rulemaking Advisory Committee painstakingly cataloged all documented decompression events to have occurred on transport airplanes (heavier than 27t) between 1959 and 2001. In over four decades’ worth of data they found a grand total of 14 cases when the cabin altitude rose above 25 000 feet. Of these, seven incidents involved the cabin altitude exceeding 30 000 feet, there being a single documented case of the altitude rising above 35 000 feet. Perhaps most signiﬁcantly though, in over four decades of commercial aviation (unrestricted by the Amendment) not a single case occurred of airline passengers being exposed to a cabin altitude of 40 000 feet or above.

2.9 The bag may not inﬂate A small, but rather signiﬁcant aspect of the legacy of the Peach Air incident (described earlier in this chapter) survives to this day in the shape of seven words most airline passengers hear in the section of the initial safety announcement preparing them for ‘the unlikely event of the loss of cabin pressure’, but probably few pay much attention to: “Note that the bag may not inﬂate.” The ofﬁcial report into the Peach Air depressurization noted that several passengers were alarmed to see that, as the masks dropped out from the overhead containers and they tugged on them, as instructed, the bags through which the oxygen tubes connected to the masks, did not inﬂate. From this they inferred, incorrectly, that the oxygen supply had failed. In order to understand how the masks actually work, it is worth considering what happens when the cabin altitude rises above 14–15 000 feet (or the deployment button is pressed in the cockpit) and the oxygen masks drop. Passenger oxygen systems are of two basic types. One type works on the principle of chemical oxygen generation, based on sodium chlorate being mixed with a catalyst when the system is activated and decomposing in an exothermic (heat releasing) reaction into sodium chloride and oxygen: 2NaClO3 → 2NaCl + 3O2 . Typically, each seat or group of two or three seats is equipped with its own generator – tugging on the mask will trigger the reaction and start the ﬂow of oxygen. The alternative is to carry oxygen in a central bank of pressurized alloy steel cylindrical bottles (normally located in the cargo hold), which are connected through a sequence of pressure regulators (the pressure in the bottles can be as high as 2000 psi!) to a manifold that distributes the oxygen to all the passenger masks12 . In both cases the ﬂow of oxygen is continuous, that is, independent of demand, independent of whether anyone inhales it. The bags ﬁtted to the oxygen masks have the role of storing the incoming oxygen while the passenger is exhaling – while inhaling, the oxygen simply ﬂows through the bag and therefore does not inﬂate it. The mask also acts as a 12

The ﬂight crew oxygen supply is always of the bottled variety, as it may be required intermittently and in chemical systems the reaction cannot be paused once it is triggered.

2.9 The bag may not inﬂate

47

diluter: the wearer breathes in a mixture of cabin air and oxygen. Incidentally, this feature renders the oxygen masks ineffective as smoke hoods in case of a ﬁre on board (though if there is a ﬁre, pumping pure oxygen into a cabin may be rather hazardous anyway). The passenger oxygen supply is generally designed to last for about 12-15 minutes, which should be sufﬁcient time for a descent to an altitude where the ambient air contains sufﬁcient oxygen. For example, the Boeing 737-200 carries two oxygen bottles of just under 2200 liters each in its forward hold, providing 12 minutes’ worth of oxygen for the passengers, a ﬁgure based on the following scenario [17]: Time required

Crew action

18s 3min 6s 7min 36s 1min

delay at 37 000 feet before commencing descent descent to 14 000 feet hold at 14 000 feet descent to 10 000 feet.

These 12 minutes include another source of potential angst for passengers already distressed by the depressurization: losing as much as 23–25 000 feet in three minutes may feel like a very precipitous descent – some passengers might interpret the rapid loss of altitude as the ‘plane going down’ and the ﬂight crew might be unable to handle the emergency and reassure the passengers at the same time. It is sometimes suggested that this could be avoided by adding a sentence to the pre-ﬂight safety announcement to the effect that “the dropping of the oxygen masks will be followed by a steep descent – this is normal.” Of course, the wording of the safety announcement has to strike a balance between reducing the chance of panic in case of a very unlikely emergency and creating a sense of unease in the nervous ﬂyer by going into the details of potential emergencies in the safety brieﬁng – most airlines, at the moment, appear to prefer to err on the side of avoiding the latter. As a matter of ﬂying technique and airmanship, fast descents from high altitude require, ﬁrst of all, a good understanding of the limitations of the aircraft. In particular, emergency descents are often performed at the speed and Mach number limits of the airframe. Maximizing the drag of the aircraft may also help sometimes and the two obvious ways of achieving this are the deployment of the airbrakes and the lowering of the undercarriage, each of these operations usually having their own speed limits (on some – especially older – aircraft even the ‘nuclear option’ of engaging idle reverse thrust is an option ). Another way of losing height quickly is by a series of sharp turns, a technique which has the advantage that the passengers will be experiencing the effect of positive g-forces pressing them into their seats, as opposed to rather unsettling sensation of being close to zero-g in a fast, straight line descent. Here the limitations of the aircraft have to be considered once again, so as to avoid over-stressing the airframe through excessively high rate turns. Modern, ﬂy-by-wire aircraft take care of most of these limitations for the pilot through their ‘ﬂight envelope protection’ algorithms, but understanding the ‘vertical abilities’ of an aircraft is still a necessary pre-requisite of a safe emergency descent. A somewhat less obvious, but often more challenging requirement is related to the topography underneath. Air crews will be, usually, performing an emergency descent over

48

2. Comfort zone

terrain that they had never meant to get close to. Of course, this is not the same as ‘never planned having to get close to’! The designation of so-called oxygen escape routes is often part of the ﬂight planning process when a ﬂight takes an aircraft over large expanses of high ground (such as much of Iran or the Tibetan Plateau), where the minimum safe altitude (as determined by the elevation of the terrain) is higher than the 10 000 feet ‘safe altitude’ to which a depressurized airplane would have to descend. These escape routes ﬁt the descent proﬁle required in case of a high altitude depressurization into terrain beneath – if such a route, avoiding high terrain, is not available, the ﬂight has to be re-routed.

2.10 Disregarded protests Another lesson of the Peach Air incident was the importance of having a two-member ﬂight crew in the cockpit. In an era of increasing automation the question is often raised whether a single pilot could safely ﬂy a modern jet airliner, in a seemingly natural progression after ﬂight crews of four were the norm on some aircraft in the 30s and three on the big jets of the 60s and the 70s. The answer appears to be yes, when everything is going to plan, but a resounding no when a crisis elevates workloads and potentially even incapacitates one of the pilots, as in the case of the Peach Air incident. Of course, the correct handling of an emergency, such as a loss of cabin pressure, is only facilitated when the pilots work together effectively, an aspiration sometimes made peculiarly difﬁcult by the potential effects of hypoxia. The signiﬁcance of Crew Resource Management (to use the term coined by the airline industry) in such unusual circumstances had been brought into sharp focus four years earlier by another near-miss, this time on an American International Airways cargo ﬂight manned by a ﬂight crew of three: a captain, a ﬁrst ofﬁcer and a ﬂight engineer. The DC-8-61 at the center of the incident departed Ypsilanti, Michigan, on 15 March 1994, on a ﬂight to Atlanta, Georgia. Soon after take-off it became apparent that the aircraft was unable to maintain pressurization. Yet, to the consternation of the ﬁrst ofﬁcer and the ﬂight engineer, the captain decided, inexplicably, that they would all don oxygen masks and carry on climbing. “Captain [...] is a strong willed person and when he is in command he is not to be questioned, [so] I followed his orders...”13 , the ﬁrst ofﬁcer later declared. As the aircraft was climbing through 29 000 feet, it became apparent to both junior crew members that the captain was no longer responding to radio calls; yet, he was still gesturing that the climb was to continue. Here is how the ﬂight engineer later recalled their unpressurized climb: “When the captain requested higher altitudes the ﬁrst ofﬁcer and I strongly protested, both verbally and by hand signals. These protests were repeated at least three times during the climb. All protests were disregarded by the captain...” The ﬁrst ofﬁcer’s fear of creating a feeling of mutiny were ﬁnally outweighed by his concerns for their safety at around 33 000 feet, when he took control from the partially incapacitated captain and initiated a descent to 10 000 feet ﬁrst, subsequently diverting to Cincinnati Airport for an uneventful landing. The captain had to be hospitalized.

13

National Transportation Safety Board Report NYC94LA062.

2.11 A bottle on the rampage

49

The depressurization turned out to have been caused by the failure of the latching mechanism of an improperly installed overwing emergency exit. The Federal Aviation Administration’s Civil Aeromedical Institute solved the more interesting riddle of why the captain exhibited the classic symptoms of hypoxia and decompression sickness (lack of judgement, overconﬁdence, the sense that everything was going well) when the rest of the ﬂight crew seemed unaffected (the captain’s oxygen mask and regulator were found to be in good working order). The answer, it turned out, was his girth: at 5 ft 10 in and 240 pounds (1.78 m and 109 kg) he was by far the most ‘substantial’ of the trio, as well as being a smoker – both aspects are known to aggravate the physiological effects of depressurization (the effects of high altitude manifest themselves in smokers at least 2000 to 3000 ft below the altitude that affects a non-smoker – they have more carbon monoxide already bound to their blood haemoglobin [129]). It almost goes without saying that the incidents described so far in this chapter are highly unusual – the robustness and multiple redundancies of modern air conditioning systems make depressurizations extremely rare. Rarer still do such failures lead to serious crisis; the majority of cases are dealt with by ﬂight crews swiftly and effectively. A good example is the following recent incident, which we describe here on account of its rather unusual cause.

2.11 A bottle on the rampage The event that made the City of Newcastle – a Boeing 747-400 operated by Quantas – the subject of the headlines of 25 July 2008 occurred at 29 000 feet over the South China Sea, about 55 minutes into a ﬂight from Hong Kong to Melbourne, Australia. The 346 passengers and 16 crew felt a sudden jolt, accompanied by a loud bang and a loss of cabin pressurization, followed almost immediately by oxygen masks dropping above each passenger seat. Most passengers began to use the oxygen masks straightaway, as did the cabin crew, most of whom sat down on empty seats amongst the passengers. It is hard to imagine the distress of a loud bang and a decompression interrupting a routine meal service while in a smooth, high altitude cruise. In fact, the ensuing panic was not limited to the passengers: several ﬂight attendants simply froze and could not do anything in the moments following the rude interruption to what was supposed to be a routine ﬂight. There was plenty to worry the ﬂight deck crew too. The computers of the 747 were generating a catalogue of failure messages, including those of the jet’s three instrument landing systems, one of the very high frequency omni-directional radio range instruments, one of the ﬂight management computers and that of the anti-skid braking system. Nevertheless, the pilots maintained their focus on the key task, that of reaching a safe altitude as soon as possible. Only 20 seconds had elapsed since the start of the crisis when they throttled back all four engines and deployed the speed brakes, launching the jet into a rapid emergency descent. The crew elected to divert to Manila for an emergency landing – they landed safely in the Philippine capital almost exactly an hour after the decompression. In the event, the

50

2. Comfort zone

approach was a smooth one, in spite of the failure of the instrument landing system, which was not needed, thanks to visual conditions prevailing throughout the 747’s ﬁnal approach. The ‘City of Newcastle’ looked somewhat worse for wear as it stood on the apron of Manila’s Ninoy Aquino International Airport (see Figure 2.7). On the starboard side the Boeing was missing several large body panels in the wing-to-body fairing area, with some cargo items clearly visible through a large hole in the fuselage, just forward of the wing root. Closer inspection revealed what was to emerge as a rather elusive, but almost certain culprit. The 747-400 carries a bank of seven large, high pressure oxygen bottles, attached to the right-hand wall of the cargo bay; these provide part of the oxygen supply to the passenger emergency oxygen system. Only six made it to Manila on-board the ‘City of Newcastle’ though – number four was, apparently, missing (see the bottom image in Figure 2.7). Clearly, the bottle must have exploded, with the sudden, localized over-pressure blowing out a section of the cargo bay wall. It would have been easy to conclude at that point that the bottle then fell out through the hole its own explosion had just created, had there not been evidence further up, inside the fuselage, that before departing it had made a brief, but memorably lively visit to the passenger cabin. Here is its itinerary, as pieced together by the Australian Transport Safety Bureau [8]. The bottle bursts open, its initial explosive decompression causing the rupture in the side of the fuselage. It rockets upwards, piercing the cabin ﬂoor, entering the cabin near one of the emergency exits. It then impacts the door frame and the large rotating handle (just failing to actually open the emergency exit door!), with the impact ﬂipping it upside down and propelling it towards the overhead paneling of the cabin. The bottle smashes into the ceiling, crushing some of the paneling and ﬁnally runs out of steam, beginning its return journey. It falls through the hole it had just punched in the cabin ﬂoor and, with a ﬁnal ﬂourish, it exits stage right, through the large hole in the side of the fuselage which had caused the decompression in the ﬁrst place. The total area of the hole punched by the explosion was estimated at 1.74 m2 . So what is the impact on the cabin pressure of such a ‘window’ suddenly opening in the cargo hold of a 747? The aircraft’s ﬂight data recorders gave a clear answer. As the ‘City of Newcastle’ was cruising at 29 000 feet, the air conditioning system was maintaining the cabin altitude at a very comfortable 3700 feet. As the oxygen bottle blew up and the hole opened, the pressure began to fall rather sharply: in the space of just under 20 seconds the cabin altitude rose to 25 900 feet, where it leveled off for the next minute or so. As we have seen, the textbook reaction of the ﬂight crew meant that the 747 now began a steep descent straightaway, initially at a rate of about 5000 feet per minute, leading to an immediate drop in the cabin altitude too. The Boeing reached the safe altitude of 10 000 feet just over six minutes after the decompression (see Figure 2.8). We have seen earlier that the paucity of reliable data on the physiological effects of decompression is the major stumbling block in the path of conﬁdent rule making – alas, the Quantas incident also furnished scant new information in this regard. While many of the passengers reported tremors, fast heartbeat, light-headedness and faintness after landing, these symptoms were as consistent with the natural anxiety brought about by the incident, as with hypoxia or decompression sickness. More readily associated with the

2.11 A bottle on the rampage

51

Fig. 2.7 The City of Newcastle after the depressurization incident: part of the wing-to-body fairing blown away (top) and the slot where oxygen bottle no. 4 used to be, as seen from the inside of the cargo hold (bottom) [8]. Images courtesy of the Commonwealth of Australia, reproduced by permission.

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2. Comfort zone

30 000

Aircraft altitude Cabin altitude

Normal cruise

25 000

Altitude [ feet ]

Emergency descent 20 000

15 000

10 000

Safe altitude

5 000

Decompression ï2

ï1

0

1

2

3

4

5

6

7

Time from decompression [ minutes ] Fig. 2.8 The ‘City of Newcastle’ incident – anatomy of a decompression. At ‘moment 0’ the cabin altitude begins to rise rapidly from around 3700 feet, reaching nearly 26 000 feet in just under 20 seconds. A textbook emergency descent follows with the cabin altitude back under 10 000 feet within about six minutes. Note that the cabin pressure never quite drops to the ambient value (data from [8]).

decompression were the ear complaints of many passengers and crew: temporary loss of hearing, ear pain or ear ‘popping’. Ultimately then, the ‘City of Newcastle’ incident is a positive note on which to conclude this chapter. Cabin depressurizations are extremely rare and much rarer still are those with any serious consequences. In spite of the substantial (and at the time unknown) damage, the crew of the Quantas 747 reacted promptly and effectively and brought the aircraft and its passengers down safely. We might spare a thought for the emergency oxygen system too: all passengers received supplemental oxygen immediately after the explosion, in spite of the damage caused by the untimely departure of one of the oxygen bottles. How ironic though, incidentally, that the system designed to ensure the safety of the passengers was the cause of the very type of emergency it was designed for.

Part II

New heights of ﬂight

3. A tale of two Comets

“We [pilots] are being paid to avoid hazard, but there are still many unexplored crevasses in our reservoir of knowledge. Our zeal for air transport is always soured when we so easily reﬂect on failures involving certain late comrades, who proved in the ﬁnal analysis to be, like ourselves, only the tip of the arrow. We are obliged to recognize our possible epitaph – His end was abrupt.” Ernest K. Gann (1910–1991) ‘Fate is the Hunter’∗

Blythe House, the Science Museum’s North London storage facility, is home to a rather unprepossessing object. Mounted on a low, horizontal wooden plinth about eight feet long and four feet wide, it is only a few inches tall and its most prominent component is a large, thin aluminium sheet, covering almost the entire plinth. A sparse framework of beams, each about an inch deep, supports the sheet from underneath. There are two rectangular cutouts near the middle, 22 inches long on each side and about 20 inches apart. Through the cutouts some wiring and coiled-up metal tubes are visible. The entire assembly looks somewhat weathered, with the once white paintwork now a dirty beige. A maze of long cracks and tears throughout the sheet create the impression of an untidy jigsaw puzzle of half-a-dozen or so pieces. Belying its underwhelming appearance, this heap of scrap metal (it must be little more in the eyes of the casual observer) is one of the most signiﬁcant artefacts of the history of the conquest of the stratosphere. For decades it has been the subject of unprecedented scientiﬁc scrutiny, analysis, controversy and debate, as well as of lazy misinterpretation and persistent myth. This is its story. ∗

Gann, E. K., “Fate is the Hunter”, a Touchstone book, Simon and Schuster, 1986.

A. Sóbester, Stratospheric Flight: Aeronautics at the Limit, Springer Praxis Books, DOI 10.1007/978-1-4419-9458-5_3, © Springer Science+Business Media, LLC 2011

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3. A tale of two Comets

3.1 A national symbol Being British in the early 1950s was a mixed experience. The terminal decline of the Empire was accompanied by the ever-increasing threat of the Cold War, made tangible by a distant, but protracted and bloody conﬂict in Korea between United Nations troops to the South of the 38th Parallel and North Korea and China to the North. At home the smog and a string of unusually violent storms were claiming thousands of lives. Yet, the rationing of tea and chocolate was coming to an end. The Commonwealth was celebrating the coronation of Elizabeth II and the conquest of Everest by Edmund Hillary and Tenzing Norgay. James Watson and Francis Crick announced their discovery of the structure of DNA. The post-war reconstruction effort was beginning to take shape, as epitomized by the Festival of Britain and its symbol, the spectacular Skylon. Arguably, however, the emblem that best represented the sense of optimism and national pride was the British-designed and built de Havilland DH.106 Comet 1, the world’s ﬁrst jet airliner and the ﬁrst airliner to cruise in the stratosphere. Even America was in awe of the sleek new bird, especially because the prototype of Boeing’s new jetliner, the Dash-80, was only a blueprint at the time, with its ﬁrst ﬂight still over two years away. United Airlines, holding out for an American jetliner, were experimenting with the rather unambitious-sounding Paper Jet, a simulated daily ﬂight between San Francisco and New York, meant to prepare ﬂight planners for the eventual arrival of the jet age1 . The ﬁrst Paper Jet ‘took off’ six months after the very real Comet entered commercial service with the British Overseas Airways Corporation (BOAC) on May 2, 1952 – small wonder Britain was celebrating. There were celebrations along the Comet’s routes too. The ﬁrst scheduled ﬂight took its lucky passengers from London, via ﬁve African stopovers, to Johannesburg, where its arrival was greeted by a cheering crowd of around 20 000 people2 . “As if poised motionless in space – but at eight miles a minute. Jet travel at 500 miles per hour through the calm stratosphere, eight miles above the world, rock-steady, vibration free – it makes every ordinary airliner a thing of the past” proclaimed de Havilland’s adverts. Indeed, when compared to its propeller-driven counterparts, the Comet’s performance was astonishing. Consider, for example, a comparison with the Canadair Argonaut, which had itself only entered commercial service less than ﬁve years previously on a large number of BOAC routes. With a typical long-range load its four Rolls-Royce Merlin engines powered it to a cruise speed of 306 mph with a service ceiling of 25 200 feet 3 , well below the stratosphere. Carrying a similar payload to a range just shy of that of the Argonaut the Comet cruised a whopping 100 mph faster, usually at around 35 000 feet4 , in the lower layers of 1

The Paper Jet is described in detail in The Next Fifty Years of Flight [12], a record of Norwegian and USAF Colonel Bernt Balchen’s aeronautical predictions made in 1954, a book from which much can be learnt, mainly that (correctly) forecasting ﬁve decades ahead is very difﬁcult indeed. 2 ‘Excess baggage’, BBC Radio 4, broadcast on 8 March 2008. 3 The service ceiling is the density altitude at which, ﬂying in a clean conﬁguration, at the best rate of climb airspeed for that altitude and with all engines operating and producing maximum continuous power, the aircraft is capable of a 100 feet per minute climb. 4 Contemporary reports differ slightly in terms of these performance ﬁgures – the numbers quoted here are those listed in the 1951–52 and 1952–53 issues of the authoritative Jane’s All the World’s Aircraft [19, 20].

3.1 A national symbol

57

Fig. 3.1 27 July 1949: the maiden ﬂight of the de Havilland Comet 1 (prototype G-ALVG). Image courtesy of the Science Museum/SSPL.

the stratosphere, above most of the weather (at least in Europe). This resulted in dramatically shorter journey times: while a trip to Tokyo had taken 86 hours, the Comet covered it in 33. Queen Elizabeth The Queen Mother was one of the early passengers. Accompanied by Princess Margaret, Group Captain Peter Townsend and others, she was taken on a special ﬂight from de Havilland’s Hatﬁeld site to Italy on board a Comet commanded by de Havilland chief test pilot John Cunningham. She was even allowed to take the controls brieﬂy, as she was the honorary air commodore of a ﬁghter squadron, and was quite taken by the idea of her ﬂying faster than her ﬁghter pilots ever had. According to a contemporary news report “clearly elated, her Majesty went back to the main cabin and told Princess Margaret, who, like the rest of the passengers, had been quite oblivious of the achievement, chatting comfortably to the Marchioness of Salisbury over a cup of tea.”5 It is thus likely that the 29 passengers boarding BOAC Comet G-ALYP at Rome Ciampino airport on the morning of 10 January 1954 were looking forward to the just over two hour ﬂight to London. Beyond the excitement of experiencing the smooth, quiet stratospheric ﬂight aboard this symbol of national pride, they had every reason to feel priv5

“Excess baggage”, BBC Radio 4, broadcast on 8 March 2008.

58

3. A tale of two Comets

ileged too – air travel in 1954 was a luxury far beyond the reach of the average traveler (to put this into perspective, fewer than one in three British households had an electric kettle at the time!). G-ALYP, commanded that day by 31-year old Captain Alan Gibson, was the same aircraft that 20 months and some 1200 ﬂights earlier had inaugurated the era of jet travel with that historic ﬂight from London to Johannesburg. This morning’s ﬂight was also part of a long-haul sequence, including Karachi, Bahrein and Beirut, the ﬂight from Rome to London being the ﬁnal leg of the trip. This was one of several ﬂights that day on the same route. In fact, another BOAC aircraft, Argonaut G-ALHJ had left for London just a few minutes before G-ALYP was cleared to taxi to the departure runway. Nevertheless, due to the great difference in performance between the two aircraft, G-ALYP was expected in London nearly two hours ahead of the Argonaut.

Fig. 3.2 A peek through the curtains: passengers relax on board a de Havilland Comet 1 in 1951. Image courtesy of the Daily Herald Archive/SSPL.

3.1 A national symbol

59

Ciampino air trafﬁc control cleared G-ALYP for take-off at half past ten. Minutes later the four de Havilland Ghost 50 turbojets were powering the Comet through thin, broken layers of cloud, allowing the passengers occasional glimpses of Tuscany: on the right the west coast of the Appenine Peninsula and on the left the granite cliffs of Isola del Giglio and Isola di Montecristo (that of Alexandre Dumas’ The Count of Montecristo fame). The Comet’s large windows made it ideal for such sightseeing – at 16.6 inches wide and 14 inches high they were amongst the major contributors to the pleasure of the ride. Meanwhile, in the cockpit, Captain Gibson and the rest of the ﬂight crew had little time to admire the scenery. The Comet was not an easy aircraft to ﬂy. While it was much loved by passengers and by BOAC executives, ﬂight crews had numerous misgivings about its design, chief amongst these being concerns about reliability. Here is how one of Gibson’s colleagues remembers the teething problems of stratospheric passenger transport: “The Comet had an equipment bay that could be reached in ﬂight from a hatch in the ﬂoor at the rear of the ﬂight deck. I noticed the amount of time our ﬂight engineers had to spend down this hole. The hydraulic systems needed reﬁlling during our short cruise periods. They leaked constantly and they were designed to be quite independent of each other, supplying main, standby and emergency power to ﬂight controls, air brakes, ﬂaps, and undercarriage. Unfortunately we found that each of the three separate systems leaked at exactly the same rate, indicating interconnection. [...] On one of my ﬂights the engineer used ﬁfteen cans of ﬂuid...”6 While the ﬂight engineers’ working conditions were thus less than ideal, the pilots’ comfort was also impaired in a number of ways. For example, the windscreens had a tendency to frost over at high altitudes and the only means of keeping them warm and clear was to run a pump blowing hot air onto them, generally resulting in dry, sore eyes in the cockpit. There were propulsion problems too. The maximum combined thrust of the four Ghost 50 centrifugal compressor turbojets was less than a ﬁfth of the all-up weight of the aircraft, which required a great deal of care on take-off and some patience during the climb. Once at cruising altitude, the engines had to be monitored very closely for signs of gradual loss of power – climbs had been known to turn into gentle descents – usually caused by small ice crystals accumulating in the fuel ﬁlters (jet fuel always contains a small amount of water – more on this and on other challenges of ﬂight at very low temperatures in Chapter 6). This had never been a problem on much lower-ﬂying propeller-driven aircraft, but the Comet, cruising in, or just below, the stratosphere was exposed to temperatures often close to −60◦ C (−76◦ F) and its fuel tanks were not heated. Icing also affected the air intakes, disrupting the airﬂow through them, in extreme cases causing the engines to ﬂame out. Let us return though to that January morning in 1954. The ﬁrst waypoint of the ﬂight to London was the radio navigational beacon at Orbetello, about 100 miles to the north-west of Rome and Gibson reported passing it at an altitude of 27 000 feet, just under 20 minutes after take-off. He also conﬁrmed to the Italian air trafﬁc controller that they were continuing the climb towards 36 000 feet, as per the ﬂight plan. In the cabin the ‘Fasten Safety Belts’ signs had gone off and the passengers were settling down for a smooth and quiet ﬂight to London. At this point the Comet had already overtaken the Argonaut 6

Excerpt taken from the memoirs of former BOAC pilot Peter Duffey[46].

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3. A tale of two Comets

G-ALHJ, which was now thousands of feet beneath it. Gibson had promised its captain that as soon as he breaks through the cloud tops he would report their altitude to the crew of the lower ﬂying propeller aircraft. After an earlier attempt to do this had been thwarted by interference, he tried again. ‘George How Jig from George Yoke Peter did you get my...’ 7 8 This truncated sentence was the last anyone has heard from those on-board G-ALYP. Shortly after, reports started coming in to the Harbour Authority in Portoferraio in the Isle of Elba of an aircraft that had exploded in the air and crashed in ﬂames into the sea South of Cape Calamita. In spite of a prompt and massive rescue effort only 15 bodies, a few mail bags and some personal items could be salvaged9 .

3.2 Tu quoque? BOAC immediately suspended all Comet operations. They also appointed a committee to examine the circumstances of the accident, though the paucity of evidence made any investigation little more than guesswork. The only clues were provided by the forensic medical report produced by the Italian authorities, which suggested that the passengers’ deaths have been caused by impact against parts of the inside of the aircraft, consistent with a possible explosive decompression of the Comet’s pressurized cabin. This provided no indications as to the cause of the disintegration of the cabin, so BOAC and de Havilland were left to speculate. Many considered sabotage as being the most likely cause, with the large number of stopovers on G-ALYP’s route providing numerous opportunities for a bomb to be placed on board. Others suspected ﬁre, possibly originating from the engines and so de Havilland proceeded to make a number of modiﬁcations on the Comet ﬂeet to “remove every possible ﬁre risk they could imagine”[97]. With such modiﬁcations (around 80 in total) complete, the Air Safety Board concluded that “everything humanly possible has been done to ensure that the desired standard of safety shall be maintained. This being so, the Board sees no justiﬁcation for imposing special restrictions on the Comet aircraft.” BOAC’s Comet ﬂeet took to the air once more on the 23 March 1954, just over two months after the loss of G-ALYP and, more to the point, before the cause of the accident was established. The 19 March editorial of the magazine The Aeroplane [3] summed up the general feeling in the airline industry: “The cautious person might ask: Why, after waiting so long could not the Comet’s return to service be postponed until the causes of the latest accident have been ﬁnally resolved? The answer to that question is simply tu quoque: What would you do? The positive cause of the 7

Radio callsigns are often made up of the ﬁrst and the last two letters of the aircraft’s registration. To avoid confusion over crackly radios, they are pronounced using the phonetic alphabet and in 1954 the so-called Able Baker alphabet was in use in Britain, wherein “A” = “Able”, “B” = “Baker”,... “H” = “How”,... “J” = “Jig”, etc. A few years later the NATO phonetic alphabet (“Alpha”, “Bravo”, etc.) replaced Able Baker. 8 Of course, we cannot be entirely certain that Captain Gibson was indeed about to repeat the cloudtop report – this is a best guess based on conversations with other BOAC pilots by journalist Tim Hewat and former RAF squadron leader William A. Waterton, as recorded in their book, The Comet Riddle [61]. 9 The reader may ﬁnd these and other details of the ﬂight in the Report of the Court of Inquiry into the accident [97].

3.3 A different era

61

accident may never be found, though every effort will be made to do so. The results of the public inquiry will undoubtedly be interesting, but they are not likely to be conclusive and it will be a relatively long time before such an inquiry can even be started. To this extent one agrees with the more vigorous shouts of a section of the Press. Now that we have done everything that the technicians consider necessary to forestall trouble in the future – let us get the Comets ﬂying again.” And ﬂy again they did – for another two weeks. On 8 April 1954 the crew of Comet 1 G-ALYY, on the Rome – Cairo leg of a London – Johannesburg run reported to be “over Naples, still climbing” a few minutes after takeoff; and were never heard from again. 14 more lives were lost in circumstances eerily similar to those of the G-ALYP accident. The Comet 1 was ﬁnished and this time the need for a full-scale investigation was evident.

Fig. 3.3 The end of the Comet 1 – G-ALYY and G-ALYP lost over Italy in the space of three months.

3.3 A different era From a 21st century standpoint the approach taken by the Air Safety Board (and accepted by the aviation establishment at the time) after the loss of Yoke Peter may almost seem cavalier. Why was the Comet allowed back into the air before the reasons behind the accident were known? The tediously cynical answer still occasionally rehearsed today cites loss

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3. A tale of two Comets

of business by BOAC and de Havilland while the Comets were grounded. Such armchair wisdom is, of course, facilitated by hindsight (had the second tragedy not occurred, few would bemoan the hurried resumption of ﬂights), but, more signiﬁcantly, it ignores the historical fact that it is precisely the story of the double tragedy of the Comet that shaped the rigorous investigation protocols we are used to and expect today. The prevailing mindset at the time was that of the editor of The Aeroplane cited earlier. Consider also the following thought experiment. A passenger airliner takes off today from Rome and, a few minutes into its climb, crashes into the sea. What does an accident investigator do in the ﬁrst few days of the enquiry? An underwater salvage operation may be commissioned – typically using a surface vessel equipped with a remotely operated submarine – to attempt to retrieve the digital ﬂight data recorder and the cockpit voice recorder (both painted a conspicuously bright orange, which makes them easier to ﬁnd and presumably annoys those who insist on calling them ‘black boxes’). Meanwhile a vast amount of potentially relevant data is collected. Recorded radar traces provide a threedimensional image of the trajectory of the ﬂight 10 . A tourist has invariably captured video footage on a mobile phone. The burning wreckage will have fallen through the ﬁeld of vision of a CCTV camera. The Italian Air Force National Meteorological Service provides an aftercast of weather conditions at the time at sea level and at the altitude the aircraft had reached in the climb. Any pilot reports of turbulence, windshear or thunderstorm activity in the area, issued at around the time of the accident, are collected. Most importantly though, once the ﬂight data recorder is found, the investigators have dozens of channels of recorded data on the positions of ﬂight controls and on the motion of the aircraft, engine performance parameters, and many other indicators that help build up a very precise history of the ﬂight. Of course, practically none of the above was available to an air crash detective in 1954 – another indication of why it took a second, similar accident for a full inquiry to be launched and what a mountain it had to climb when it was.

3.4 The Hall inquiry Arnold Alexander Hall, director of the Royal Aircraft Establishment at Farnborough, was a tall, thin man with angular features and a nose that, according to a contemporary account [61] “looked ready-made for prying into things”. His selection, however, for the daunting task of solving the Comet riddle had much less to do with having the correct nasal attributes for the job, than with being a brilliant engineer, in charge of an institution that, at the time, was uniquely qualiﬁed for the massive task.

10

Strictly speaking, the radar-based reconstruction of the trajectory is only guaranteed to be accurate in terms of latitude and longitude, as conventional (primary) radar can only ‘see’ in two dimensions. The third dimension, altitude, is captured by a secondary radar, but this relies on information broadcast by the aircraft’s transponder and is only as accurate as the altimeter(s) of the aircraft. Thus, if an accident is caused by, say, the pilots ﬂying on the wrong altimeter setting (wrong reference barometric pressure value dialled into the altimeter), the secondary radar will have seen the aircraft at the same incorrect altitude value as the pilots themselves will have read off their altimeter.

3.4 The Hall inquiry

63

Hall had outside help too. The University of Pisa was working on the forensic examination of the bodies of the victims of the Elba crash. The Admiralty had ordered the Royal Navy to attempt to recover as much of the wreckage of G-ALYP as possible (G-ALYY had crashed into waters much too deep to attempt any form of salvage). While all this was going on, investigators at the Royal Aircraft Establishment set about examining the histories of the two crashed aircraft and of the Comet in general. And there was a great deal to look at. The brief, trailblazing history of the world’s ﬁrst jet airliner had been rather eventful, with a catalogue of mishaps ranging from small incidents to fatal crashes. While this didn’t make it unique amongst other types of that pioneering era, any of these events could have provided useful clues as to what had gone wrong with G-ALYP and G-ALYY. A word that crops up frequently in these incident reports is over-rotation, a symptom of the Comet’s somewhat temperamental nature. Here is how over-rotation happens. The aircraft begins its take-off run. At a pre-determined speed the pilot lifts the nose gear off the runway (rotates the aircraft) to a speciﬁed angle between the fuselage and the surface of the tarmac. The acceleration then continues until the lift generated by the wings exceeds the weight of the aircraft and thus the aircraft becomes airborne. Should the pilots over-rotate, that is, raise the nose beyond the correct angle, the drag of the rotated wings may exceed the thrust the engines are capable of generating and the aircraft will begin to slow down. Worse still, the steady ﬂow of air over the wings may break down altogether, resulting in buffeting at ﬁrst and, ﬁnally, a wing stall, with the nose falling back onto the tarmac. Either way, the red lights marking the end of the runway will soon be too close for comfort and a prompt decision will be required about whether to abandon the take-off attempt. Judging the correct angle of rotation (particularly at night) is not easy and the vast majority of aircraft are designed to forgive an error of a few degrees. Not the Comet though – the somewhat underpowered jet was quite unforgiving and numerous crews fell victim to this foible11 . Two such cases ended in disaster. First, Captain R. E. H. Foote wrote off G-ALYZ on an aborted night take-off and the resulting overrun in October 1952. On that occasion no one was seriously hurt. Sadly though, four months later a similar over-rotation cost the lives of eleven people, this time in Karachi (the location is signiﬁcant, because jet engines produce less thrust in hot, thin air, thus aggravating the problem). The Comet involved, CF-CUN, operated by Canadian Paciﬁc, was, once again, written off. The most mysterious and most tragic of these earlier accidents occurred only two months after the Karachi overrun. G-ALYV took off from Calcutta with bad weather forecast in the vicinity of the airport. There are conﬂicting accounts as to just how bad the conditions turned out to be, but what is known for certain is that approximately six and a half minutes after departure eyewitnesses saw the burning wreckage of the Comet falling to the ground. 43 people lost their lives. The date was 2 May 1953 – the Comet had been in service for precisely one year. The Indian accident investigators suggested that the aircraft had been over-stressed in the storm and this may have been due to a fundamental design ﬂaw, but neither was there ﬁrm evidence for any of this, nor was the location or type of 11

You may ask, why not keep accelerating with the nose gear on the runway for longer, thus generating a reserve of speed and momentum that could overcome a subsequent over-rotation? This technique has been tried, but the Comet’s long, rather fragile nose gear strut was thought to be overloaded by the high speed impacts with bumps in the tarmac.

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ﬂaw made clear. Nonetheless, here was another piece for the hugely complex puzzle Sir Arnold Hall (knighted during the investigation) had to solve. A fairly common complaint of Comet pilots had been a certain lack of responsiveness in the controls and, even worse, a lack of ‘feel’. Unlike slower, propeller driven aircraft, the Comet had hydraulically assisted controls (similar to the power steering on a car), that is, the pilots’ control inputs were only indirectly converted into control surface movements. There was little design experience available at the time on this novel system and the result was an aircraft that could behave unexpectedly in technically demanding manoeuvres, such as crosswind landings. In fact, the very aircraft that crashed at Elba had been damaged in a heavy landing in the summer of 1952. Incidentally, there was another remarkable entry in G-ALYP’s short logbook: an explosion and ﬁre in one of the engines, just over a year prior to its eventual demise [61]. Could either of these events have caused undetected damage? There was another type of often unseen damage that was, at the time, at the center of structural engineering thinking, even ﬁnding its way into literary ﬁction, as we shall see on the small de-tour we are about to dedicate to it. The phenomenon was also very much in the minds of Hall’s investigators, as an ogre that had been known about for nearly a century, but was not quite fully understood.

3.5 The W¨ohler curve 8 May 1842 entered the history books as the date of one of the ﬁrst major railway disasters. A train traveling from Versailles to Paris derailed at Meudon when one of the axles of its steam engine fractured, killing 55 passengers. The phenomenon of shafts and axles failing suddenly near stress concentrators (e.g., near shoulders and diameter step changes) had been heard of at the time – at least by those who read, for example, the work of the great Scottish physicist William Rankine (of Rankine thermodynamic cycle fame). There was also some understanding in scientiﬁc circles of the fact that a load that would, statically (applied constantly), not cause failure, could still lead to sudden brittle fracture after large numbers of repeated applications of that load. However, it was not until the 1860s and the work of another railway engineer, August W¨ohler, that the ﬁrst serious efforts were made to quantify this effect and to try and understand its signiﬁcance from the point of view of engineering design. Working for the Prussian Railway Service, W¨ohler designed and built the ﬁrst full scale railway axle fatigue testing rig. More importantly though, his work led to the characterization of mechanical components in terms of their cyclic stress amplitude versus length of life (S-N) curves – sometimes referred to to this day as W¨ohler curves. Figure 3.4 is an idealized example of a W¨ohler diagram, which highlights a key difference between ferrous (iron based) and non-ferrous materials. Ferrous materials have what is now known as a fatigue limit. That is, if never exposed to loads generating stresses higher than this value, they can safely operate forever. Conversely, the safe service life of components made of non-ferrous materials, such as the aluminium alloys used in many parts of the Comet, will keep increasing as the intensity of their cyclic loading decreases, but they will always have a ﬁnite life, no matter how low the stress.

65

^ƚƌĞƐƐĂŵƉůŝƚƵĚĞ;^Ϳ

3.6 No Highway

&ĞƌƌŽƵƐŵĂƚĞƌŝĂů &ĂƚŝŐƵĞ ůŝŵŝƚ

EŽŶͲĨĞƌƌŽƵƐŵĂƚĞƌŝĂů EƵŵďĞƌŽĨĐǇĐůĞƐƚŽĨĂŝůƵƌĞ;EͿ Fig. 3.4 W¨ohler diagram illustrating the difference between the fatigue behavior of a ferrous and a nonferrous material.

It is, perhaps, worth issuing a ‘health warning’ here. An alarmingly large number of books and articles offer a ‘popular’ illustration of the concept of metal fatigue, inviting the reader to take a piece of wire or a paperclip and bend it and straighten it out repeatedly until it breaks – ‘fatigue failure in action!’, they announce triumphantly. It is, in fact, pseudoscience in action. The experiment could be called ‘how to break a paperclip without good reason’, as metal fatigue only occurs in elastic deformation, that is, when the test specimen is only deformed to the extent that, when released, it ‘springs’ back to its original shape (as opposed to the plastic deformation, after which the paperclip would stay in its bent shape). You could perform the paperclip experiment to actually see metal fatigue in action by only bending it very slightly each time, such that no permanent deformation occurs that has to be ‘straightened out’, but it may take a few months or years until failure occurs (incidentally, a phenomenon called work hardening is responsible for the failure of the aggressively bent and straightened paperclip).

3.6 No Highway Here is the plot, in a nutshell, of No Highway, a 1948 novel by aeronautical engineer and writer Nevil Shute Norway12 . Theodore Honey is an engineer assigned to study the fatigue behavior of the aluminium airframe of the Reindeer, a modern (ﬁctional) trans-Atlantic airliner. He ﬁnds that, according to his calculations, the tailplane of the Reindeer is likely to have a much shorter fatigue life than that predicted by its designers. Alarmingly, much of the ﬂeet is already approaching the critical number of ﬂight hours and, sure enough, a Reindeer crashes in mysterious circumstances. Although the ofﬁcial enquiry blames pilot error, Honey persists with his conviction that the real cause was metal fatigue and... 12

Shute is better known today for his post-apocalyptic novel On the Beach [117], the chilling story of the survivors of a nuclear world war.

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3. A tale of two Comets

It is best to stop the trailer here for fear of spoiling the reader’s enjoyment of No Highway or of its 1951 movie version No Highway in the Sky (starring James Stewart and Marlene Dietrich), by revealing its nail-biting twists or its ending. The story is worth a mention here, however, as an indication of the presence of the ogre of metal fatigue in the minds of engineers and of the general public in the years preceding the Comet crashes. Nonetheless, in the days immediately following the loss of G-ALYP, the aviation industry focused on a number of seemingly likelier culprits. The editorial of the next issue of The Aeroplane magazine [4] speculated about sabotage, as well as the possibility of ﬁre or turbine wheel fracture – we have already seen that these were BOAC’s principal concerns too. “A little less worrying to the traveling public”, it went on to say, “is the idea of metal fatigue, which has become a topic of conversation in the most untechnical circles following the introduction to the world by the author of “No Highway” of Mr. Honey and his theories. However, as we recently reported, the Royal Aircraft Establishment have been testing the components of the ﬁrst Comet extremely thoroughly.” In spite of these re-assuring words though, the results of the tests were somewhat worrying. One of the fatigue-critical areas is the structure of the wings. As the aircraft encounters gusts in ﬂight, the wings ﬂex, adding each time a fatigue cycle to their lifetime tally. BOAC estimated the number of such gust encounters to be about 25 per ﬂight. The engineers at de Havilland had sought to gain a reliable estimate of the fatigue life of the wings of the Comet and had handed over the prototype (G-ALVG) to the Royal Aircraft Establishment (long before the two crashes) for the “extremely thorough” tests mentioned in the article. A rig had been set up at Farnborough, designed to reproduce the ﬂexing effect of gusts on the wings. Somewhat unexpectedly, after the equivalent of 6700 ﬂying hours, a fatigue crack had occurred near one of the undercarriage bays [61]. De Havilland engineers estimated that the ominous crack still had many hours to go before posing any great danger (besides, none of BOAC’s Comets had ﬂown more than 3500 hours at the time), but an inspection programme was put into place. G-ALYY had been inspected, G-ALYP’s turn had not come at the time of its crash. The other major fatigue-critical component is the pressurized cabin. The design constraint that air pressure in an airliner cabin could never be allowed to fall below what one would experience outside at an altitude of 8000 feet, was already in place in the 1950s. This meant that ﬂying at its cruising altitude in the thin air of the stratosphere every square inch of the inside of the Comet cabin experienced an outward force of 8.25 pounds. The skin stretching effect of the pressure difference, as we have seen earlier, peaks at the cruising altitude and then slowly diminishes during the ﬁnal descent, disappearing completely at 8000 feet. A fatigue cycle is thus added with each ﬂight. So, if the Comet’s designers had overestimated the fatigue life of the cabin, what would the eventual in-ﬂight failure have looked like? A fatigue crack will have reached its critical length and, under the massive pressure load from the inside, it will have suddenly grown with explosive speed, tearing the fuselage to pieces in an instant. When was such an explosive failure most likely to occur? During the climb, anywhere above 8000 feet... Clearly, this was a scenario that the enquiry had to give serious consideration to.

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3.7 Consternation It was quite clear from the early days of Hall’s investigation that most of the details of both accidents were consistent with an explosive fatigue failure of the pressurized cabin. Yet, there was a rather awkward fact in the way of the ﬁnger of blame pointing at fatigue. It was clear that de Havilland had gone to great lengths to test the assumptions that underpinned their fuselage design methods by subjecting a 26 foot front fuselage section to just about any strength and durability trial they could think of. They had sealed the aft end and ‘inﬂated’ the cabin to twice the pressure it would experience at its cruising altitude – let us call this 2P (assuming a cruising altitude of 40 000 feet, P = 8.25 psi). This had been followed by 20 pressurization cycles to between P and 2P and 2000 additional cycles to the expected cruise pressure differential P and back. At this point they had cleared the Comet to ﬂy, but carried on punishing the still intact test fuselage. It had eventually suffered a catastrophic fatigue failure after 16 000 simulated climbs to its cruising altitude. While today’s jet airliners are still considered to be in their prime at that age, in the 1950s no one dreamt of a pressurized jet airliner exceeding a service life of 10 000 take-offs. At the rate BOAC were using them, this would have meant roughly ten years. No wonder then that the engineers at de Havilland were not losing any sleep over the fatigue life of the Comet cabin. Here is the enigma Arnold Hall was faced with. The test section failing after 16 000 cycles did not necessarily mean that the aircraft had a fatigue life of 16 000 take-offs. After all, many considered the ‘laboratory’ conditions of the tests more benign than the real ﬂights typical of everyday operations, with hot, vibrating engines, turbulence encounters and rough landings. Moreover, manufacturing differences could be expected between aircraft, which could have caused some variability in terms of their fatigue life. While no one could quantify what this all meant in terms of the actual service life to be expected, surely the experimentally obtained value could not possibly be off by a factor of well over ten? G-ALYP exploded after 1290 cycles and G-ALYY was even younger at the time of its demise (a mere 900 cycles old) – even assuming that de Havilland’s design calculations had been wrong, what could possibly account for such a huge discrepancy? A major step the Royal Aircraft Establishment took towards clarifying this was to repeat de Havilland’s tests – this time on a complete aircraft (1230 cycles old G-ALYU), just in case the front fuselage section used in the original tests was not fully representative of the airframe. A rig was set up to vibrate the wings at a rate of 25 times per pressurization cycle – this was also intended to make the experiment more realistic by simulating the effect of gust encounters. Instead of pumping air into the cabin, for every simulated cycle they ﬁlled it with water up to the cruise altitude cabin pressure value P. The rationale behind using water is that it practically does not compress under pressure and therefore a potential fatigue failure will simply take the form of a small tear and a leak near the crack that started it, as opposed to the explosive failures typical of highly compressible air, which leave little evidence of the location of the original crack. Pumping water into the cabin until the pressure gauge indicated 8.25 psi, vibrating the wings 25 times and draining the water took about ﬁve minutes [61], allowing the scientists to compress the equivalent of hundreds of ﬂights into each day. The test protocol also included the replication of a scheduled proof test BOAC had conducted on their Comets once every thousand ﬂights,

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which simply involved a simulated cycle going 30% above the normal cruise pressure differential of 8.25 psi. To the consternation of all involved, on only the third such proof test of its lifetime, the cabin of G-ALYU gave way on the morning of 24 June after a combined total of just over 3000 ﬂights (of which the last 1830 were in the water tank). The location of the failure became obvious when the tank was drained: there was a long crack in the fuselage near the forward escape hatch. Another incipient crack was found at the top corner of one of the windows and the metallurgists agreed that both of these were fatigue cracks. This was a shocking result and de Havilland could take little comfort in the (today largely forgotten) fact that the fatigue failure occurred at a simulated altitude the Comet cabins only ever experienced in ground tests. The disturbing and now undeniable fact was that never again would a Comet 1 be allowed to return to airline service.

2P test + 20 Pï2P tests + 16 000 simulated cycles Cleared to fly after 2000 simulated cycles

Test section

GïALYP

Crashes at Elba after 1290 flights

GïALYY

Crashes at Naples after 900 flights

1230 flights + 1830 simulated cycles

GïALYU

0

5000 10 000 Pressurization cycles

15 000

Fig. 3.5 Fatigue cycles to failure – a comparison of four Comet 1s.

The outcome of the water tank experiment, compelling as it was, still did not allow Hall to draw a line under the Comet riddle. He still needed direct evidence that the two crashed aircraft disintegrated in the same way as G-ALYU did in the water tank. It was not long before he got it. It was one of thousands of corroding, foul smelling pieces of G-ALYP’s wreckage salvaged by the Royal Navy: the heap of metal described at the beginning of this chapter. The Comet had a pair of antennae on its roof, designed to pick up the signals emitted by ground-based radio navigational transmitters (so-called non-directional beacons). This signal was then converted by the aircraft’s automatic direction ﬁnder (an electronic device) into the deﬂection of a needle on a compass-like cockpit instrument, such that this pointed towards the beacon (while largely obsolete, the system is still used occasionally by pilots of light aircraft). The antennae had to be placed outside the Comet’s aluminium cabin,

3.7 Consternation

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which would have shielded them from the signal, but they could not protrude too far out, as they would have increased drag – something the fuel-thirsty and slightly under-powered Comet could ill-afford. They were therefore mounted in two cutouts in the aluminium skin, covered with ﬁbreglass lids (which are more transparent to radio waves), almost ﬂush with the surface of the skin. The crucial piece of the wreckage, which now lives in London’s Science Museum, was the section of the cabin roof containing the two cutouts. Though torn apart like the rest of the fuselage, it could be, jigsaw puzzle-like, pieced back together to reveal two tell-tale cracks that started near the the edges of the cutouts. The Royal Aircraft Establishment’s metallurgists identiﬁed one of them, showing a classic fatigue failure pattern, as the most likely origin of the fatal tear that ruptured G-ALYP’s cabin (see Figure 3.7). The complete history of the crack could not be determined, but here is the Hall enquiry’s best guess. It started during the manufacturing process. It was subsequently identiﬁed and, this much can be seen on the recovered sheet, a hole was drilled into its path to stop it from spreading. This was a fairly standard and usually effective practice at the time, but it appears to have failed on this occasion. The crack seems to have continued and, upon reaching a critical length, just as G-ALYP’s cabin was beginning to ‘inﬂate’ during the climb from Rome, it began to spread at an enormous speed with the results we have already seen. The scenario of the crack having started during the manufacturing process and halted later, temporarily, by the hole, may or may not be correct, but this is largely irrelevant. Fatigue cracks can also start at microscopic impurities that exist in any metal and spread at a speed determined by the intensity (amplitude) of the cyclic stress in the material. If the stress peaks are relatively high, the crack will grow quickly and if it is low, as we have seen earlier, the crack will either grow slowly (in aluminium) or not at all (in a ferrous material). The Comet’s aluminium fuselage was engineered following a philosophy now usually referred to as safe life design. This means that the the skin was designed to experience stress levels that are so low that even if a crack were to start early in the life of the aircraft, it would never have reached the critical length, the point where it would have turned into a rapid, catastrophic tear through the just under 1 mm thick aluminium sheet. This philosophy, however, is predicated on an accurate estimate of the stress levels in the skin and this is where de Havilland’s calculations were found to be wrong. There were densely riveted areas close to the edges of the antenna cutouts and some of the windows and escape hatches, where the stress values had been underestimated. This is probably a good time to clear up a persistent myth, which can be found in countless books on the subject. This theory aims to boil down the solution of the Comet riddle to the stress-concentrating effect of the supposedly sharp corners of the airliner’s windows. While it is true that making a cutout with sharp corners in a pressure vessel, such as an airliner cabin, will amplify stresses around its corners, the Comet’s windows had been designed with rather generous ﬁllets – in fact the radii of these corner ﬁllets were not smaller than many found on current aircraft. In other words, the shape of the windows did not make the Comet unsafe. However, underestimating the combined stress-enhancing effect of the dense riveting (each rivet hole introducing its own stress concentration) and the presence of a window (which could have been of almost any shape) did. Presumably the myth owes its existence, at least in part, to de Havilland having used elliptical and

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3. A tale of two Comets

Fig. 3.6 A number of diagrams were drawn onto massive pieces of canvas for the public inquiry into the loss of the two Comets. This one shows the pieces of the wreckage of G-ALYP that have been recovered from the sea near Elba (in grey) – a label indicates the location where the initial crack is believed to have started: in the aft starboard corner of the automatic direction ﬁnder antenna cutout. Image published with the permission of the Science Museum, London.

circular windows on later marks of the Comet, even though it is the inclusion of reinforcing plates, the local thickening of the skin and the re-design of the riveting pattern that actually increased the safe life of these. In fact, it is probable that merely changing the shape of the Comet 1’s windows to the ellipse seen on the revised Mark 1XB model would actually have increased the stresses associated with the presence of the cutout! Of course, while this all makes theoretical sense, it still leaves one major question unexplained. Why did de Havilland’s test fuselage ‘live happily’ through 16 000 cycles before failing? The answer is, once again, rather inconveniently multi-faceted. Firstly, as we have already seen, a fair degree of variability can be expected from one fuselage to the next. No two airframes were made of exactly identical materials, no two workers used exactly identical riveting techniques and, according to some accounts, there even were differences between the precise layout of the riveting around the cutouts of different airframes – as a result, the test section could simply have happened to be slightly stronger. Secondly, de Havilland began their ‘torture’ of the test section by pressurizing it once to a massive 16.5 psi – twice the maximum stress it was expected to carry in service. This, somewhat counter-intuitively, appears to have had a strengthening effect on the test section, a phenomenon that is relatively well known now, but it had not been heard of at the time when the Comet 1 was designed. A third reason could have been that the original experiment

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Fig. 3.7 The only remaining piece of the wreckage of G-ALYP (aft part to the right), now looked after by the Science Museum in London. The initial crack is believed to have started in the location indicated by the arrow; detailed examination of the broken skin (inset) shows tell-tale signs of fatigue. Image published with the permission of the Science Museum, London.

Fig. 3.8 Comet 1 automatic direction ﬁnder window, as seen from the inside.

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3. A tale of two Comets

was a less than perfect simulation of reality, mainly through not accounting for gusts and only testing the front section of the aircraft. Our brief account of the investigation conducted by Hall’s team merely hints at their thoroughness in exploring all conceivable chains of events. Thousands of pages’ worth of technical reports were presented to the subsequent Public Inquiry, detailing countless experiments. These included many hours of ﬂight tests with Comets ﬁtted with strain gauges (electronic probes used for measuring strains and thus inferring stresses in a structure), experiments on scale models of the Comet used to simulate the fall of the disintegrated aircraft, explosions simulated on scale perspex models of the cabin, allowing the investigators to ascertain that the movements of the small crash test dummies therein were consistent with the injuries seen by the Pisa coroner on the victims of the Elba crash, fuel system tests on full scale aircraft with radioactive markers enabling the tracing of all fuel leaks (recall that at ﬁrst ﬁre was seen as the most likely cause of the crashes), and so on. The result: not only was a credible chain of events pieced together to explain the Comet crashes, but a standard was set for all subsequent air accident investigations. In terms of design practices, the Comet disasters have had a lasting impact. Modern fatigue testing is much more realistic than de Havilland’s initial experiments with the front fuselage section. Materials have improved and their properties are much more consistent, as are the manufacturing processes. The riveting in high stress areas is designed with much more care. Crack inspection processes are much more advanced. Most importantly though, the industry has taken a key step beyond the ‘safe life’ approach with what is termed a failsafe design philosophy, where no single fatigue crack can lead to catastrophic decompression. Fail-safe design usually means designing structures with multiple load paths, which maintain high strength even in the the presence of a crack or damage. The structure of fuselage skins includes tear-stoppers, usually based on a cellular layout, where a tear in a cell will only damage that cell, without spreading, Comet-like, across the fuselage13 . At the same time, it cannot be said that the pressure cabins of today’s airliners are fundamentally different from that of the Comet. Ultimately, passengers traveling on most current airliners are still only separated from the freezing, low pressure air of the stratosphere, rushing by at close to the speed of sound, by a sheet of aluminium, just over a millimeter thick. The overall structural design philosophy has not changed much either. This indicates two things. Firstly, what de Havilland created in the early 50s was a wonderful, game-changing, pioneering piece of engineering, marred by a small, yet fatal misjudge13

Metal fatigue yielded another famous ‘tale of the unexpected’ three decades after the Comet disasters, which appeared, at ﬁrst sight, to suggest shortcomings in the implementation of the fail-safe philosophy. This was one of the great white knuckle rides of the history of aviation: the infamous Aloha 243 incident, when explosive decompression tore off the top half of the front third of the fuselage of a Boeing 737-200 on a short hop between two Hawaiian islands. Boeing had demonstrated as part of the certiﬁcation process of the 737-200 that fatigue failures would merely lead to a ‘ﬂap’ tearing open, leading to controlled decompression. This was, however, an ancient aircraft, which had operated nearly 90 000 ﬂights (far more than its ‘economic design life’ of 75 000 cycles) in a highly corrosive environment. The National Transportation Safety Board concluded that this resulted in multiple fatigue failure sites, which, of course, reduces severely the effectiveness of the fail-safe philosophy. There is an alternative view too, acknowledged by the Board: some believe that initially only a ﬂap opened, as designed, but the decompression sucked the body of one of the ﬂight attendants into the opening and this impact lead to the rapid spreading of the failure [93].

3.8 Epilogue

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ment. Secondly, conveying passengers through the stratosphere, between distant locations and in reasonable comfort is a very delicate balancing act, where potential disaster is only a small step away from a safe design. Thankfully, today we have a fairly reliable idea of what this small step is.

3.8 Epilogue On 13 July 2009, a Southwest Airlines Boeing 737-300 was approaching its 35 000 feet cruising altitude near Charleston, West Virginia, when the passengers and the crew heard a loud ‘pop’, followed by a ‘whooshing’ sound, as the aircraft depressurized. The only indications, at that time, of what had happened was a deformed ceiling panel near the aft end of the cabin, around rows 20-21, and the wind noise, which was loud enough that conversation between passengers required shouting. Here is how an off-duty airline Captain, traveling in the cabin as a passenger, remembers the event: “This was not a Hollywoodtype depressurization event. Ear trauma was minimal. No items ﬂew around the cabin. No one was sucked towards the ceiling of the plane. In fact, I noticed that there was not even a hint of an unusual breeze or temperature change within the cabin. Items (including paper) on seat back trays were not disturbed at all. I found no trouble breathing at any time, with or without the oxygen mask [these had deployed moments after the depressurization, a.n.] (I did utilize the mask all-the-same).”14 The ﬂight crew declared an emergency and diverted to Charleston, where they landed uneventfully (the ﬂight had originally been en route from Nashville, Tennessee to Baltimore, Maryland). What had happened became clear upon landing, as the deformed ceiling panel fell off to reveal a view of the blue sky, through a small ﬂap, where the fuselage had evidently cracked open (see Figure 3.9). As the National Transportation Safety Board investigation was to reveal, a fatigue crack had developed in the 0.9mm thick outer sheet of the skin of the 737. As the pressure differential rose on the 13 July ﬂight the crack reached a critical length and lead to a tear, which immediately stopped, as designed, at the edge of its cell on the wafﬂe-patterned skin. If there ever was an aircraft incident with an ultimately positive message, this must be it, both from the point of view of the crew having handled the event in a textbook fashion, and from an aircraft design perspective. A crack, similar to those that brought down the two Comets in 1950s, was, on this occasion, essentially, a non-event. The learning curve of stratospheric aircraft cabin design is probably not at an end yet, but it has become become much, much shallower. To a large extent we have the pioneering work of de Havilland and the Hall investigation to thank for that.

14

NTSB Docket ID DCA09FA065.

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Fig. 3.9 The three-sided ‘ﬂap’ on the roof of the 737 (a) and the fuselage skin structure in the area surrounding the failed ‘tile’, featuring a continuous outside skin and a chemically milled doubler on the inside. Image courtesy of the NTSB.

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“Airmanship is a combination of characteristics. It is hard to describe precisely; it is one of those things that you ‘know when you see it.’ It involves at least one part pilot skill, one part judgement, one part knowledge, and one part experience.” Paul A. Craig ‘The Killing Zone’ [42]

4.1 The ‘410’ club Jesse Rhodes’ career was going well. Aged only 31, he was already an airline Captain, ﬂying Canadair CRJ-200 jets for regional carrier Pinnacle Airlines. Most people who had shared a cockpit with him regarded him as a talented pilot and an easy man to get along with – one ﬁrst ofﬁcer even went as far as saying that Rhodes was ‘the best stick and rudder man’ he had ever ﬂown with. His previous employer had once sent him a letter of commendation acknowledging his competent handling of an in-ﬂight emergency1 . The evening of 14 October 2004 found him ﬂying, as a passenger on a Pinnacle Airlines jet, from his home base, Detroit Wayne County Metropolitan Airport to Little Rock International Airport in Arkansas. ‘Dead-heading’ (airline jargon for traveling on one of one’s own company’s ﬂights as a non-revenue passenger) with him was 23-year old First Ofﬁcer Peter Cesarz, a young man aspiring for a similar career in aviation. The two had never met before, but they did have something in common: they had both worked for the same airline before joining Pinnacle and they had ﬂown the same type of aircraft there. Having both been on standby duty that day, they had received instructions in the afternoon to ﬂy to Little Rock, where their actual ﬂying duty would begin. One of Pinnacle’s CRJ-200s had earlier that day had to abandon a take-off attempt at Little Rock due to a sensor failure in 1

The details regarding the pilots, as well as the reconstruction of the circumstances and the events of 14 October 2004, as described here, are based on [96].

A. Sóbester, Stratospheric Flight: Aeronautics at the Limit, Springer Praxis Books, DOI 10.1007/978-1-4419-9458-5_4, © Springer Science+Business Media, LLC 2011

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one of the engines, and, although in the meantime the technicians had replaced the faulty part, the aircraft was now at the wrong airport for the following day’s ﬂying schedule. It was needed the next morning for a ﬂight out of Minneapolis and Rhodes and Cesarz’s task for that evening was to ﬂy the empty Canadair jet to the Midwestern city. While airlines dislike having to resort to such repositioning ﬂights (a lot of fuel is burnt, yet no tickets are sold), pilots tend to like them, mainly for the simple reason that not having to worry about passengers makes for a more relaxed ﬂight. There is a further, more mischievous reason though and this was probably beginning to feature quite prominently in the thoughts of the two young pilots as, upon landing in Little Rock, they exchanged cheerful greetings with the gate staff and headed towards their aircraft. This second attraction of a repositioning ﬂight is that the performance of a jet designed for carrying 50 passengers and their luggage, food, water, etc. can be quite ‘sporty’ when only carrying the two pilots instead of its usual payload. This is quite an exciting prospect for a thrillseeking pilot, especially on an aircraft like the CRJ-200. This, the smallest of the Canadair (now Bombardier) family of regional airliners, could best be described as looking like ﬂying even when sitting on the tarmac. It is hard to tell whether the sleek, slender fuselage, the comparatively large tail-mounted engines, the long, pointed nose or the aggressivelooking ground stance are the chief culprits for this illusion, but few would dispute that the CRJ looks like a speed machine (Figure 4.1). It was clear from the start of the take-off roll that having this multi-million dollar toy to themselves for the evening excited Rhodes and Cesarz. The Little Rock controller cleared

Fig. 4.1 Elegant and agressive – the ‘sporty’ Canadair (Bombardier) CRJ-200.

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Flagship 3701 (the radio callsign allocated to their ﬂight) for take-off at 21:20 and the jet blasted off into the chilly, clear night with ﬁghter-like urgency. Within seconds, unusually for a passenger airliner departure, the two men were pressed into their seats by nearly twice their body weight as the two General Electric turbofan engines accelerated the CRJ upwards at a rate of climb exceeding 3000 feet per minute. In fact, in an attempt to maximize the climb performance of the airliner Rhodes and his young co-pilot were pulling back on the control columns so aggressively that the angle at which the wings were meeting the oncoming airﬂow was above the limit that the software running on the on-board ﬂight management computer was designed to tolerate and, sensing the impending complete loss of lift force on the wings, it activated the stick shaker. This is a system that induces vibrations in the control column to alert the pilot that the approved performance limits are close and wing stall may follow (the physics of how a wing generates the lifting force that enables ﬂight and how the phenomenon of stall can disrupt this is rather interesting and we shall return to it later). A few seconds later the computer switched to the second line of defence: it pushed the control columns forward, allowing the nose of the aircraft to drop slightly, throwing Rhodes and Cesarz into near-weightlessness for a moment as a result of the sudden break in the vigourous ascent. After leveling off brieﬂy at 15 000 feet, the two pilots were up for some more rollercoaster action. They put the jet into yet another steep climb, gaining another ten thousand feet in just over three minutes, the ‘seat of their pants’ brieﬂy feeling almost two and a half times their body weights as they were powering towards their allocated cruising altitude of 33 000 feet. Had there been any passengers in the cabin behind them, they would have been struggling to hold onto their coffee cups (not to mention to hold down their dinners!). Rhodes and Cesarz were mesmerized by the CRJ’s instruments. They were looking at rather impressive vertical speed readings, yet the lightly loaded engines were getting through the three and a half tonnes of fuel on board with economy they had previously considered unthinkable. ‘Dude, I have seen this thing eat up like four thousand pounds an hour...’, said the ﬁrst ofﬁcer as they were both ﬁxated on the ﬂow meters displaying a fuel consumption of less than a thousand pounds per hour on each engine. As exciting as all this may have been though, on this evening of personal bests an even more desirable trophy still beckoned: membership of the ‘410’ club. This was the ‘underground’ clique of Pinnacle Airlines CRJ-200 pilots who have taken the twin jet to its maximum operating altitude of 41 000 feet (Flight Level 4102 ). The cruise altitude the airline had planned for ﬂight 3701 that night was eight thousand feet below this target, but the crew decided not to allow the difference to stand between them and the kudos of having taken a CRJ as deep into the stratosphere as the manuals permitted. ‘Dude, we can do it. Forty-one it’, said Cesarz.

2

Technically, Flight Level 410 is the altitude at which an altimeter set to the ISA reference pressure value of 1013.25 mbar (29.29 inHg) reads 41 000 feet. The concept of a ‘Flight Level’ reﬂects the aeronautical convention that all altitudes above a certain transition value (which varies from country to country, but is generally less than 20 000 feet) are understood to be measured in terms of the standard pressure, as opposed to the actual sea level pressure value measured at that point (which, of course, must be used on landing if the altimeter is to indicate the exact elevation of the airport at the moment of touchdown).

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And indeed, a few minutes later, just over half an hour after departing Little Rock, a triumphant Captain Rhodes made his next routine call to the Kansas City en-route air trafﬁc control center from 41 000 feet. ‘Kansas City center, good evening, Flagship 3701, 410.’ The controller acknowledged the call and Rhodes turned to his equally excited co-pilot: ‘You’ll do the next one. To say ‘410’. Want anything to drink?’ Half a minute later they were both holding cans of Pepsi, which Rhodes had fetched from the galley, and their eyes were ﬁxated on the ticker-tape style altimeter on the edge of the digital screen, displaying the coveted ‘410’. However, for the ﬁrst time that evening, they were experiencing the slightly unsettling sensation that all was not going quite as expected. The jet was in a rather pronounced nose high attitude and it was decelerating. The wings, working near the limits of their lift generating capability to keep the jet in level ﬂight in the thin air of Flight Level 410, were far above their optimum angle relative to the oncoming airﬂow and were therefore generating slightly more drag than the engines were capable of canceling with their thrust. Still, there was one more opportunity for basking in the glow of their achievement as the Kansas City controller called again. ‘Flagship 3701, are you an RJ-200?’ ‘3701, that’s afﬁrmative’, Rhodes responded. ‘I’ve never seen you guys up at 41 there.’ ‘Yeah, we’re actually... ah... We don’t have any passengers on board so we decided to have a little fun and come on up here.’ ‘I gotcha.’ ‘This is actually our service ceiling’, the captain added. By then it was, however, becoming clear that the wings were losing their battle to hold the jet at Flight Level 410 at their dangerously reducing speed and if a stall was to be avoided, the two-man crew had to lower the nose to trade some of their altitude for extra speed. A slightly more humble sounding Rhodes called the en-route controller for permission to descend. ‘Yeah, just as you said, it looks like we’re not even going to be able to stay up here, look for maybe ... ah...390 or 37...’ He had barely ﬁnished the sentence when the stick shaker activated and the autopilot suddenly disconnected. The triumph of having just joined the ‘410’ club evaporated in an instant. The two pilots were struggling to make out the controller’s response as, amidst the accompaniment of a range of warning horns, beeps and warblers the wings ﬁnally gave up the struggle with the thin air and stalled. The aircraft began to sink with unpleasant rapidity, like a roller-coaster car that has just passed the apex of the track and is beginning to pull its passengers into the drop with all the discomfort of ‘that falling feeling’. At the same time the jet began a roll to the left, which continued until the wings were at almost right angles to the horizon. First Ofﬁcer Cesarz instinctively pulled back hard on the control column, trying to arrest the dive. A few moments later, even more worryingly, the engine oil pressure aural warning began to sound, alternating with the master caution alert, which had also joined in the general pandemonium. A wing stall, especially one that happens at such a high altitude, is recoverable, generally with some altitude sacriﬁce – in fact, this is an exercise any trainee pilot will have practiced countless times as a student. However, the oil pressure warning signaled something much more serious.

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‘We don’t have any engines!’ Apart from a few expletives and a terse emergency call to Kansas City, these were the ﬁrst words spoken in the cockpit since the crisis began. There was little doubt about the complete loss of engine power as most of the display screens suddenly went blank, all the lights went out and the reassuring hum of the engines was replaced by the unfamiliar whirr of the air driven generator. This is a small, windmill-like device, which is normally stowed inside the fuselage and it is deployed into the airstream in such emergencies. Its role is to provide enough electrical power to run a few key instruments, such as the altimeter, which Cesarz was now concentrating on as he was trying to level off after the vertiginous drop from the height that had caused them all that exhilaration moments earlier. Meanwhile, Rhodes, after groping around for a while in the back of the dark cockpit for a ﬂashlight, was ready to go through the ‘double engine failure’ checklist, which he had located in the aircraft’s operating handbook. ‘Okay. Continuous ignition on. Thrust levers shut off....’ The cabin altitude warning cut in, indicating that the air conditioning system, starved of its usual supply of warm air from the engines, was unable to maintain cabin pressure and the equivalent altitude inside the aircraft was now above ten thousand feet and rising. Rhodes called Kansas City control again. ‘Yeah, we’re still descending, we’re gonna need to descend down, ah... to about 13 000 feet, is that okay?’ ‘Flagship 3701, afﬁrmative, descend and maintain 13 000 feet.’ Rhodes continued reading the engine restart procedure. ‘Airspeed not less than three hundred knots... No, we’re not getting any N2 at all.’ N2 is the rotation speed of the core of the engine, that is, the speed of the shaft that the compressor and the turbine are connected to. The core not rotating was disturbing news, as, by now, the air rushing through the compressor should have spun it up sufﬁciently for a windmilling restart. The controller came back on, enquiring about the nature of the emergency. ‘We had an engine failure up there at altitude’, Rhodes responded, ‘...ah...airplane went into a stall and one of our engine’s failure [sic]... so we’re gonna descend down now to start our other engine.’ Of course, this halting reply contained only half of the much more alarming truth. Behind Rhodes’ hesitation, apart from the stress of the moment, was that the CRJ had, in fact, been reduced to a glider – worse still, a glider that had now lost almost 30 000 feet of altitude since the engines had stopped. Having ‘dropped out’ of the thin air of the stratosphere did have one advantage though. High pressure air generated by the aeroplane’s auxiliary power unit (a small gas turbine engine mounted inside the tail of the aircraft) could now be used to attempt a restart – this is similar to the method that would be used on a normal start-up, on the ground. Rhodes quickly briefed Cesarz (who had been ﬂying the aeroplane since the start of the incident) on the engine restart plan, though communication was now a little difﬁcult as they both had to shout through the intercoms inside their oxygen masks, which they had had to put on because of the loss of cabin pressurization. ‘You want a hundred and seventy knots... [...] You with me on this? You clear? You clear? All right, we’re gonna get this going. Don’t worry bro. All right? You okay? Seriously?’

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In spite of the earlier conﬁdent messages from ﬂight 3701, the controllers at Kansas City center were now getting concerned about the rapidly reducing altitude displayed on their radar screens underneath the triangular symbol representing the Pinnacle Airlines jet. Planning an emergency landing is up to the captain, but they could, at least, keep him informed about his options. ‘ [You] have a lot of choices up ahead. Columbia’s right up ahead. JEF [Jefferson City, Missouri] is up ahead. And they’re best to accommodate you.’ ‘Roger, 3701, thank you’, Rhodes acknowledged the call. Another two precious minutes and three thousand feet of altitude went by as the pilots were engaged in a futile attempt to relight the engines using the auxiliary power unit. With the instruments now indicating a mere 10 000 feet of altitude, Rhodes ﬁnally decided it was time to come clean to the controller about the extent of their problems. ‘Tell her’, he said to Cesarz, ‘we need to get direct to airport, any airport, neither engine is started right now’. Cesarz relayed the information to the controller as instructed. They were immediately given instructions for an approach into Jefferson City, now about ten miles away. With yet another failed restart attempt and the altimeter needle now falling through 8000 feet, this was suddenly looking desperately tight. ‘Dude, we’re not gonna make this...’ There was just enough time for one more desperate call to air trafﬁc control. ‘We’re not gonna make the runway. Is there a road?’ At almost exactly a quarter past ten, just under an hour after Flight 3701 had taken off from Little Rock, it disappeared off the radar screens. Ten seconds later, two and a half miles to the South of Jefferson City Memorial Airport the calm evening of a residential neighborhood was disturbed by a series of loud blasts as the CRJ tore through a line of trees, slid to a halt behind a row of bungalows and exploded. It is possible that a last minute decision by the pilots not to lower the landing gears delayed the impact for long enough to avoid a direct hit on a number of houses, thus saving many lives. In fact, almost incredibly for an impact in a densely populated area, Jesse Rhodes and Peter Cesarz were the only victims of the accident. The National Transportation Safety Board (NTSB) enquiry, launched immediately after the crash, had a rather difﬁcult task ahead. The cockpit voice recorder and the ﬂight data recorder were recovered and they did provide a number of answers, but raised new questions too. What caused the double engine ﬂame-out? After all, the control and fuel systems of the two engines being almost entirely independent, this is an exceptionally rare occurrence. Even more unusually, why did all the restart attempts fail? Why did the cores not windmill as the air was ﬂowing through the engines of the rapidly descending CRJ? The unusual behavior of the ﬂight crew was also bafﬂing. There was no doubt that they had demonstrated lack of good airmanship by climbing to Flight Level 410 purely for their own entertainment, but, even after losing all engine power and sinking a few thousand feet in the stall, their altitude was still enough to place several airports within easy gliding range – so how come they still fell short of their chosen emergency landing site? Perhaps even more remarkably, why did they not tell air trafﬁc control that they had no power at all until it was too late?

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Cesarz’s stall recovery attempts came under scrutiny too. Most pilots will instinctively allow (or, if necessary, force) the nose of a stalling aircraft to drop in order to reduce the angle of incidence of the oncoming airﬂow, increase speed and thus restore lift on the wings – yet, the analysis of the ﬂight data recorder revealed that the young ﬁrst ofﬁcer did the exact opposite on each occasion. Finally, there was a more technical riddle. Leaving aside the lack of justiﬁcation for the high-climbing stunt, the CRJ had been designed to be capable of ﬂying safely at 41 000 feet – why did it stall then, in spite of its light payload? It took the NTSB the best part of two years to unravel these mysteries and establish the probable cause of the accident – or, rather, as it is almost always the case in a safetyconscious industry such as air transportation, the highly improbable combination of even separately improbable events that caused the loss of Flight 3701. Armed with the ﬁndings of this enquiry today we are in the position to reconstruct these events. Before we do that though, we need to take a closer look at some of the technical issues involved.

4.2 Into thin air The main role of an aircraft wing is to generate lift. This is a force perpendicular to the direction of the airﬂow (relative wind) encountered by the wing3 and it is meant to keep the aircraft on its desired trajectory (whether this is straight and level, turning, climbing, etc.) by counter-balancing other forces (for example the weight of the aircraft in level ﬂight). Considering that the aircraft ﬂies through an air mass of uniform pressure (this is the ambient pressure in that point in time and space), the wing achieves this by creating a distortion in this uniform ﬁeld. Lift is generated if the distortion is such that the pressure above the wing is lower than the pressure below it. Clearly, the position and the shape of the wing determine whether this pressure difference occurs. The airfoil – a section through the wing, lined up with the airﬂow – either has to meet the oncoming airstream at an angle (nose up) – the angle of attack – or it has to be cambered (curved). Usually a combination of a small angle of attack and a slight camber ensures the most efﬁcient cruising ﬂight (efﬁcient here meaning that the wing generates the least amount of drag in cruise, while producing an amount of lift equal to the weight of the aircraft)4 . So what is a ‘small’ angle of attack? A passenger jet, such as the CRJ-200, cruises at around 2 to 10 degrees, depending on altitude, weight and speed. The amount of lift generated by the wing increases proportionally with the angle of attack up to a threshold value, where the rate of increase reduces brieﬂy, before the lift falls away to zero, that is, 3

Technically, both an aircraft in ﬂight and the air mass it ﬂies in, are, usually, in motion. However, it is sometimes easier to create a mental image of the aircraft as being ﬁxed and the air ﬂowing over it, as if it were in a wind tunnel – this generally makes the description of various aerodynamic phenomena both conceptually and mathematically more straightforward. Hence, when we talk of relative wind, we mean the airﬂow as seen by an observer attached to the aircraft. 4 A staggering amount of research has been devoted to the question of how to design the most efﬁcient airfoil for a given aircraft – in fact, it is one of the fundamental questions of aeronautics and it was the subject of scientiﬁc debate even before the Wright brothers’ ﬁrst ﬂight.

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the wing stalls. The physics of the viscous ﬂow in the boundary layer around a wing, the mechanisms of its transition from the laminar to the turbulent regime and the onset of its separation from the upper surface at high angles of attack (which causes stall) is covered in any standard text on aerodynamics – here we shall merely consider what is ‘special’ about the stratosphere in terms of stalling. It all comes down once again to the low ambient pressure and thus the low density of the air. The amount of lift generated by a wing is proportional with the square of the airspeed, the density of the air, the surface area of the wing and, for most ‘normal’ ﬂight conditions, the angle of attack5 . Now consider a scenario that became the central theme of the NTSB investigation into the loss of Flight 3701. The wing of a given aircraft is required to generate the lift needed to maintain a speciﬁed rate of climb. As the altitude increases during the climb and thus the density decreases, with the wing surface area ﬁxed the remaining means of maintaining this rate of climb are either to increase the angle of attack to compensate for the falling density – provided we stay below its stall value, of course – or to increase the airspeed. The latter is clearly limited by the maximum thrust the engines are capable of – which, again due to the decrease in density and the decrease in temperature, reduces with altitude – so once the throttles hit the ‘fully open’ stops, we are only left with the angle of attack as a variable to play with – up to the critical value where stall occurs. Some aircraft (low speed trainers, in particular) stall in a rather benign way: as lift is lost, the nose tends to drop, which has two consequences. First, the angle of attack falls way below its stall value. Second, the aircraft accelerates and lift increases with the square of the speed. Both phenomena will therefore tend to restore normality – prompt return to normal ﬂight is usually possible with some loss of height. It is worth noting here that stall has killed many pilots even in such benign aircraft, mainly when that lost height would have been more than the altitude they were ﬂying at – usually too slowly and/or at too high an angle of attack. Never be low and slow is therefore the mantra that is hammered incessantly into every student pilot. In other, less benign aircraft stalling can be a more memorable experience. Firstly, the two wings do not necessarily stall at the same time. This means that as one wing nears stall, it starts losing lift and (as another standard by-product of stalling) its drag will increase. As a result, that wing will drop, perhaps excessively (recall the moments after the CRJ of Rhodes and Cesarz stalled at 41 000 feet) and, in particularly unlucky circumstances, the additional drag will tend to ‘hold back’ that wing momentarily, inducing the altogether more sinister phenomenon of spinning. Secondly, not every aircraft is balanced in such a way that its weight distribution will facilitate an automatic recovery, in which case the pilot needs to be much more proactive, pushing the control column forward – this will deﬂect the elevators on the tail downwards and thus lift the tail and lower the nose. Additionally, more power may also be required to force an increase in airspeed, which will expedite the recovery and minimize the height loss. A third aspect of the the stall behavior of an aircraft is the abruptness of the onset of stall. Some aircraft will give plenty of natural warning of the airﬂow over the wing 5

In the classic lift equation the angle of attack is incorporated into a lift coefﬁcient, which lumps together all other factors, most importantly the shape of the wing.

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breaking down. This usually takes the form of a pre-stall buffet, a vibration that can be felt throughout the airframe and on the controls. In other aircraft the only indication of the impending stall is a loss of effectiveness in the controls. In such aircraft the vibrations have to be ‘engineered in’ artiﬁcially – this is done by means of the already mentioned stick shaker. This is an electric motor that vibrates the control column and it is usually accompanied by a loud beeping sound. Its working principle is relatively simple: there are angle of attack sensors mounted on the fuselage and the stick shaker is triggered by the wing angle of attack nearing its stall threshold. We have seen that stall recovery begins on almost any aircraft by a lowering of the nose – should this not happen immediately after the triggering of the stick shaker, most electronic stall prevention systems will make sure it does: a similar electrical mechanism (the stick pusher) will eventually force the control column forward. While the term ‘stall’ is generally used as shorthand for ‘wing stall’ or ‘lifting surface stall’, it is also used sometimes in the context of the airﬂow into the air intakes of gas turbine engines. At high angles of attack the ﬂow can separate over the lower part of the lip of the intake, causing a distortion in the pressure ﬁeld presented to the compressor. In extreme cases this can lead to a breakdown of the airﬂow along the compressor and, as a result, an engine ﬂame-out. On certain types of aircraft wing stall and engine stall are related (beyond the fact that they are both caused by exceeding a critical angle of attack) by virtue of the fact that the engine may ingest some of the low energy, turbulent wake of the stalled wing, thus increasing its own stall-proneness. The reconstruction of the events leading up to the Jefferson City crash began by determining what led to the high altitude stall of the Pinnacle Airlines CRJ-200 – once investigators had a clear picture of this triggering event, they were able to piece together the rest of the tragic story. Here is how the 54 minutes of Flight 3701 unfolded: 21:21 The CRJ-200 is airborne. An extremely aggressive initial climb results in an excessive nose-up attitude, which triggers the stick shaker and soon after, as the aircraft comes dangerously close to stalling, the stick pusher. 21:26 Unusually, and for reasons that are unclear to this day, the two pilots swap seats6 . 21:27 and 21:32 The stick pusher has to intervene two more times to prevent the aircraft from stalling due to the excessively high angle of attack. The CRJ is now passing 24 600 feet. 21:35 Following a request from Captain Rhodes, air trafﬁc control approve a change in the planned cruising altitude to 41 000 feet. 21:48 ‘We can do it. Forty-one it.’, says First Ofﬁcer Cesarz. At approximately this time the crew set the autopilot to maintain a rate of climb of 500 feet per minute, with a target altitude of 41 000 feet. This is the gravest misjudgement of the night – one that will have 6

The cockpit voice recorder runs on a 30 minute loop and therefore it only provides evidence for what happened in the last half hour of the ﬂight. As the impact occurred at 22:15, we have no audio evidence of what happened in the cockpit before 21:45. This leaves two somewhat academic questions unanswered: why did Rhodes and Cesarz swap seats only ﬁve minutes into the ﬂight and whose idea the climb to 41 000 feet was.

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disastrous consequences. According to company manuals the minimum safe speed for high altitude climbs is 250 knots. This is to account for the fact that density of the air is decreasing and therefore lower speeds would demand angles of attack that can come dangerously close to the critical value; it also ensures that the aircraft does not reach a point in its operating envelope where the decaying performance of the engines (also caused by the decreasing density) is not compounded by a potentially perilous loss of momentum. Therefore, the standard procedure is to select the ‘speed hold’ mode of the autopilot with at least 250 knots as the target speed, allowing the computer to determine the maximum rate of climb possible at that speed. By forcing the CRJ into the excessively steep 500 feet per minute climb instead, Rhodes and Cesarz inadvertently slow down the airliner, which thus enters the stratosphere in a dangerously low energy state. 21:51 The jet levels off at 41 000 feet. The crew fail to realize that they are now ﬂying at just over 160 knots, 90 knots below the safe speed. The captain tells air trafﬁc control that they have climbed to Flight Level 410 ‘to have a little fun’. He then brieﬂy leaves his seat (which is now the right-hand seat since the swap) to get a celebratory drink. 21:53 The crew notice that, in order to maintain level ﬂight, the autopilot now has to keep the jet in an extreme nose-high attitude. They fail to realize though that the airspeed is now 150 knots and diminishing. 21:54 The stick shaker and the stick pusher intervene three times in quick succession. Puzzlingly, Cesarz pulls back on the control column each time, instead of lowering the nose to regain some airspeed. Investigators speculate that this could be an attempt to minimize the height loss caused by the near-stall condition. Nevertheless, the result is that the computer’s attempts to avert a stall are thwarted. 21:55 The incorrect recovery technique forces the aircraft into a nose-high attitude, with the angle of attack sensor running out of its measurement range at 27 degrees. Stall is now unavoidable. In addition to the wings losing their ability to generate lift, the extreme angle of attack disrupts the airﬂow into the engines, which begin to slow down. As a result of the stall the nose now drops to 32 degrees below the horizontal. The aircraft also rolls rapidly to the left. Both engines ﬂame out. Their core is very hot after the aggressive climb and the cold stratospheric air now rushes through them unheated. This subjects the turbomachinery to extreme thermal stress. 21:56 The crew recover from the stall, having dropped to 34 000 feet in the process. The fuel ﬂow meters on both engines now indicate zero. 21:57 The emergency generator only provides enough electrical power to run the instruments on the left-hand side, where First Ofﬁcer Cesarz is now sitting. The Captain can only see the instruments if he leans over, which he is unlikely to do, as he is busy attempting to locate the ﬂashlight ﬁrst and then the ‘double engine failure’ checklist. This could be part of the reason why they make the second disastrous oversight of the evening. A minimum speed of 240 knots is required to keep the engines turning while the crew are preparing for the windmilling restart procedure – yet, Cesarz is holding the nose too high and the CRJ is now gliding at less than 180 knots.

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21:58 The airliner is descending through 30 000 feet. If the crew started to plan a gliding descent for an emergency landing at this moment, they would have a choice of six major airports within gliding range. Neither crew member is heard mentioning an emergency landing on the cockpit voice recording. 22:00 Having located the checklist, Rhodes is ready for the windmilling restart attempt, for which they need a minimum speed of 300 knots. In spite of two reminders from the Captain, Cesarz only reaches a maximum of 236 knots. Nonetheless, this new error is now irrelevant – unbeknownst to them, their earlier failure to maintain 240 knots after the ﬂame-out has lead to the cores of the overstressed engines seizing up. This phenomenon, known as core lock, will prevent the cores from rotating, thus precluding any further restart attempts. 22:03 The Captain tells air trafﬁc control that one of their engines had stopped and they are now attempting to restart it. The aircraft is now below 13 000 feet and the auxiliary power unit can be used to attempt to restart the engines. Four consecutive attempts at this fail – as we now know this is due to the cores having locked earlier. 22:06 The controller asks about the crew’s intentions. The Captain still does not mention any landing plans. 22:09 With the core speeds at zero in spite of all the restart attempts, Rhodes ﬁnally instructs Cesarz to tell the controller about the double engine failure – ﬁfteen minutes after both engines ﬂamed out. The ﬁrst ofﬁcer makes the radio call – according to the cockpit voice recording, when he makes this transmission he is back in his normal seat, on the right. 22:10 An emergency landing is now clearly inevitable. The controller gives them instructions for landing at Jefferson City, including navigational information and the local weather conditions. 22:14 Cesarz reports that he has the runway in sight. 22:15 The jet impacts terrain, with the crash forces exceeding 100 times the pilots’ body weights. They are dead by the time the violent post-crash ﬁre engulfs the wreckage.

4.3 Cofﬁn corner The drag versus speed curve of most aircraft exhibits a fairly clearly deﬁned minimum between the descending and ascending arms of a ‘U’ shape – see, for example, Figure 5.1 on page 97. Economical operation generally demands that the cruise is ﬂown near the speed corresponding to the bottom of this ‘drag bucket’. While it is not always practical (or, for a variety of reasons, advisable) to always ﬂy at this speciﬁc speed, it is very important to err on the side of exceeding it, rather than falling short. The reason is the simple principle of speed stability: when ﬂying below this minimum drag speed, if the speed of the aircraft suddenly reduces – say, as a result of encountering a gust – this will push it further up

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the left-hand side of the ‘U’, increasing the drag and therefore further reducing speed. This, in turn, will further increase the drag and ultimately the vicious circle can end in the aircraft slowing to below its stall speed, that is, the speed at which the angle of attack needed to sustain it in level ﬂight would exceed the stall angle. On the other hand, ﬂying too far above the optimum speed – which is a stable strategy, as any speed upsets can merely bring the aircraft back to the optimum speed – has its own danger: exceeding the maximum operating speed or maximum operating Mach number. These are deﬁned for each altitude and they are caused by compressibility effects. As an aircraft approaches the speed of sound, pockets of supersonic ﬂow appear around it – this typically happens at a critical Mach number lower than one (that is, before the speed of sound is actually reached). Exceeding this speed, indicated on the airspeed indicators of high altitude aircraft by a red and white striped needle (hence also known as the barber pole speed), is dangerous because the shock waves appearing around the surfaces of the wings and control surfaces may cause sudden separations in the ﬂow. This leads to instabilities that manifest themselves as a buffeting similar to that encountered just before a stall. The physics of this over-speed buffet is different from the equivalent ‘under-speed’ (stall) phenomenon, but the consequences are similar and can be as severe as structural damage even in seemingly undramatic circumstances. Consider the case of a FedEx McDonnell Douglas DC-10 freighter that landed on 9 April 1997 at Memphis airport after what the crew described as an uneventful, turbulence-free ﬂight from San Juan. Ground personnel noticed that something was wrong with the elevators of the large tri-jet – closer inspection revealed wrinkling on both the upper and lower skins and a small over-stress crack in one of the ribs of the left elevator. The NTSB investigation established that these had to have been caused by buffeting, though the ﬂight data recorder contained no clues as to which type of speed anomaly was to blame7 . In addition to such ﬂow instabilities, older subsonic aircraft also used to feature the terrifying Mach tuck phenomenon, caused by a sudden shift in the position of the center of lift near the critical Mach number – this violent pitch-down effect has now been largely engineered out of modern aircraft. Speed selection is therefore a matter of staying close to the lowest drag point, while avoiding the lower and upper limits determined by the stall speed and the critical Mach number respectively. And herein lies one of the great challenges of high altitude ﬂight. As the altitude increases, so does the stall speed. At the same time the true airspeed corresponding to the critical Mach number tends to decrease with altitude. In other words, as an aircraft climbs to high altitudes, the two limiting conditions gradually close in on it, with the safe window of operating speed getting narrower and narrower. The point where the stall speed coincides with the critical Mach number is referred to as cofﬁn corner and, along with the structural and physiological constraints of cabin design, this is the key factor limiting the safe operating altitude of an aircraft. In fact, in most cases it is necessary to stop a certain safety margin short of the cofﬁn corner to allow for manoeuvring, which, through increasing the load factor of the wings, can also reduce the stall speed (typically a load factor of 1.3g is allowed for).

7

NTSB investigation MIA97LA125.

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87

The perils of high altitude manoeuvering are neatly illustrated by an incident that occurred in 2008 in the skies above Raymond, Pennsylvania. A McDonnell Douglas/Boeing MD-10 freighter was instructed by air trafﬁc control to enter a holding pattern at 33 000 feet. The captain commanded the ﬂight management system to maintain 240 knots, 15 knots above the minimum safe speed for that altitude, but, as the aircraft entered the ﬁrst 90 degree turn of the racetrack pattern, the speed began to bleed away, decaying by about 20 knots. Although near the stall speed now, with the engines at 100% power, the captain did not intervene, expecting the aircraft to accelerate along the straight section of the holding pattern. It did not. In spite of the crew attempting to extend the leading edge slats (in an effort to decrease the stall speed of the aircraft) and commencing a descent, the massive freighter began buffeting, stalled and entered a dive. After losing around 5000 feet of altitude the aircraft ﬁnally regained its safe speed and, following an uneventful approach, it landed at JFK International Airport in New York. As in the case of the DC-10 incident, the stabilizer and the elevators suffered signiﬁcant structural damage as a result of the pre-stall buffet8 . Such incidents serve to further illustrate that high altitudes demand extreme caution, as the width of the safe speed window can be as little as 20 knots between the stall speed and the maximum operating Mach number. In fact, the safe margin can be so narrow that, upon the onset of the buffeting, the Mach number might provide the only clue as to whether too much or too little speed is the cause. As we shall see in Chapter 8, this becomes a particularly pressing issue when high altitude gusts are likely (for instance above high storm clouds), which can easily knock an aircraft past either limit. The cofﬁn corner therefore joins pressure cabin design and the uncertain physiological limits of the human passenger on the list of complex trade-offs upon the very careful balancing of which safe stratospheric ﬂight depends. The rewards of climbing to higher altitudes are, however, enormous and numerous. Next we look at arguably the most important of these, at least from an airline passenger’s point of view. This involves returning brieﬂy to the de Havilland Comet one more time.

4.4 The trans-Atlantic race The stratospheric comfort of the Comet 1 was, to a large extent, compromised on long-haul routes by the need for a tediously large number of stop-overs. The large propeller-driven aircraft of the era had longer non-stop ranges than the Comet, but they were unwieldy, ﬂew at turbulent, low altitudes, slowly, noisily and carried few passengers. Consider, for instance, the enormous Bristol Brabazon. Powered by eight 18-cylinder Bristol Centaurus piston engines, it weighed over 130 tonnes. Cruising at just over 220 knots, it would take over 15 hours to cross the Atlantic. The airlines began to perceive a demand from passengers for faster travel and the only way to achieve that economically was to ﬂy higher. A race to build the ﬁrst high altitude jet with trans-Atlantic range ensued. The European competitors were de Havilland, who evolved the ill-fated Comet 1 into the larger, longerrange, more powerful Comet 4 (Figure 4.2). In the United States Boeing and Douglas were 8

NTSB investigation DCA08FA075.

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developing the Dash-80 and DC-8 airliners respectively. In the end, de Havilland got there ﬁrst, inaugurating the ﬁrst high altitude trans-Atlantic jet service on 4 October 1958, a mere three weeks ahead of the competition. Thus, de Havilland won the battle, but would, ultimately, lose the war. On a visit to Boeing’s plant in Seattle in 1955, Lord Hives, the head of Rolls-Royce, remarked after walking around the Dash-80: “This is the end. The end of British aviation.” [39] The Dash-80, which would subsequently evolve into the legendary 707, was a more economical proposition, mainly because it could carry more passengers.

Fig. 4.2 A British European Airways de Havilland Comet 4 at Stockholm Arlanda airport in December 1967. Image courtesy of Lars S¨oderstr¨om.

We shall delve a little more deeply into the performance aspects of high speed, high altitude ﬂight in Chapter 5 – for now, let us merely consider the attraction of high altitudes in numbers – more speciﬁcally, in fuel burn numbers, for this is where climbing high truly pays off. Figure 4.3 is the fuel burn chart of a typical, 1980s two-engined passenger airliner with a maximum take-off weight of just over 60 tonnes. Clearly, the weight has huge inﬂuence on fuel burn: cruising at a weight of 50 tonnes at an altitude of 35 000 feet the aircraft will burn around 2150 kg of fuel per hour at cruising speed. At a weight of 34 tonnes, the ﬁgure drops to 1600 kg per hour (a good argument, if there ever was one, to keep the weight of passenger luggage to a minimum!). What is even more striking, however, is the impact of altitude changes on these cruise ﬁgures. Descend a mere 3000 feet or so at a 50 tonne weight and the fuel burn rises by about 120 kg per hour in the cruise. The holding (ﬂying at the minimum fuel burn speed) ﬁgures are worth considering too: at a weight of 36 tonnes (the lowest curve on the left hand side of the chart), at 30 000 feet

4.4 The trans-Atlantic race

89

about 1600 kg of fuel will be burnt in a holding pattern. While holding at 5000 feet instead, a pilot of this aircraft would have to expect to get through 1900 kg of fuel per hour. It is also clear from Figure 4.3 that for every weight there is an optimum altitude (from the fuel burn point of view) and the lower the weight, the higher the optimum altitude. Consider the 62 tonne curve – the optimum altitude is around 32 000 feet. By the time 12 tonnes of fuel are burnt off, the optimum altitude approaches 37 000 feet. On a long cruise the best fuel burn strategy is therefore a steady, gentle climb. Unfortunately, the air trafﬁc management system can rarely accommodate such ﬂight paths (we shall see a rather remarkable exception in the next chapter), so the cruise climb is usually broken down into several steps – as in the example altitude proﬁle shown in Figure 4.4.

2800 62t 62t 2600

60t

60t 58t

Total fuel burn [ kg / hour ]

58t

56t

56t

2400

54t

54t

52t 2200

52t

50t 50t

48t

48t

46t

2000

46t

44t

44t

42t 40t 38t 36t

1800

42t 40t 38t 36t

1600

Flaps up hold in a racetrack pattern Long range cruise

34t

00 ,0 35

00 ,0 30

00 ,0 25

00 ,0 20

00 ,0 15

00 ,0 10

0 00 5,

Altitude [ ft ] Fig. 4.3 Fuel burn of a typical single aisle twin engined jet airliner (maximum take-off weight of just over 62t), as a function of weight, altitude and mode of ﬂight (cruise – heavy lines – or hold).

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4. Higher 40,000 35,000

Altitude [ feet ]

30,000 25,000 20,000 15,000 10,000 5,000 SL 0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

Time elapsed since takeïoff [ hours ]

Fig. 4.4 A typical stepped cruise climb proﬁle on a medium-long haul ﬂight: climbs of 2000 feet every hour or so.

4.5 The Swede who could see air After World War II the aeroplane as a reconnaissance and surveillance platform went through a strange identity crisis, oscillating between the ideas of very low level ‘under the radar’ penetration and very high altitude overﬂight. After only moderately successful applications of the former idea (the Lockheed P-2 Neptune is a good example), the focus shifted towards pushing the limits of high altitude ﬂight as a means of getting close to enemy targets, both for purposes of delivering bombs or taking photographs. A remarkable early representative of this pioneering era was the English Electric Canberra, which, introduced in 1951 was to remain in service in various guises and variants for over half a century (see Figure 4.5, a photograph of an American variant, the Martin B-57, used to this day by NASA as a high altitude research aircraft). The Canberra’s altitude capabilities were hugely impressive, as demonstrated by several records in the early 1950s. On 29 August 1955, test pilot Walter Gibb reached an altitude a new record altitude of 65 876 feet over Bristol, England. He later commented: “The last 500 ft took an awfully long time. It was the most difﬁcult ﬂying I have ever experienced.” [121] Eventually the Canberra also became the ﬁrst aircraft to pass the 70 000 feet mark. Amongst aircraft powered by air-breathing engines this still makes the ‘ancient’ Canberra a member of a very, very small club. The most iconic high altitude surveillance aircraft of this era (and, indeed, one of the most recognizable and widely admired of all time), however, was to be the Lockheed U-2 ‘Dragon Lady’. The driving force behind the development of this remarkable aircraft was the legendary Clarence ‘Kelly’ Johnson. Leader of Lockheed’s Skunk Works team, he was a tremendously gifted designer who, by the time the need for a high altitude reconnaissance aircraft was formulated by the US Air Force, had already been instrumental in the design of a string of successful aircraft. Lockheed boss Hall Hibbard referred to him as “That damned Swede”, who could “actually see air” [69]. The potential customer, Gen-

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Fig. 4.5 In this NASA Ames-Dryden Flight Research Facility photograph taken in 1982 the B-57B Canberra is shown making atmospheric measurements near a mountain range. Image courtesy of NASA.

eral Curtis LeMay, head of the Strategic Air Command, may or may not have agreed with that assessment, but was unconvinced by the design put forward by Johnson. The Lockheed proposal was just too unusual: it was essentially a jet-powered glider – a high altitude aircraft (ﬂying above 70 000 feet to avoid interception) with long, high aspect-ratio wings, capable of little more by way of manoeuvering than transport aircraft, powered by a single General Electric J73 engine (the U-2 would, in the event, be re-engined several times), expected to produce about 6% of its sea level thrust at the aircraft’s cruising altitude [100]. Eventually, after long negotiations, the U-2 project (it was not called that at the time) was to go ahead with joint support from the CIA and the Air Force. The design challenges were tremendous and what resulted (Figures 4.6 and 4.7) was unlike any other military aircraft before. Consider the high aspect ratio, glider-like wings: in contrast with standard design practice, where the two wings share a main spar that goes through the fuselage, the U-2 has a separate spar for each wing, attached to the sides of the fuselage, thus leaving space in the middle for the large, long focal distance camera. The downside of this solution was that the U-2 ended up being a very fragile aircraft – not a problem in the still conditions to be expected at 70 000 feet, but a grave concern for the climb and the descent. The wings also double as fuel tanks, carrying the large amounts of fuel demanded by the requirement for intercontinental range. The bicycle undercarriage with ‘fall-away’ sticks with small wheels on the wing tips, as well as a tail assembly attached to the main fuselage with three bolts are further idiosyncratic features of this ground-breaking design. Extreme altitudes raise extreme physiological issues, including, as we saw in Chapter 1, that of ebullism, the evaporation at the very low ambient pressures of the water contained

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Fig. 4.6 The U-2 ‘Dragon Lady’. Image courtesy of the US Air Force.

Fig. 4.7 The view from 70 000 feet – over half a century after “that damned Swede” dreamt it up, the Lockheed U-2 is still in operational service. Image courtesy of the US Air Force.

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in human tissues. A strange parallel to this problem arose during the design of the U-2’s unpressurized fuel tanks – how to prevent the fuel from evaporating? Shell had to be asked to develop a special, low volatility fuel mixture, with a boiling point of around 150◦ C (302◦ F) at sea level – this, in turn, required a redesign of the aircraft’s ignition systems (used to start and, if necessary, re-light the combustion chambers)9 . Flying close to its cofﬁn corner, the U-2 had also has a delicate trim balance at high altitudes, which requires the help of the fuel system to maintain the stability of the aircraft, through a network of pipes and pumps that have to move the fuel around as required (in the early days of U-2 operations this caused serious difﬁculties in keeping track of available fuel quantities [100]). At the root of the static stability issue lies the need for a wing with a high lift coefﬁcient and therefore a large camber. This, in turn, leads to a large pitching moment, which needs to be counteracted by the tail. Damage to the tail can, therefore, ﬂip the aircraft on its back, which is, as Johnson would later speculate, what happened to the U-2 piloted by Francis Gary Powers, shot down over the Soviet Union on 1 May 1960 [69]. The Powers incident was a turning point in the history of the U-2. In its early years, operating at up to 72 000 feet, it was out of the range of all anti-aircraft weapons and it could therefore operate almost at will over the Soviet Union, bringing back precious camera footage. After the loss of Powers’ aircraft the U-2 would become the center of attention once more during the Cuban missile crisis (triggered by U-2 footage showing Soviet missiles stationed on the Caribbean island), but its vulnerabilities were now becoming clear, as was the solution. A replacement was needed that could ﬂy even higher and, in view of the Soviet development of high altitude interceptors, much, much faster. And once again, Johnson’s Skunk Works would provide the solution: another ground-breaking, innovative and awe-inspiring aircraft, the praises of which we will sing in the next chapter.

9

Producing this special fuel required the use of petroleum byproducts that Shell normally used in the production process of the insecticide ‘Flit’. Diverting these to the production of the low volatility fuel of the U-2 lead to nationwide insecticide shortages – not easy to explain with a CIA-funded secret project as the client [100].

5. Faster

“Cocooned in Time, at this inhuman height, The packaged food tastes neutrally of clay, We never seem to catch the running day But travel on in everlasting night...” John Betjeman (1906-1984) ‘Back from Australia’∗

5.1 Making waves The “Mach 0.999 Study” – so read the title of a 1944 report into the comparative merits of various high speed aircraft engines by Major Ezra Kotcher of the Wright Field Air Corps Engineering School [110]. This somewhat tongue-in-cheek reference to the ‘sound barrier’ 1 is an indication of the apprehension surrounding supersonic ﬂight for much of the history of aviation. The existence of a ‘barrier’ in the minds of the aerodynamicists of the early decades of the ﬁrst century of ﬂight is often over-stated, implying a ‘here be dragons’ mentality – any such reasoning was generally conﬁned to the public imagination. To be fair, as far as popular misconceptions go, the existence of an impenetrable barrier at Mach 1.0 ranked amongst the more plausible ones, in the light of reports of World War II ﬁghters accidentally diving past the magic number and losing control or even their structural integrity. Nonetheless, by the time of Kotcher’s study two things were clear about the sound barrier: ﬁrst, that it was mostly made of money – that is, novel, expensive and probably vastly inefﬁcient propulsion systems were required to penetrate it – and second, that the easiest path through it lay in the thin air of the stratosphere. Over half a century later both statements are still largely true, though at different scales. Thanks to advances in aerodynamics (special, high speed wing sections and a design principle known as area ruling) and propulsion technology, supersonic ﬂight does not demand ∗

c John Betjeman 1967, 1968, 1969, 1970, 1971, J. Betjeman. A Nip in the Air. John Murray, 1974. 1972, 1973, 1974. Reproduced by permission of John Murray (Publishers). 1 A Mach number of one describes the ﬂow of air meeting an object moving at a speed equal to that of sound in dry air at a given temperature and pressure.

A. Sóbester, Stratospheric Flight: Aeronautics at the Limit, Springer Praxis Books, DOI 10.1007/978-1-4419-9458-5_5, © Springer Science+Business Media, LLC 2011

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the astronomical resources that it used to swallow up in the post-war era, but a sound barrier made of wads of banknotes is still there and crossing it is largely the privilege of men and women in military uniforms (though this was not the case for the glorious ‘Concorde interlude’ between the seventies and the early noughties – more on this shortly). Moreover, the concept of a critical Mach number represents the start of a gradually steepening ramp well short of the barrier, which limits most of commercial aviation to below Mach 0.9 or so. The fundamental reason is that the airﬂow around parts of an aircraft generally exceeds the speed of sound well before the aircraft itself does with respect to the atmosphere. The critical Mach number (usually to be found around 0.7 or 0.8) marks the speed where this happens. The emergence of locally supersonic pockets and associated shock waves causes drag divergence, that is, a sudden, steep increase in the drag opposing the progress of the airplane. At high subsonic speeds the energy absorbed by the production of the shock waves manifests itself as additional drag, as do the interactions between the boundary layer around the aircraft and the shocks, which increase the proﬁle drag (this is a component of the parasitic drag, so-called because it is unrelated to the useful process of generating lift). These phenomena mark out the region between around Mach 0.7 and 1 – the so-called transonic domain – as the ground where propulsion and aerodynamic engineers earn their money. Beyond the drag rise, a number of additional factors conspire to limit the cruising speed of a subsonic aircraft and these are all related, in often quite complicated ways, to altitude. As the altitude increases and the air density decreases, the angle of attack at which the wings must meet the oncoming airﬂow gradually increases and, beyond a certain point, the aerodynamic efﬁciency of the aircraft begins to fall away. At the same time, the maximum allowable thrust setting of the engines decreases – this is limited by the maximum temperature that the turbines can sustain and fan and compressor aerodynamic constraints. The air inlet total temperature2 increases with speed, though it decreases with altitude. In turn, the total air temperature is directly proportional to the inlet total temperature. For a given airspeed the Mach number – which determines the onset of drag rise – increases with altitude through the troposphere, but it stops increasing in the stratosphere (at least where the atmosphere behaves largely as modeled by the International Standard Atmosphere). The density of the air mass meeting the aircraft, however, keeps falling... It is difﬁcult to draw an obvious line under this intricate web of interactions, mainly because their balance depends on so many variables – let us, instead, consider a typical example of what all these effects boil down to. Figure 5.1 is a plot of the total drag force on a large, four-engined transport aircraft, as a function of speed and altitude. The three ‘drag buckets’ correspond to three different altitudes. Their right-hand, ascending arm steepens at about Mach 0.8 almost regardless of altitude (though, of course, this means different airspeeds at different altitudes), but the effects listed above compensate, to the extent that the sea level drag curve, the 20 000 feet drag curve and the 40 000 feet drag curve are offset at roughly equal intervals along the speed axis. The upshot of this is that the minimum drag is roughly the same at any altitude (145 kN – just 2

The total air temperature is generally higher than the ambient temperature, the difference being the result of converting the kinetic energy of the air into heat by bringing it to rest.

5.1 Making waves

97 300 Sea level 20 000 feet 40 000 feet

280 260 Drag [ kN ]

240 220 200 180 160 140 150

200

250

300 350 400 450 True airspeed [ knots ]

500

550

Fig. 5.1 Drag of a large, four-engined transport aircraft – high speed cruise at the bottom of the ‘drag bucket’ is only possible at high altitudes.

under 15 tonnes), but it occurs at very different speeds: 240, 340 and 440 knots – roughly 100 knots apart for sea level, 20 000 feet, and 40 000 feet respectively. It is generally wise to cruise slightly above these drag-optimal speeds. As we have seen in Chapter 4, speed stability considerations are a good enough reason to do this, particularly at high altitudes, near the cofﬁn corner. Additionally though, the shallowness of some of these minima (see, for instance, the middle bucket in Figure 5.1) suggests that a reasonable speed gain (often as much as 5%) might be possible with only a slight sacriﬁce on fuel burn (say, 1%). To further complicate matters though, as with much else in aviation, all this reasoning can be nulliﬁed by the weather. Often the winds at the altitude for maximum speed or for maximum range are unfavorable enough to cancel any theoretical advantage that may have been anticipated on the assumption of relatively uniform wind velocities across the ﬂight levels. A typical example is an east to west crossing of the Atlantic with a 100 knot west to east jet stream centered near the maximum speed altitude of, say, 35 000 feet, but only low winds at 40 000 feet – a crossing at the latter height will almost certainly be faster (not to mention more economical!). Another caveat that accompanies charts like Figure 5.1 is that they are based on International Standard Atmosphere conditions; we have seen that real temperature proﬁles can deviate from the idealized standard atmosphere model by a signiﬁcant margin (see the charts on page XX). These deviations translate into inlet total air temperature deviations, which, in turn, lead to turbine entry temperature deviations. In any case, as far as airliners and other large transports are concerned, the grand total of all of the speed-related performance penalties amounts to a ‘soft’, economical sound barrier at around Mach 0.85, even at high altitudes. An offer of Mach 0.98 was on the table for the airlines in 2001 in the utterly gorgeous shape of the Boeing Sonic Cruiser concept – in the end, they opted for better fuel economy over higher speed and, to the chagrin of aviation enthusiasts the world over, the Sonic Cruiser was shelved. This leaves

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the manufacturers of business jets to ﬁght the speed battle in the Mach 0.85 and Mach 1 range on behalf of their wealthy customers – and ﬁght they do! The protagonists of this epic high speed showdown for the second decade of the new century are set to be Cessna’s ‘The Ten’ (successor to the Citation X, the reigning holder of the ‘fastest civilian jet in the stratosphere’ crown), Gulfstream’s G650 and the Bombardier Global Express 7000 and 8000 – all aiming for slightly different payloads and ranges, but all keen to impress with savings on time, that all-important business commodity [66]. This is a high altitude battle, waged almost entirely in the stratosphere, but there are some interesting trade-offs to consider. While speed is certainly extremely important, even the rareﬁed world of high performance business aviation cannot avoid altogether the mundane constraints of cost and environmental impact. Figure 5.2, a cruise speed versus fuel burn chart of a high speed business jet, illustrates what this means in terms of altitude selection. The complex web of aerodynamic and propulsion effects strikes again here: alas, the altitude at which these jets are capable of their eye-watering top speeds are often much lower than the altitudes at which their fuel consumption is optimal (this is, of course, true, to some extent, to large transports too – in fact, Figure 5.1 is an example of this: for a speed of 500 knots the drag is considerably lower at 20 000 feet than at 40 000 feet). 530 35 000 feet 37 000 feet 39 000 feet

525

Cruise airspeed [ knots ]

520 515

41 000 feet

33 000 feet 31 000 ft 29 000 feet

510 43 000 feet

505 500

45 000 feet

495 490 485 480 3

47 000 feet 3.5

6 5.5 5 4.5 4 Fuel burn [ pounds / nautical mile ]

6.5

7

Fig. 5.2 Maximum cruise speed versus fuel burn diagram of a high performance business jet at mid-cruise weight. 34 000 feet is the place to be if the passengers are in a hurry, but climbing far into the stratosphere and sacriﬁcing around 10% of the cruise speed can mean huge fuel savings.

Moreover, cruising at 34 000 feet might satisfy the all-out speed requirement, but being far above virtually all other trafﬁc at, say, 50 000 feet might enable the air trafﬁc management system to ﬁnd a more direct and ultimately quicker routing (though, of course, the added time and cost of the additional 16 000 feet climb would have to be factored in). When one also considers weather – thunderstorms and other sources of turbulence are much rarer

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at higher ﬂight levels (we shall consider this in much more detail in Chapter 8) and, as already mentioned, jet streams might be more favorable at some levels than others, as well as the distance to be covered (there is no point in climbing deep into the stratosphere for a 150 mile hop3 ), the picture becomes even more complicated. Once again, the viability region of stratospheric ﬂight might be a small sliver of the altitude–speed–payload–range space, determined by multiple intersecting constraints and limitations. As we have seen, economic considerations prevent most of us from ever exceeding the speed of sound. This is, of course, not to belittle the importance of supersonic ﬂight, a glorious technological achievement, the story of which began as the world was emerging from the dark days of its bloodiest conﬂict.

5.2 Supersonic men “10...9....8...” Major Roberto “Bob” Cardenas of the United States Air Force began counting down as his Boeing B-29 Superfortress leveled off at an altitude of 20 000 feet above Muroc Army Air Field (now Edwards Air Force Base) in the morning of the 14 October 1947. In the bomb bay of the B-29 hung a cigar-shaped, bright orange colored rocket plane: a Bell X-1, tail number 46-062, “Glamorous Glennis”. “...3...2...1... Drop!” As the X-1 fell away, its pilot ignited the four liquid oxygen and ethanol fueled rocket motors. “Glamorous Glennis” accelerated sharply under the savage thrust of its barely controllable, primitive propulsion system and began a rapid climb towards the stratosphere. Captain Charles “Chuck” Yeager, the man at the controls, had been ﬁghting Nazi Germany in the skies over Europe only three years earlier – in fact, he had done so in a way that had earned him the Distinguished Flying Cross, the Bronze Star, the Silver Star with oak leaf cluster, the Air Medal with six oak leaf clusters and the Purple Heart [110]. Now he was facing different dangers: rapidly approaching 42 000 feet over the Mojave desert, he was on the verge of becoming the ﬁrst person ever to go faster in level ﬂight than the speed of sound. The 14 October ﬂight was the culmination of a long, gradual test programme conducted by Bell, the military and the NACA4 and the engineers had a reasonably clear idea of the supersonic ﬂow physics involved. Yet, Geoffrey de Havilland Jr., son of the great British aviation pioneer of the same name, had died while testing a research aircraft at high speeds (his DH Swallow suffered a catastrophic structural failure after developing pitch oscillations at Mach 0.9) only a year earlier and the X-1’s own test programme had not been entirely without white knuckle moments, many of them to do with partial losses of elevator effectiveness and pitch control around Mach 0.95. Thus, Yeager must have shared some of the apprehension of the NACA and Air Force staff watching their radars on the ground as the aircraft began to feel unstable at around Mach 0.9 and, as on some of the previous ﬂights, he started to note a marked decrease in the effectiveness of the elevator at Mach 0.94. 3

...though the distances over which it is worth climbing to very high altitudes are often counter-intuitively short! 4 National Advisory Committee for Aeronautics.

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Fig. 5.3 The Bell X-1. Image courtesy of the US Air Force.

Nevertheless, he was now in the thin air of the stratosphere and, in spite of only running two of the motors at this point, the X-1 kept accelerating. Once at 42 000 feet he ignited the third rocket. A fresh burst of acceleration took the needle of the Mach meter to 0.98 and, after a brief ﬂuctuation, off the scale. In his subsequent report Yeager estimated that it may have swung out to 1.05 (there were no markings past 1.0 on the dial!) – the ofﬁcial ﬁgure recorded for the 14 October ﬂight is variously reported as Mach 1.06 or 1.07. Either way, a new page was turned in the history of high altitude ﬂight. It was a page written by visionaries like the Caltech ﬂuid dynamicist Theodore von K´arm´an5 , who had laid the theoretical foundations of supersonic ﬂight, Bell Aircraft chief designer Robert Woods, who had committed the company to the massive research programme that led to the development of the X-1, the already mentioned Major (later Colonel) Ezra Kotcher of the US Air Force, countless other engineers, scientists and test pilots from Bell, the US Air Force and the NACA and, of course, Yeager himself6 .

5

Originally K´arm´an T´odor, a Hungarian Jew, who, upon ﬂeeing likely persecution in Europe in the 1930s, worked at Caltech, later becoming one of the founders of the Jet Propulsion Laboratory. 6 Now Brigadier General (Ret.) Yeager.

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Fig. 5.4 Captain Yeager and “Glamorous Glennis”. Image courtesy of the US Air Force.

5.3 Of triumphs of technology and white elephants Once in a while remarkable aircraft emerge that capture the public imagination. These are not necessarily the fastest, the most powerful, the noisiest, the quietest, the most successful or the most... anything. They simply come to represent a moment in the history of aviation in such a compelling way that their names are forever imprinted into the minds of millions. Beyond this level of fame (or, in some cases, notoriety), however, there is one stratospheric (pun half-intended) accolade that only one such aircraft has managed to achieve over more than a century of ﬂight. This is a special distinction that has eluded even the Wright Flyer, that started it all, the Spitﬁre, the Avro Lancaster, the P-51 Mustang, the Mitsubishi Zero and the Messerchmitt Bf 109 of the second World War, the Bell UH-1 “Huey” helicopter of the Vietnam War and the bubble-canopied Bell 47 of M.A.S.H. fame, the staggeringly long-serving workhorse that is the DC-3, the Boeing 707 that opened up long haul travel to the masses, the mighty Vulcan bomber of the Cold War, the alien-looking B-2 Spirit Stealth Bomber that never fails to draw gasps from air show spectators. It is the loss of the deﬁnite article ‘the’ that marks Concorde out even in such august company7 . 7

One needs to leave Earth’s atmosphere for other, less widely used examples: the International Space Station, sometimes simply referred to by its crews as “Station”, and the Space Shuttle, known to its crews as “Shuttle”.

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Fig. 5.5 Concorde – once ﬂying at 22 miles a minute above a planet rotating at a tangential speed of 11 miles a minute, today a static display at Heathrow Airport...

This is a reﬂection, perhaps, of Concorde’s almost unique ability to elicit passion and public interest, especially in Britain and France. As with other aspects of British public life, the Hansard (the ofﬁcial transcript of the proceedings of the UK Parliament) is usually a fairly reliable gauge of the most serious preoccupations of the nation at any given time – the chart in Figure 5.6 gives an insight into how Concorde’s role waxed and waned but never disappeared from the forefront of the public consciousness from its inception in the 60s to its (sadly possibly ﬁnal) retirement in 2003. With all the ups and and downs of Concorde’s history though, it is hard to debate its status as a transport icon, in the same league as the Orient Express and the QE2. In fact, for a while it was possible to take what is probably the greatest voyage of all time: Orient Express to Southampton, QE2 to the US and return at 60 000 feet at twice the speed of sound on board Concorde. The stratospheric cruise was essential to stretch Concorde’s range sufﬁciently for an Atlantic crossing at Mach 2.04. The high altitude allowed the supersonic airliner to cruiseclimb continuously (as opposed to following a stepped altitude proﬁle, like other airliners), as it was practically the sole user of the higher ﬂight levels of the stratosphere – this gave it the ability to be at the exact optimum altitude at all times and thus eke out its fuel burn target. After a London Heathrow departure, leaving Wales and accelerating to supersonic speeds over the South of Ireland the optimum altitude for its weight was around the 50 000 feet mark. Concorde would then begin a steady cruise climb over the ocean, which saw it reach around 60 000 feet by the time it was passing Sable Island, southeast of Nova Scotia, about two hours later. It would then decelerate to reach subsonic speeds over the continental United States and proceed to lose its over 11 mile altitude for the arrival into New York JFK airport.

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"we should spell "The Concorde "Concord" is a white elephant" without an "e"

"Concorde" references

"the bang from Concord would be louder"

"Is the commercial success of Concorde now doomed?" "any progress on overflying rights?"

500

"now concentrating our efforts on securing routes "Total UK Government to Melbourne and Tokyo" expenditure in support of Concorde was £1,358 million gross" "no recent enquiries about purchases..."

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"sonic boom currently affecting SW of England"

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"expressing our sadness at the terrible crash in Paris"

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Fig. 5.6 Concorde as a matter of varying public interest in Britain, as reﬂected by the annual number of references in the House of Commons (with a few of these references quoted from the Hansard above the graph).

Cruising at such altitudes raised myriad design and certiﬁcation questions at the time of Concorde’s development. In fact, the certiﬁcation documents themselves were rendered irrelevant in some parts, as the technical regulations establishing the requirements for the safe operation of passenger airliners had not been conceived with Flight Level 600 in mind. The regulations had to be rewritten to ensure that a supersonic airliner was to be no less (and, indeed, had to be no more) safe than a subsonic one entering service at the same time. The need for stratospheric operation took the British and French engineers into territory that was not altogether unchartered, but was certainly much less well trodden than the 30–40 000 feet band of subsonic airliner operation and it raised numerous questions, as did the sustained high speed of the aircraft (the military jets capable of Concorde-like speeds could only reach these for very short periods of time, on full afterburner). One such ‘known unknown’ was: is it possible to encounter weather above 50 000 feet, perhaps in the shape of thunderstorms, hailstones, rain, or steep temperature gradients? Some of these phenomena were actually found to be relatively common. For instance, temperature changes of 11◦ C (20◦ F) over one nautical mile (barely more than three seconds at Concorde’s speeds!) were to prove a serious headache in initial ﬂight testing, due to the effect they had on the stability of the ﬂow in Concorde’s extremely complex engine air intakes [29, 122].

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Other high altitude meteorological menaces were rare, but their risks were considered substantial: what would happen, for example, if Concorde was to strike a hail stone thrown high into the stratosphere by powerful convection – while traveling at over twice the speed of sound? In fact, what does it ‘feel’ like to hit a rain drop at 1300 miles an hour? North American F-100 Super Sabre aircraft ﬂown by the US National Severe Storms Laboratory had detected hailstones up to 2.5 inches across in the stratosphere: would Concorde’s skin withstand a strike by an object that size? At the very least, would hail be capable of damaging the special paint that played such a key role in Concorde’s temperature control (more on which later)? Somewhat surprisingly, the possibility of raindrops hitting the exposed rivet-heads on the external surfaces of the aircraft at Mach 2.04 turned out to be the greatest of these concerns. Following tests with rocket sleds at the Pendine test track, where precise collisions between water droplets and rivets were engineered, the shape of the rivet heads had to be changed [29]. Of course, these were extremely rare contingencies that the design and certiﬁcation engineers had to worry about – the everyday experience of operating Concorde was of ﬂight in an extremely calm environment, deep in the stratosphere. Mike Bannister, Chief Pilot of British Airways’ Concorde ﬂeet, recalls how it was possible to stand a coin on its edge on the back of the throttle quadrant and it would just stay upright – while the aircraft was covering 22 miles every minute (or, to use a popular ‘non-SI’ unit, the length of six football pitches a second!) [49]. In many ways, therefore, Concorde can be said to have passed the sound barrier (twice over!) by climbing into the upper reaches of the atmosphere. While its performance there was unmatched even by most military aircraft, it still fell short of the original design target, which envisaged a Mach 2.5+ airliner. There are several reasons for the design team having had to concede 20% of the planned cruise speed. On the one hand, Concorde, like any other supersonic aircraft, is a compromise between low speed performance – in the ﬁrst and last segment of each ﬂight – and high speed performance in the supersonic cruise. This is because there is simply no getting around the fact that an airframe geometry that favors feasible supersonic cruise will be inefﬁcient in the subsonic regime and will present enormous technical challenges from the point of view its two inevitable parts: the take-off and the landing (think of Concorde’s drooping nose!). The greater the supersonic cruise speed, the tougher this compromise becomes and Concorde’s rather marginal fuel economy could have taken a further hit. Additionally, the complex air intake system of the aircraft was sensitive to sideslip at higher Mach numbers, causing pressure distortions on the engine face. But there was a third, even harder constraint looming large not far beyond the Mach 2 mark: a whole new type of barrier, the appearance of which can be delayed by climbing high into the stratosphere, but cannot be avoided altogether.

5.4 Burning through the skies When high speed airﬂow is brought to rest, its kinetic energy is transformed into heat. This warms the air locally to the so-called stagnation temperature, which, at subsonic speeds, is practically insigniﬁcant, but at supersonic speeds can be high enough to present serious engineering challenges. The speed of the airﬂow around an aircraft comes to stagnation in

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several places, for example, just upstream of the nose or the leading edges. Most signiﬁcantly though, the air is also brought to a halt by viscous forces in a very thin layer along all surfaces wetted by the ﬂow and here, although due to convective effects and turbulent mixing the temperature does not quite reach the stagnation temperature, is where aerodynamic heating (or kinetic heating) causes serious problems. In fact, it was this effect that limited Concorde’s cruising speed to Mach 2.04. The temperatures arising from aerodynamic heating decrease signiﬁcantly with altitude for a given speed and ambient temperature – a handy rule of thumb valid at high speeds and high altitudes is that every additional 5000 feet of altitude will reduce the surface temperatures by about 7◦ C (15◦ F) [77]. Nevertheless, in spite of cruising at up to 60 000 feet in the extremely cold air of the stratosphere, the temperature of Concorde’s metal skin would reach over 90◦ C (194◦ F) around the fuselage, 105◦ C (221◦ F) along the leading edges of its slender delta wings and around 130◦ C (266◦ F) around its sharp nose. With every tenth of a Mach number adding around 14◦ C (25◦ F) to these temperatures, the Mach 2.5 cruise speed of the initial design brief would have meant having to switch from aluminium (which Concorde’s skin is made of) to a much more expensive material, such as titanium. Even at the eventual cruise speed of Mach 2.04 the management of the kinetic heat was a pressing design issue across several departments in the British Aircraft Corporation and in A´erospatiale. Most importantly, a very high amplitude cyclic thermal loading was added to the cabin’s pressurization cycles, which, on the one hand, demanded an extremely expensive testing rig (with provisions for heating in addition to the usual mechanical loading simulation apparatus that had become the norm since the development of the de Havilland Comet) and, more signiﬁcantly, meant that the structure of the pressure cabin had to be made even stronger than Concorde’s very high altitude cruise (and therefore vast ambient pressure changes) would have demanded. In aircraft design it is not uncommon that satisfying extreme requirements on one account has serendipitous payoffs elsewhere – this is largely the case with Concorde’s aerodynamic heating. As we saw in Chapter 2, the strong airframe demanded by the heavy cyclic loads allowed Concorde’s designers to make the argument that the extreme physiological impact on the passengers of a possible depressurization at 60 000 feet was mitigated by the reduced probability (compared to subsonic airliners) of such an event occurring in the ﬁrst place, as a result of the extremely strong cabin structure. The designers of the air conditioning system (Hamilton Standard in the US and Hawker Siddeley Dynamics in the UK) also had to deal with the unusual thermal conditions. As a result of the kinetic heating, the intense solar radiation and the presence of internal heat sources (such as electronic equipment and the passengers themselves) Concorde’s air conditioning system had to produce around eight to 17 tonnes of refrigerated air an hour, as opposed the three tonnes or so required by a subsonic airliner of similar size. Here is a rough outline of how the system worked. Fresh bleed air from the compressors of the four mighty Olympus engines entered the primary heat exchanger at a temperature of around 580◦ C (1076◦ F). The air was cooled here to 200◦ C (392◦ F) by releasing some of its heat into ram air entering the exchanger from the outside. A high speed turbo-compressor intercooler refrigerator unit followed, with the ﬁnal stage of the cooling involving yet another heat exchanger, this time using the fuel stored in the aircraft’s tanks as a heat sink. The

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resulting cool air passed through the cabin, after which it is circulated around the walls within the insulation before it was discharged [41, 60]. Our presence in the stratosphere took a major step back with Concorde’s retirement on 26 November 2003, but not before the iconic airliner laid down a stunning technological milestone and a number of impressive records. To quote just one such achievement, a British Airways Concorde still holds (and, no doubt, will continue to hold for a long time to come) the record for the fastest commercial ﬂight between New York and London, with the time of 2 hours, 52 minutes and 59 seconds, recorded on the 7 February 1996 (this amounts to an average ground speed of 1193 miles per hour). One record Concorde does not hold, however, is the fastest crossing of the Atlantic. That race has been won by an entirely different beast; and the winning margin is massive. We have already dedicated some of Chapter 4 to the Lockheed SR-71 Blackbird (for this is the transatlantic record holder, with a time of 1 hour 55 minutes), the legendary reconnaissance aircraft, designed by Kelly Johnson’s team at Skunk Works in the 1960s to climb deep into the stratosphere, out of the reach of Soviet missiles. Its maximum altitude of 90 000 feet and its typical cruise band of 70 000 to 85 000 feet also enabled it to pass the Mach 3 marker, although the ‘heat barrier’ still meant that aluminium could no longer be used to build the airframe structure. Stainless steel had ﬁrst been considered as an alternative – the North American XB-70 Valkyrie Mach 3 bomber (Figure 5.7), being developed at the same time, used a steel honeycomb construction – but was eventually discarded in favor of titanium, a material Skunk Works had already been experimenting with for a decade. While a simple construction had been seen as the chief advantage of titanium, the design of the structural components ended up taking a very long time to perfect. As Kelly Johnson recalls, when the ﬁrst wing section was heated up to the temperatures it would encounter in ﬂight, “it wrinkled up like an old dishrag” [69]. Eventually, corrugations and dimples had to be designed into the wing surfaces, which merely deepened or ﬂattened in response to changes in temperature (see Figure 5.8). So what temperatures were encountered by the Blackbird in ﬂight? Figure 5.9 is a ‘heat map’ of the surface of the SR-71 at Mach 3, with temperatures ranging from 250◦ F (121◦ C) on the aft section of the fuselage to 500–600◦ F (260–315◦ C) around the leading edges and on the nacelles. Retired US Air Force Colonel Richard Graham, one of the select few pilots who had the opportunity to ﬂy the SR-71 regularly on operational missions, recalls how at Mach 3 he could only hold his pressure-suit gloved hand to the windscreen for about 15 seconds before it became too hot to stand. The small, angular windows were also routinely used by Air Force pilots to heat their meals on long endurance missions [54]. Like on Concorde, the fuel has an important heat management role to play on the Blackbird, but, in terms of using fuel as a heat sink, the difference between Mach 3 and Mach 2.04 is large enough for some extreme measures having been needed on the high altitude ‘eye in the sky’. The fuel tanks are part of the fuselage structure of the SR-71: in several places the titanium skin of the aircraft doubles as the wall of one of the six fuel tanks. The extreme cycles of heating and cooling and the resulting expansion and contraction made it impossible to ensure watertightness in all conditions, so the fuel tanks leak to some extent under the pressure of the nitrogen used to inert them. Inerting was an important safety measure, as the fuel temperature could reach 148◦ C (300◦ F) at Mach 3 (in fact, in case the

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Fig. 5.7 The North American XB-70 Valkyrie, a Mach 3+ high altitude bomber of which only two prototypes were ever built, before its long and fraught development program was cancelled. To cope with the thermal demands of ﬂying at three times the speed of sound, most of the airframe was built of stainless steel. Image of the maiden ﬂight of the XB-70, courtesy of the US Air Force.

Fig. 5.8 Avoiding the ‘dishrag’ phenomenon – corrugations on the titanium skin on the upper surface of the wing of the SR-71. Image courtesy of USAF/NASA.

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Fig. 5.9 Upper surface temperatures on the Blackbird at Mach 3 [67]. Sketch courtesy of NASA.

inerting system failed or the nitrogen ran out from the dewars located in the nose wheel well, the Mach number had to be limited to 2.6). The nitrogen atmosphere of the fuel tanks was also pressurized for the eventuality of a rapid emergency descent; a steep increase in the ambient pressure from the extremely low cruise values, say, 27 millibars at 80 000 feet, to, the ‘safe to breathe’ level of around 700 millibars (around 10 000 feet) would crush an unpressurized fuel tank [54]. Incidentally, an interesting side effect of the fuel-based cooling of the aircraft, found early on in the Blackbird test programme, was that uneven depletion of the fuel in the various tanks lead to uneven cooling of the airframe and therefore uneven expansions and contractions, which, by virtue of distorting the shape of the long chines alongside the forward fuselage slightly altered the aerodynamic characteristics of the aircraft [86]. The Blackbird’s J58 engines were also pushed to their limits by the high stagnation temperatures encountered beyond Mach 3. The J58 had originally been developed by Pratt and Whitney for the Glenn L. Martin Company’s enormous ﬂying boat strategic bomber, the Martin P6M SeaMaster. The P6M never went into service, but the hugely powerful J58 found fresh use in the Blackbird [69, 100]. In standard conditions (that is, at an ambient temperature of −56.5◦ C (−69.7◦ F)) the air temperature at the compressor inlets of the the J58 was at around 120◦ C (248◦ F) at Mach 2, but rose to 340◦ C (644◦ F) at Mach 3 and over 400◦ C (752◦ F) at Mach 3.3. In common with other supersonic aircraft (such as Concorde) the J58’s inlet itself acts as a compressor as it slows the Mach 3+ incoming airﬂow to subsonic speeds, effectively making the powerful Pratt and Whitney engine a hybrid between a ramjet and a turbojet at high speeds. The wide speed range of the SR-71 required an elaborate variable geometry system to achieve efﬁcient compression of the ram air in the intake at all operating regimes. The main component of this is a translating conical spike in the inlet throat (see Figure 5.8), which generates the shock waves that

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slow down and compress the air. The optimum positioning and stability8 of these shocks at various speeds requires the spike to move in and out through a range of about 25 inches. The payoff for the carefully designed ramjet/turbojet hybrid is that the aircraft can take off under its own power (a pure ramjet needs a high speed airﬂow to start and thus has no static thrust) and ﬂy relatively efﬁciently at speeds exceeding Mach 3 (something a turbojet is not capable of). In fact, the efﬁciency of the hybrid even improves with increasing speed in the latter part of the speed range: at a weight of around 45 tonnes an SR-71 burns 17 tonnes of fuel an hour at Mach 3, but nearly a tonne less at Mach 3.15 [54]. Perusing the operating manuals of the SR-71 is an exercise in getting one’s head around numbers that create the impression of an incorrectly placed decimal point – as even the cursory description above illustrates, many of its performance metrics are nearly an order of magnitude above those of everyday ﬂying machines. We shall conclude this chapter on speed at high altitude with one ﬁnal large number related to the Blackbird. On 28 July 1976 Captain (now retired Major General) Eldon W. Joersz of the 9th Reconnaissance Wing of the US Air Force entered the SR-71 into the record books of the FAI (F´ed´eration A´eronautique Internationale) by taking it to a speed of 1906 knots (3530 km/h) near Beale Air Force Base, California. Three and a half decades later, long after its ofﬁcial retirement, the Blackbird is still the fastest aircraft powered by an air-breathing engine. Of course, there have been faster ﬂying stratospheric vehicles – notably, the Apollo command module still traveled at around 15 times the speed of sound as it was descending through the higher layers of the stratosphere on re-entry – but none of these are ‘aircraft’ in the true sense of the word. It is also worth remembering that, beyond the eye-watering performance, the Blackbird fulﬁlled the strategic reconnaissance role it had been designed for highly effectively, while advancing with several great leaps the science and technology of aeronautics in the process.

8

At certain ﬂow conditions and throttle settings all supersonic engines run the risk of ‘spilling out’ the shock waves, a phenomenon known as unstarting.

Part III

‘Above the weather’

6. Deep freeze

6.1 A cold morning 17 January 2008 started off as a slow news day in the British media. In fact, some newspapers were dedicating more column inches to the ﬁrst Democratic Party primaries in the US presidential elections (held the previous week) than to domestic events. Then, shortly after mid-day, all that changed. A newsﬂash appeared on the BBC: a British Airways Boeing 777 crashed at Heathrow airport. This seemed unreal. Many incredulous minds were rehearsing the same instinctive, subjective arguments in the face of overwhelming evidence to the contrary. British Airways aircraft don’t crash – the only serious accident in the company’s history – a mid-air collision caused by an air trafﬁc control error – had happened over 30 years before. Boeing 777s don’t crash – in its lifetime in service of nearly a decade and a half, the large longhaul twin jet, the ﬁrst aircraft ever to have been conceived entirely on a computer-aided design system, had never had a hull-loss accident. Accidents don’t happen at Heathrow – the world’s busiest international airport had not had a single crash in over a decade. Yet, Triple Mike, G-YMMM, Flight BA38 from Beijing, was lying on the grass undershoot area, about 330 meters short of the paved runway. Its nose landing gear and the two main landing gears had collapsed, with the right gear actually having separated from the aircraft, and the passengers were being evacuated through the emergency slides. There were no fatalities or serious injuries, but it had been a close call. Just how close it had been, however, was only clear at that moment to Captain Peter Burkill, Senior First Ofﬁcer John Coward and First Ofﬁcer Conor Magenis, whose day at the ofﬁce had just ended with 34 seconds of sheer horror and the relief of having managed to bring the jet to a touchdown a mere 110 meters inside the airport perimeter fence1 . So what had happened over those last few seconds? After an uneventful ﬂight from Beijing, the 777 was on ﬁnal approach into Heathrow’s runway 27L. The ﬁrst sign of all not being right came at 430 feet above ground level, when, to the captain’s announcement that the approach was stable, Senior First Ofﬁcer Coward, the pilot ﬂying at the time, responded somewhat uncertainly: “Just.” Uncertainty turned to serious concern moments 1

Details based on [1] and [23]

A. Sóbester, Stratospheric Flight: Aeronautics at the Limit, Springer Praxis Books, DOI 10.1007/978-1-4419-9458-5_6, © Springer Science+Business Media, LLC 2011

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later when the jet began to decelerate to below its 135 knot approach speed and, in spite of the autothrottle being on, the engines did not seemed to respond. The attempts of the autopilot to maintain the aircraft on the pre-selected glideslope in spite of the engines remaining on idle (the crew had, by now, overridden the autothrottle by advancing the throttle levers manually, but to no avail) resulted in further loss of speed – down now to 115 knots. With the engines producing idle power, a certain sign that the aircraft was unable to maintain both the glidepath and a safe speed at the same time was that the stick shaker came on, warning of the impending stall. Captain Burkill decided to try and reduce the drag of the aircraft and thus reduce the angle of their descent by the only means at his disposal: he retracted the 777s massive double-slotted ﬂaps from 30◦ (the normal approach setting) to 25◦ . As it turned out later, this only extended their glide by about 50 m [1] and it thus made little difference to the ﬁnal outcome. Nonetheless, the 150 tonne jet made it over a petrol station, the busy A30 carriageway by Hatton Cross Tube Station and, crucially, over the perimeter fence of the airport. The impact and the subsequent 372 m slide caused enough damage to write off the 200 million dollar jet, but the 135 passengers, 16 crew and numerous others on the ground were safe. All that remained was to discover what caused the nearly unthinkable double rollback on the two Rolls-Royce Trent 800 engines? It was clear that the aircraft had not run out of fuel, there had been no signs of mechanical problems on either engine during the ﬂight (besides, the probability of both engines simultaneously developing independent mechanical failures was remote enough to be ignored) – the only clue to what may have happened was an unusually cold lower stratosphere over Asia that morning. Could the fuel of the jet have come close to its freezing point?

6.2 Ten hours earlier It was a cold morning in Beijing, with snow ﬂurries forecast for the afternoon. The airport weather station was showing a ground temperature of −7 ◦ C (19.4 ◦ F). G-YMMM was fuelled with 79 tonnes of Jet-A-1 for its ﬂight to Heathrow. Once in the tanks, the temperature of the fuel settled at −2 ◦ C (28.4 ◦ F) as the engines of the 777 were started. This was signiﬁcantly warmer than the −20 ◦ C (−4 ◦ F) recorded by the fuel temperature probes of the aircraft on arrival from London the previous day and certainly high above the −44 ◦ C (−47.2 ◦ F) freezing point of Jet-A-1. Of the two fuel types commonly used in aviation, Jet-A-1 has the lower freezing point – the other, Jet-A, freezes at around −37 ◦ C (−34.6 ◦ F). The electronic Engine Indicating and Crew Alerting System in the cockpit of the 777 generates a warning a few degrees before the freezing temperature is reached, giving the crew time to descend into warmer air if necessary. Of course, stratospheric (and even tropospheric) temperatures can fall well below those values. We saw, however, in Chapter 5, that in the speed range where the compressibility of air becomes a factor it is important to take into account the kinetic heating effect of the fast moving airﬂow being brought to rest (with respect to the aircraft) upon encountering the aircraft. The ambient (static) air temperature must therefore be corrected for speed. This

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adjusted, so-called total air temperature (TAT) is a much better indicator of what the fuel temperature might be on a long ﬂight and it rises with the square of the Mach number. For instance, the TAT around an aircraft travelling at Mach 0.6 through a −56 ◦ C (−68.8 ◦ F) airmass is around −40 ◦ C (−40 ◦ F). Speeding up to Mach 0.7 will increase the TAT to −35 ◦ C (−31 ◦ F), while by Mach 0.8 it will be −28 ◦ C (−18.4 ◦ C). In addition to the speed correction, the insulating effect of the boundary layer around the aircraft demands a further adjustment, to the extent that the fuel will be up to 3 ◦ C (5.4 ◦ F) colder than the TAT. With all this taken into account, it is clear that the fuel of high altitude aircraft still needs to be heated before it is fed into the engines. A fuel/oil heat exchanger, comprising a matrix of ﬁne tubes, accomplishes this – the oil passes through the tubes, the fuel passes around them. This helps keep the oil temperature down (typically around 90 ◦ C (194 ◦ F)) and the fuel temperature far enough above the freezing point to ensure safe operation (as noted in Chapter 3, the absence of such a system made the Comet prone to ice crystal accumulation in its fuel tanks and therefore loss of thrust). The weather brieﬁngs received by the ﬂight crew of G-YMMM before their 17 January ﬂight to Heathrow brought all this into sharp focus, as the forecast was for ‘extreme cold’. The ﬂight plan was devised around this warning, featuring a rather unusual step descent proﬁle: initial cruise at 34 100 feet was to be followed by a descent into warmer layers at 31 500 feet above Mongolia. In the event, as G-YMMM was climbing out of Beijing, air trafﬁc control changed the plan and requested that they climb to 34 800 feet and remain there. Captain Burkill and his crew accepted this, resolving to keep a close eye on the fuel temperatures. The forecast turned out to be largely correct. About seven hours into the ﬂight the thermometers of the British Airways jet, now at 38 000 feet above the Arhangelsk region in Russia, were recording a rather remarkable −73 ◦ C (−99.4 ◦ F). Figure 6.1 shows a snapshot of temperature proﬁles from upper atmosphere soundings from around the globe, taken an hour or so after G-YMMM crossed the Ural mountains and headed towards Scandinavia on its way to London – as the Arhangelsk sounding shows, the Russian winter was making its presence felt in the stratosphere too. Yet, there seemed to be no need for a descent, as the TAT remained above −45 ◦ C (−49 ◦ F) and the fuel temperatures in the tanks of the 777 also stayed above the warning limit. The Jet-A-1 continued to ﬂow normally, at an average rate of around 3.5 tonnes an hour into each of the Trent 800 engines, never cooling below −34 ◦ C (−29.2 ◦ F). Almost exactly at midday the crew of G-YMMM began their descent into London, levelling off ﬁrst at 11 000 feet, then continuing to descend in a holding pattern above Lambourne, with the fuel consumption down to about half a tonne an hour on each engine. Upon exiting the hold, the ﬂight received radar vectors for Runway 27L and, with the ﬂaps extended to 30 degrees, the 777 began its ﬁnal descent. The autothrottle was working fairly hard to maintain the target airspeed of 135 knots, changing the power setting as required, in response to gusts and attitude changes along the somewhat choppy approach path. Some of these accelerations involved sudden, short bursts of power, with the fuel ﬂow – the Jet-A-1 was now at−22 ◦ C (−7.6 ◦ F) – peaking at around 5.6 tonnes an hour. Then, with around three and half miles to touchdown, in response to the next power burst command, the right engine failed to respond. Seven seconds later the left engine rolled back to near idle thrust too – with the results we have already seen.

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Fig. 6.1 A snapshot of the temperature proﬁle of Earth’s atmosphere at midday (Greenwich Mean Time) on 17 January 2008. BA38 encounters unusual cold between the Urals and Eastern Scandinavia.

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6.3 Data mining Arhangelsk’s stratospheric cold spell, which the British Airways 777 had ﬂown through en route to London, was rather unusual, but by no means unique. At ﬁrst sight, none of the other air and fuel temperatures or fuel ﬂow numbers saved on G-YMMM’s ﬂight data recorder on its last ﬂight seemed too extraordinary either. Yet, by a process of elimination, some effect of the cold on the fuel system seemed to be left as the only possible cause. Of the possible temperature-related scenarios, the most straightforward one could be eliminated straightaway: waxing. As jet fuel begins to freeze, its long straight chain hydrocarbons are the ﬁrst to form wax crystals, which remain suspended in the liquid. For this, of course, the fuel temperature would have had to drop below −44 ◦ C (−47.2 ◦ F) and G-YMMM had not even come close to this value. What about water crystals forming in the fuel? This is obviously possible as soon as the temperature of the fuel drops below 0 ◦ C (32 ◦ F). The Jet-A-1 loaded into the tanks of the aircraft in Beijing was estimated to have contained around 3–4 litres of water – free, suspended or dissolved; however, just like the temperatures, this was not an unusual number. While the jet itself provided little useful evidence – with the exception of some cavitation damage to the fuel pumps, the fuel system seemed to be in working order – the careful study of another, similar incident, which occurred while the British Airways crash was being investigated, was to provide a signiﬁcant breakthrough. On 26 November 2008, a Delta Airlines Boeing 777 was cruising at 39 000 feet, enroute from Shanghai to Atlanta, when the power on its right engine suddenly reduced sharply. In spite of several commands from the autothrottle, the power remained low for the next 23 minutes. Following descent into warmer air, at idle power, the engine recovered and the ﬂight resumed without further incident. The fuel temperature at the moment of the uncommanded rollback was −22 ◦ C (−7.6 ◦ F), just like in the case of the British Airways ﬂight. Furthermore, a signiﬁcant increase in power – in the climb from 37 000 to 39 000 feet – preceded the rollback, once more, mirroring the G-YMMM event, where high fuel ﬂows were commanded on the ﬁnal approach. More interesting numbers were to emerge from a data mining study conducted by the investigation. 35 000 ﬂights of Trent-powered Boeing 777s, including the 17 January Beijing to London ﬂight, were examined, with a variety of ﬁlters applied to their data. These ﬁlters included constraints on fuel temperatures, total air temperatures, fuel ﬂow rates, ﬂight times, throttle settings and many other parameters and combinations thereof, most of them letting through large subsets of the 35 000 ﬂights. Eventually though, a set of criteria was found that only allowed one ﬂight through: low cruise fuel ﬂow, high fuel ﬂow during approach and low temperature on approach. The only ﬂight that ﬁtted this description was G-YMMM’s last ﬂight. Ultimately, thus, it appeared that the low temperatures during the cruise were not directly responsible for whatever phenomenon disrupted the fuel ﬂow into the Boeing’s engines – their effect was merely that the fuel temperature remained quite low during the approach. A picture began to emerge regarding the likeliest scenario that led to the rollbacks. During an eight hour cruise, the fuel temperatures on the 777 remained below −20 ◦ C (−4 ◦ F)

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and, as a result of relatively gentle step climbs, the fuel ﬂows never exceeded 4 tonnes an hour. They did, however, peak at relatively high values on the ﬁnal descent. The accident investigation involved extensive laboratory testing designed to replicate these conditions. These showed that ice could form in the cold fuel feed pipes. Moreover, it became clear that ice could be released from the fuel feed pipes when the ﬂow rates suddenly increased (as on G-YMMM’s ﬁnal descent) and form a restriction as it reached the matrix of pipes in the fuel/oil heat exchanger. The experiments also conﬁrmed the conjecture raised by the data mining exercise, namely that the fuel temperature dropping to −34 ◦ C (−29.2 ◦ F) during the cruise was not critical to ice formation. In fact, somewhat counter-intuitively, the most dangerous temperature range was found to be between −20 ◦ C (−4 ◦ F) and −5 ◦ C (23 ◦ F) – this is the ‘sticky’ range of ice, when it is most likely to adhere to the walls of the components of the fuel system, with −12 ◦ C (10.4 ◦ F) appearing to be the ‘stickiest’ [1]. Before the G-YMMM incident fuel system icing sat quietly on the scientiﬁc landscape of high altitude ﬂight in the ‘unknown unknowns’ corner. Although a B-52 bomber crashed over half a century ago as a result of ice accumulating in its fuel ﬁlters, the problem was thought to have ‘gone away’ as a result of two measures. Fuel heating systems (such as the fuel/oil heat exchanger on the 777) were introduced on the majority of high altitude aircraft, while chemical icing inhibitors2 , effective for fuel temperatures down to about −40 ◦ C (−40 ◦ F), were prescribed as fuel additives on the rest. The Heathrow crash, involving a modern aircraft with fuel of the correct speciﬁcation in heated tanks, unexpectedly brought the problem back... The thorough investigation conducted by the Air Accidents Investigation Branch, RollsRoyce, Boeing, QinetiQ and others made huge leaps towards a better understanding of the phenomenon (and, at the very least, showed the way to the re-design of the heat exchanger), but it can also be seen as having moved the problem to the ‘known unknowns’ ﬁeld on the landscape. Much of the mechanics of the deposition and release of ice crystals in fuel lines is still not completely understood. For instance, it is not entirely clear whether there are any signiﬁcant differences between the laboratory experiments and real ﬂight. Additionally, the exact temperature distributions and their variations inside large volumes of fuel are not always known with great accuracy. Nevertheless, at least a number of areas have been highlighted where further research may be due.

6.4 Melting evidence At mid-day on 12 July 2004 a Raytheon Beechjet 400A3 was cruising at 41 000 feet over the Gulf of Mexico, about 100 miles from Sarasota, Florida, about to commence the descent into its destination, Fort Myers airport. Air trafﬁc control cleared the ﬂight to 33 000 feet in the ﬁrst instance. As the Beechjet was descending through 39 000 feet the pilots felt a jolt and heard a bang, accompanied by the unsettling realisation that the cabin pressure was diminishing. By 35 000 feet the situation got much worse: every warning light was illuminated in the cockpit and it became clear that both engines had stopped (in fact, as the 2 3

Typically diethylene glycol monomethyl ether. Twin-engined business jet with a service ceiling of 45 000 feet (originally developed by Mitsubishi).

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subsequent investigation would reveal, they stopped at the same time as the pilots heard the initial ‘bang’ [109]). Several unsuccessful restart attempts followed, but, upon descending through 10 000 feet, the crew ﬁnally managed to get one of the engines started and made an uneventful emergency landing. Much to the surprise of all concerned, detailed examination of the jet revealed no faults in the two Pratt and Whitney Canada JT15D engines or, indeed, in any of the aircraft’s other systems, with the only anomaly being a lower than prescribed concentration of the fuel icing inhibitor in the (unheated) fuel system of the jet4 . On 28 November 2005 the crew of another Raytheon Beechjet 400A powered by JT15D engines began descending from 38 000 feet near Jacksonville, Florida, when they hear a loud bang and lost all power. Three attempts at engine restarts failed, but the pilots managed to glide the aircraft down for a safe landing at Jacksonville. Once again, apart from a battery somewhat depleted by the multiple restart attempts, the jet had to be given a clean bill of health5 . As bizarre as it may seem, we could go on. In fact, we will. 23 April 2000: A Beechjet 400A, en route from Curacao, Netherlands Antilles, to Belem, Brazil, was cruising at 41 000 feet when both engines ﬂamed out in quick succession. The pilots were able to restart one of them as the aircraft was descending through 26 000 feet and made a safe emergency landing in Macapa, Brazil. 14 June 2006: A Raytheon Beechjet 400A was descending from 38 000 feet near Norfolk, Virginia, when suddenly all power was lost. At 24 000 feet one of the engines restarted on its own. Upon landing, no faults were found on the aircraft, which then, after refuelling, continued on to Charleston. As in the case of the British Airways 777 crash, investigating this rather intriguing cluster of incidents amounted to ﬁnding the variables they all had in common. Beyond the obvious observation that the same type of aircraft, powered by the same type of engine was involved in each case, it was also clear that all of the engine failures occurred at high altitude, as the pilots throttled back the engines for the ﬁnal descent. It was, however, an analysis of the meteorological conditions that provided the critical clue. Each incident happened in the vicinity or directly above high altitude convection, associated with cumulonimbus cells, with their anvil-shaped tops spreading out into the stratosphere as high as 55 000 feet6 (see Figure 8.9 for an image of a high altitude cumulonimbus). So how can convective weather cause a high altitude ﬂame-out? Here we can draw another parallel with the fuel icing accident that brought down GYMMM, as once more we appear to be dealing with the surfacing of an ‘unknown unknown’ that was, until recently, thought to be a ‘known known’ peril: airframe and engine icing. Icing has been a menace to aviation ever since its early days and it still is, at least to the extent that it requires a great deal of attention from aviators and meteorologists. An understanding of what conditions can lead to airframe icing is an essential item on any ﬂying school syllabus – and herein lies the rub. Conventional wisdom has it that the strato4

National Transportation Safety Board incident ID ENG04IA012. National Transportation Safety Board incident ID DCA06IA007. 6 The conditions could not be established with certainty for the Brazilian incident, but such powerful convective activity is a daily occurrence in that part of the world [109]. 5

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sphere is a safe place when it comes to the danger of icing (22 000 feet is widely regarded as the limit above which icing is not a signiﬁcant concern [82]) – in fact, this is one of the main advantages of ﬂying high (Figure 6.2 shows the complex wing-de-icing system of a low-ﬂying turboprop).

Fig. 6.2 De-icing boot in action on the leading edge of the wing of a Bombardier Dash-8 Q400 turboprop. The boots are deployed in cycles – the left-hand (inboard) segment is shown deployed, that is, inﬂated. When the boot is depressurized, the ribs deﬂate (as seen on the outboard section). This repetitive action helps shed the ice resulting from supercooled droplets running up the leading edges. High altitude jets typically have heated wing leading edges instead.

Several different icing mechanisms are known, but super-cooled droplets cause the greatest danger to aircraft. These can, theoretically, be as cold as −42 ◦ C (−43.6 ◦ F) and still remain in a liquid state (they result from very rapid cooling of atmospheric water). If such supercooled water hits the surface of an aircraft (itself below freezing), it can run along it and freeze. Sometimes a seemingly small amount of such ice can disrupt the ﬂow over wings and control surfaces and cause a catastrophe. One of the most abrupt and most tragic accidents of aviation’s meteorological learning curve was the ﬁrst event to truly bring home the dangers of supercooled droplets. On 31 October 1994, an ATR 72 regional airliner was in a holding pattern at 8000 feet above Roselawn, Indiana, when ice accumulation on one of the wings caused an abrupt stall and a fatal spin all the way down to the ground. 62 passengers and four crew died. This, and the vast majority of all other incidents caused by supercooled droplets occurred, however, at temperatures between −15 ◦ C (5 ◦ F) and 0 ◦ C (32 ◦ F) – at stratospheric temperatures super-cooled water simply cannot exist. What can exist though in the upper layers of the troposphere and the stratosphere, typically above thunderstorm cells, is a ﬁne mist of tiny ice crystals lifted up there by powerful

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convection, sometimes to altitudes of over 40 000 feet. It is thought that their concentration above moderate thunderstorms is usually about 0.4 g/m3 , but above the more serious storms can rise as high as 3 g/m3 [78]. Once ingested into a turbofan aircraft engine, a large proportion of the ice crystals will ﬂy straight through the bypass duct and cause no harm. What happens, however, with those crystals that enter the core of the engine? Here is the scenario, pieced together by the National Transportation Safety Board, that is most likely to have led to incidents described above. The ice crystals are sucked into the air inlet of the engine. As they pass through the fan they begin to melt. They next hit the stator vanes at the entrance to the JT15D’s low pressure compressor, on the leading edges of which they can re-freeze and slowly accrete as the aircraft ﬂies across the top of a cumulonimbus anvil or, perhaps, downwind of the top of one. Next, the pilots close the throttles for the descent. This causes a sudden change in the angle of attack of the stator vanes, which dislodges the accumulated ice. If the quantity of ice is large enough, this, in itself, may cause the engine to surge (the ‘bang’ featuring in several of the pilots’ reports) and, eventually, ﬂame out. An alternative failure mode is that the ice blocks one of the critical pressure sensors inside the compressor, which will then transmit spurious readings to the engine control system. This, once again, can cause a surge and a ﬂame-out. It is somewhat unfair, perhaps, to single out the Beechjet 400A or its JT15D powerplants, on the subject of the ice crystal phenomenon, which, as a result of such events, is now more of a ‘known unknown’ or, rather, a ‘known incompletely known’. Such events have happened to other aircraft, powered by other engines too. The exact mechanism of the phenomenon can differ from one engine to the next, but, the pattern is believed to be similar in many cases. First, frozen ice crystals hit a warm surface in the engine. Some of them melt, leading to so-called mixed phase conditions, where ice and super-cooled water are present simultaneously. Further crystals fall onto the surface thus wetted and stick to it, gradually cooling it below 0 ◦ C (32 ◦ F) – ice can now begin to accrete. Once the ice is shed, surge, compressor stall and airﬂow reduction may result or, if the ice reaches as far downstream as the combustor, it can quench the ﬂame. The ice may also hit and damage compressor blades, which may later lead to vibrations across the turbomachinery. In any case, thrust loss and/or ﬂame-out may result [82]. The consensus across the industry appears to be that more research is needed for a better understanding of how ice crystals interact with turbofan engines. The detection of airmasses containing a high density of crystals is also an interesting technological challenge, as standard weather radar cannot see them (though, interestingly, they are sometimes visible to the naked eye, in the form of mist). Until such time as research in this direction progresses, however, all that remains is to re-iterate one of aviation’s most fundamental rules: steer clear of thunderstorms – an adage we shall revisit in Chapter 8.

7. Rivers of air

“The attack against the United States mainland in the near future by means of bomb-carrying stratosphere balloons manned by death-defying Japanese pilots was predicted by Lieutenant Colonel Nakajima, chief spokesman of Imperial forces in the southern regions, at his regular press conference.” Japanese propaganda broadcast, 4th June 1945 cited in [85]

7.1 Meteorology as a weapon of war World War II saw the advent of the high performance, high altitude, long range bomber. Flying routinely at altitudes approaching 30 000 feet, these bombers were capable of reaching deep into enemy territory. The aircraft that best exempliﬁes this aspect of 1940s warfare is, perhaps, the Boeing B-29 Superfortress. This enormous four-engined bomber arguably ended the war in the Paciﬁc, by delivering a series of devastating blows to the Japanese Empire, ﬁrst through conventional bombing raids on Tokyo and other targets, later by the apocalyptic nuclear strikes on Hiroshima and Nagasaki. With such long distance, high altitude missions becoming commonplace, pilots began to encounter a rather strange weather phenomenon, as they cruised at altitudes above 30 000 feet or so: unexpectedly ferocious, steady winds, in particular from the west. Sometimes exceeding 150 knots, these were extremely bad news for bomber crews if they encountered them head on. Of course, on the other hand, catching a 150 knot tailwind was a welcome development, but not knowing the altitudes at which these winds could be expected made for difﬁcult ﬂight planning either way. The notion that a fast moving stream of air was to be found at high altitudes was not lost on the Japanese either – in fact, as a post-war assessment of their upper atmosphere meteorological studies would reveal, they probably had the best grasp of the phenomenon.

A. Sóbester, Stratospheric Flight: Aeronautics at the Limit, Springer Praxis Books, DOI 10.1007/978-1-4419-9458-5_7, © Springer Science+Business Media, LLC 2011

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An exact explanation of the causes of these winds was only to emerge decades later, but the wind speed proﬁles of the upper atmosphere, as estimated by the Japanese Aerological Observatory, proved to be astonishingly accurate. The director of the Observatory, a physicist by the name of Wasaburo Ooishi (1874–1950), began to see patterns in the directions and seasonal variations of these winds too. Trained at Tokyo Imperial University and having worked in the US and in Europe (at the Lindenberg Aerological Observatory), Ooishi meticulously observed, recorded and averaged high altitude airﬂows over Japan throughout the 1920s. Unfortunately, his research was not disseminated very widely (if at all), owing perhaps to his rather puzzling insistence on writing many of his papers in Esperanto [80]. A turning point of the war in the Paciﬁc theatre was the Doolittle raid. On 18 August 1942 a wing of US Army Air Forces bombers commanded by Lieutenant Colonel James “Jimmy” Doolittle launched from the aircraft carrier USS Hornet and struck, for the ﬁrst time, at the Japanese mainland. With Japanese morale seriously dented, the military were looking for an effective means of striking back. Suddenly, Ooishi’s work seemed to provide the basis for a rather ingenious solution. Another scientist, Hidetoshi Arakawa of the Central Meteorological Observatory of Japan, combined low altitude wind patterns and observations of ocean currents across the Paciﬁc with Ooishi’s high altitude wind observations and came up with an estimate of a high altitude wind pattern that, during the winter months and in the early spring, could potentially carry a balloon from Japan all the way to the United States [80]. The ambitious threats of Lieutenant Colonel Nakajima cited at the top of this chapter never materialised, but a rather ingenious unmanned weapon was developed. As entrepreneur and Paciﬁc-crossing balloonist Richard Branson notes, this was, effectively, the ﬁrst inter-continental ballistic missile [18]. The ‘missiles’, consisting of payloads of up to 27kg of incendiary and high explosive bombs, were carried aloft by hydrogen balloons. The balloons, 10 meters in diameter, made of mulberry bark paper (see Figure 7.1), were ﬁlled with about 540m3 of hydrogen. Originally they were meant to be launched from ships and submarines, but this plan had to be abandoned because of the severe depletion of the Imperial Navy. Launch sites thus had to be set up in mainland Japan instead. The balloons were designed to control their own altitude during the Paciﬁc crossing in a band between 15 000 and 30 000 feet – this was to be achieved through the venting of some of the hydrogen and the release of sand ballast bags. The journey to the United States was expected to take about ﬁve days, at the end of which they would either descend to the ground or the ballast control mechanism would release the bombs [85]. To say that the ‘balloon enterprise’ had limited success is an understatement. Estimates vary as to the number of balloons that made it across the Paciﬁc, but of the 9000 launched, there are records of only about 300 balloons having been located in the United States and Canada. The ﬁnding of one of the balloons resulted in the only fatalities caused by this new weapon – a woman and ﬁve children were killed near Bly, Oregon, when one of the bombs exploded as they approached the strange craft [85]. In spite of the failure of the Japanese paper balloons to change the course of the war (or indeed, to have any signiﬁcant impact on it), there is little doubt as to the scientiﬁc signiﬁcance of the episode: there was now direct evidence of the existence of the high altitude wind pattern known today as the jet stream.

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Fig. 7.1 World War II Japanese ‘ballistic missiles’. Image courtesy of the US Air Force.

We shall not delve too deeply into the mechanics of Earth’s jet streams, as a proper treatment of the subject deserves a book of its own. From an aeronautical perspective it is worth, however, distinguishing between two major types of streams: both hemispheres have polar and subtropical jet streams. The subtropical jets, meandering above subtropical high pressure systems usually occur at higher altitudes, owing to the fact that the tropopause is, as we saw in the Prologue, higher at the lower latitudes. High temperature gradients are usually associated with the jet stream, marking the location of frontal zones. Correspondingly, the polar jet stream (lower, but generally stronger than the subtropical jet, on account of the stronger temperature gradients nearer the poles) is on the polar front, on the boundary between cool polar and warmer mid-latitude air. Both types of jet streams sometimes break up into several, smaller jets, but often there is a clearly deﬁned, central core. In certain fortunate circumstances this becomes visible to the naked eye, though not usually from the ground – the images shown in Figure 7.2 were taken from low Earth orbit. Of course, as Ooishi’s example indicates, charting the jet streams at a given moment does not necessarily require observations made from space (though they certainly help!) – in fact, there is a rather simple tool, which has been the cornerstone of upper air meteorology for well over half a century.

7.2 A half a kilogram stratospheric aircraft The vertical proﬁles of atmospheric variables (temperature, wind speeds, etc.) seen throughout this book (for instance, those in Figure 0.2, page XX) and hundreds of others plotted each day around the globe, come from what is, perhaps, the simplest stratospheric aircraft – indeed, one of the simplest aircraft of any type: the weather balloon. Wasaburo Ooishi also made his ﬁrst observations of the jet stream by releasing these small hydrogen or helium-ﬁlled gas balloons into the atmosphere and following their trajectories with a theodolite, inferring wind speeds and directions from it – in fact, between 1923 and 1925 he launched a total of 1288 weather balloons [137].

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Fig. 7.2 High altitude cloud layers sometimes reveal the location of the jet stream (NASA images taken from the Space Shuttle and the International Space Station).

The modern version of this simple device is the radiosonde. A small instrument package, generally weighing no more than about 300 g, the radiosonde is launched into the atmosphere on a latex weather balloon. The balloon is inﬂated on the ground with helium or hydrogen and its natural buoyancy lifts it, along with the attached radiosonde, steadily through the troposphere, the tropopause and into the stratosphere. As the ambient pressure reduces, so does the pressure inside the balloon, which means that the lifting gas expands, steadily increasing the diameter of the balloon. This is the standard weather balloon’s downfall (as it were), as the stress in the latex skin eventually reaches its tearing limit and the balloon bursts. The radiosonde then begins a long plunge towards the surface, with its descent slowed by a small parachute. Figure 7.3, a series of images taken by a camera attached to a weather balloon, illustrates the rather spectacular climb and descent of a radiosonde. Weather balloons come in different sizes and thus have different burst diameters. Routine atmospheric soundings (atmospheric variable observations through a range of altitudes) only require a latex balloon weighing about 350 g (the weight of the entire ‘aircraft’ adding up to barely more than half a kilogram). These are inﬂated to a diameter of just over

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Fig. 7.3 The pictorial journey of a weather balloon over Hampshire, courtesy of a University of Southampton student project. From top left to bottom right the images document the launch into the cold, gloomy morning, the ascent through a thin layer of stratus cloud into the bright sunshine and then all the way up into the stratosphere. The last four snapshots correspond to balloon burst and the long fall through the troposphere.

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one meter – the mass of the air thus displaced (and therefore the buoyancy of the balloon, as per Archimedes’ law) is sufﬁcient to lift the small radiosonde through the atmosphere at a rate of about 800-1000 feet per minute. A balloon this size bursts at around 80 000 feet, expanding to a diameter of about four meters – see Figure 7.4.

Fig. 7.4 Latex weather balloon ultimate strain test at the University of Southampton, Faculty of Engineering and the Environment. This 350 g balloon burst at a diameter of 4.2 m.

By this altitude the temperature, the humidity and the winds (the three key items recorded by most radiosondes) will have exhibited most of their interesting variations, so, in most parts of the world, a sounding up to 80 000 feet is sufﬁcient for the purposes of providing the input (boundary condition) data required by weather forecasting models. Reﬁnements in the manufacturing technology of latex balloons now allow larger sizes (up to about 4 kg), with correspondingly greater burst diameters and thus higher burst altitudes – for instance, a 1 kg balloon should be ‘good for’ about 110 000 feet. It is quite interesting to note that such a light, low cost, simple device can climb far higher than Concorde – in fact, routinely exceeding the cruising altitude of the SR-71 Blackbird!

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By ascending to 80 000 feet a radiosonde will also have pinpointed the location of the jet stream. Consider, for example, the set of atmospheric proﬁles shown in Figure 7.5, as mapped out by a radiosonde ﬂight. The temperature proﬁle shows a very clearly deﬁned stratosphere above 40 000 feet or so. The temperature lapse rate in the troposphere begins to diminish slightly at around 37 000 feet, indicating the start of the tropopause. Looking now across to the wind speed chart on the left, this is almost exactly where we ﬁnd the high speed core of the jet stream. The ‘river of air’ begins at around 23 000 feet, with the wind speed suddenly beginning to increase steadily, reaching 120 knots or so by 30 000 feet. Here the rate of increase reduces slightly, but a further sharp increase clearly deﬁnes a central, fast moving streak, which, as the temperature curve indicates, separates the troposphere and the stratosphere. The bottom row of plots shows the effect of this fast stream on the ﬂight of the weather balloon: during its 90 000 foot ascent it covered nearly 200 miles (a rather remarkable mission proﬁle for an aircraft mostly made of latex and styrofoam)! While one would expect the trajectory of a lighter-than-air aircraft, such as a weather balloon, to be so strongly inﬂuenced by the jet stream, its impact on a heavy jet airliner may be less intuitive. Consider the example of an airliner crossing a strong jet stream,

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Fig. 7.5 Wind and temperature proﬁles through the atmosphere in a location and at a time with a strong jet stream (top row) and the reconstruction of the radiosonde ﬂight that generated these proﬁles (bottom row). The symbols indicate: ‘o’ – launch, ‘*’ – balloon burst and ’+’ – touchdown. Note the strong (150 knot) core of the jet stream being positioned almost exactly at the tropopause (at around 40 000 feet), as indicated by the turning point in the temperature proﬁle.

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as reﬂected on its Electronic Flight Instrument System display shown in Figure 7.6. In order to maintain the desired ground track of 138◦ , the approximately westerly 152 knot jet stream is forcing the aircraft to crab along on a heading of 158◦ . Of course, even more dramatic are the effects of the jet stream when an aircraft is having to ﬂy through it head on – on an Atlantic crossing this typically results in a ﬂight time difference of about an hour between westbound and eastbound ﬂights between the same two airports. As we saw in Chapter 4, this sometimes raises interesting trade-offs in terms of altitude selection: higher altitudes may take the aircraft nearer to its minimum fuel burn point for a given weight, but the jet stream may completely nullify such calculations. World War II bomber crews, the ﬁrst ﬂyers to encounter the jet stream, were painfully aware of this, but, in the absence of reliable forecasts, they never knew how to revise their ﬂight plans – today’s pilots and airline dispatchers are in a much easier situation. The jet stream forecast models of today are extremely accurate, in spite of the tremendous complexity of the Earth’s atmospheric ﬂows. It is estimated that the average ﬂight time calculation error resulting from jet stream prediction inaccuracies on a Los Angeles – London Heathrow ﬂight is around one minute [70]. Yet, few passengers appreciate the enormous scientiﬁc achievements behind their captain being able to provide them with a pre-departure estimated arrival time that almost always turns out be accurate two meals and three in-ﬂight movies later.

Fig. 7.6 Crossing the jet stream. In ﬂight image of the Electronic Flight Instrument System display of an Airbus A320 aircraft showing winds approximately from the west (264◦ ) at 152 knots (175 mph), as the aircraft (depicted by the large cross symbol, bottom center) is having to crab along at about 20◦ to the right of its intended ground track (depicted by the long diagonal line). The tailwind component of the jet stream is helping the aircraft along though: the true airspeed is 438 knots, while the ground speed is 461 knots (top left corner). Photo courtesy of Captain Dave ( f lightlevel390.blogspot.com).

8. Rough ride

“Of course, a ﬁnal theory [of physics] would not end scientiﬁc research, not even pure scientiﬁc research, nor even pure research in physics. Wonderful phenomena from turbulence to thought would still need explanation, whatever ﬁnal theory is discovered.” Steven Weinberg (1933– ) ‘Dreams of a Final Theory’∗ In May 2000 the Clay Mathematics Institute of Cambridge, Massachusetts published a set of seven problems. Of all the problems of mathematics that have eluded solution so far, the Clay Institute regard these as being the most important; so important, in fact, that the successful solver of any of them can expect a one million dollar cheque from the Institute to fall through their letter box. The complete solution of at least one of these problems, however, promises scientiﬁc breakthroughs that might ultimately make the cash prize feel almost triﬂing by comparison: this is the question of the existence of exact solutions to the Navier–Stokes equations. The equations, bearing the names of Frenchman Claude-Louis Navier and Irishman Sir George Gabriel Stokes, describe the motion of ﬂuids. They have been known since the 19th century – indeed, they can be obtained quite readily by applying general conservation principles (conservation of momentum, mass and energy) to an inﬁnitesimally small ﬂuid element. They continue, however, to defy attempts at exact solutions (if, indeed, such solutions exist). According to Charles Fefferman, the author of the Clay Institute’s ofﬁcial problem statement, “There are many fascinating problems and conjectures about the behavior of solutions of the [...] Navier–Stokes equations. Since we don’t even know whether these solutions exist, our understanding is at a very primitive level. Standard methods from [the theory of partial differential equations] appear inadequate to settle the problem. Instead, we probably need some deep, new ideas.” ∗

Dreams of a Final Theory: The Search for the Fundamental Laws of Nature by Steven Weinberg, published by Vintage Books. Reprinted by permission of The Random House Group Ltd.

A. Sóbester, Stratospheric Flight: Aeronautics at the Limit, Springer Praxis Books, DOI 10.1007/978-1-4419-9458-5_8, © Springer Science+Business Media, LLC 2011

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Pending these new ideas, approximate, numerical solutions have been made possible by the advent of high performance computing. The parameters describing the ﬂow of air around aircraft or, indeed, around the planet, can be computed increasingly accurately and, especially in some particular cases and with certain simplifying assumptions, affordably. There is, however, a phenomenon, encountered in almost any type of ﬂow of any practical signiﬁcance, which limits the accuracy of such approximations, sometimes to the point of rendering them nearly meaningless.

8.1 The natural state of things At most length scales of interest in aircraft design (up to tens of meters) or meteorology (up to thousands of kilometers) the motion of air has a chaotic, random element: turbulence. Turbulence is ubiquitous – as Professor P.A. Davidson of the University of Cambridge remarks in his popular textbook on the subject [44], it is quite simply the natural state of things. The most compelling manifestation of turbulence is the familiar multitude of usually ephemeral and chaotically moving eddies (or whirlpools, vortices) emerging ceaselessly in a range of sizes (see Figure 8.1). The term ‘chaotic’ is meant here in its mathematical sense: a tiny change in the initial conditions of the airﬂow can have a very signiﬁcant impact on its subsequent behaviour. It is this feature that makes any predictions we are capable of today unsettlingly uncertain in both space and time.

Fig. 8.1 Turbulence occurs in nature at vastly different length scales: the spiral galaxy M51 in the constellation Canes Venatici (left), the ever-present, monumental storms of Jupiter (top right) and cloud vortices above the Paciﬁc. Images courtesy of NASA.

8.1 The natural state of things

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Horizontal average temperature

The further away we are in space and time from any known initial conditions (say, the temperatures, barometric pressures, wind speeds, etc. measured at a set of locations at a given moment in time), the greater this uncertainty. In fact, turbulent ﬂows can be viewed as rapidly losing all memory of their initial states, with any subsequent perturbations adding further unpredictability. This is a relatively recent observation, due to MIT meteorologist Edward Lorenz, whose 1963 paper ‘Deterministic Nonperiodic Flow’ [81] is widely regarded as one of the earliest milestones of modern chaos theory. Lorenz’s seminal article considers a much simpler problem than atmospheric ﬂow, that of the behavior of a convection cell cooled from above and heated from below1 , a system governed by three simple, ordinary differential equations. A helpful way of analyzing the solution of these equations2 is to plot the behavior of a ﬂuid particle over time in the so-called phase space, whose three dimensions are temperature, velocity and temperature averaged horizontally across the cell. The resulting trace is an iconic image, forever associated with the history of chaos theory: Lorenz’s butterﬂy attractor (Figure 8.2).

Velocity

Temperature

Fig. 8.2 The history of states visited by a ﬂuid particle in the convection cell, tracing the so-called ‘butterﬂy attractor’.

1

When a ﬂuid is heated from below and cooled from above, the rising warmer and descending cooler ﬂuid will organize into a pattern made up of cells of ﬂuid rotating as a result of the temperature (and density) differential – this is a simple example of a convection cell. 2 Like the much more complicated Navier–Stokes equations, in most cases these convection equations cannot be solved exactly either, though very accurate approximations can be obtained comparatively readily.

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The solution to the convection equation has a number of intriguing properties. First, the curve never intersects itself, that is, the system never revisits any of its previous states, regardless of the length of time we run the simulation for. Second, the solution keeps drawing one wing of the butterﬂy for a number of loops, then the other for a few more, before returning to the ﬁrst for more cycles, thus continuing forever – but it seems to be impossible to predict when these transitions will occur. In other words, the system swings back and forth between two states (represented by the two wings), without any apparent regularity. Furthermore, if we repeat the experiment with the initial conditions slightly altered (say, starting with a different initial temperature and/or velocity), an entirely different sequence of swings will almost certainly result. Summed up in one word: chaos. The signiﬁcance of Lorenz’s observation was that, for the ﬁrst time, it became evident that the accuracy of our predictions of turbulent atmospheric ﬂows (a much more complex problem than the convection cell) may not keep on improving endlessly with the advent of ever faster computers. The equations governing the ﬂow (as we understand them today) appear to have an insidious intractability that becomes increasingly pernicious with the passing of time since the last known measurements – the initial conditions of the problem. While the chaotic nature of airﬂow thus poses extreme difﬁculties in terms of the accurate numerical solution of the Navier–Stokes equations, other mathematical tools have, over the last half century, contributed to an increasingly clearer picture of some of the key features of turbulent ﬂow. Most notably, regarding a turbulent ﬂow as a mean motion corrupted by some random3 ﬂuctuations opens the door to statistics, as an essential investigative tool. Thus, statistical models applied to experimental observations of turbulent ﬂows have highlighted the mechanism of energy transport across the various length scales of turbulence, as a possible key to understanding and forecasting the turbulent motion of air. The turbulent ﬂow gains its energy at the largest scales, on its largest eddies. In atmospheric ﬂows these can be as large as a thousand kilometers or more across – they are the objects of synoptic scale meteorology. Inertial forces drive the motion of air here. The kinetic energy then cascades down to ever smaller eddies, eventually dissipating into heat at the Kolmogorov microscales (thus named after Andrey Nikolaevich Kolmogorov, one of the 20th century’s most versatile mathematicians). The heat emerges here from the work done by the ﬂow against the viscous stresses in the shearing layers of the air. Turbulent ﬂows and their mathematical intractabilities affect aeronautics in different ways across this spectrum, but perhaps the larger scales of the free atmosphere are where their impact is the most dramatic – a point illustrated by the events of 9 December 1992.

3

There is no consensus as to what ‘random’ actually means here. For example, theoretical mechanician Richard Scorer insists in his classic environmental aerodynamics text [114] that one should read no more into the word than ‘containing details we shall not enquire into’, while others would go slightly further by deﬁning ‘random’ as ‘unpredictable in detail’.

8.2 ‘Just so you know...’

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8.2 ‘Just so you know...’ Denver lies at an altitude of almost exactly one mile (‘the mile-high city’), as does its northern neighbor, Boulder. Rising a further 9000 feet above them and dominating the skyline to the west is the imposing Front Range of the Rocky Mountains, stretching across northern Colorado in a north-south direction. Approaching from the east, this is where the Great Plains come to an abrupt end. The peaks of the Front Range also form part of the Continental Divide: all rivers to the West of here drain into the Paciﬁc Ocean, while rivers on the Great Plains side of the mountains drain either into the Arctic or the Atlantic Ocean. On 9 December 1992 Boulder awoke to stiff, cold winds blowing down the slopes of the Rockies with a frontal zone approaching from the northwest [34]. While winters are generally moderate in the region, the locals are used to wind storms – indeed, each winter brings, on average, seven or eight signiﬁcant wind storms with blasts of up to 90 knots recorded on several occasions on the eastern approaches to the Front Range.4 Just over a thousand miles to the east, in Dayton, the three-man crew of cargo ﬂight Connie Kalitta 926 were waiting in the much calmer Ohio morning for 20 tonnes of cargo destined for San Jose, California to be loaded onto their Douglas DC-8 jet. Their day had begun egregiously early, at 4.00 am, when they reported for duty at Toledo airport. The ﬁrst, ‘red eye’ trip of this December day was a twenty-minute hop to Dayton, where they landed at 5:35 am, still in pre-dawn darkness. With the freight loaded onto the large, fourengined jet, they took off once again two hours later for the approximately six-hour ﬂight to the West coast. The ﬂight plan had called for an initial cruising altitude of 28 000 feet. The ﬁrst couple of hours of the ﬂight were largely uneventful, before the crew began experiencing episodes of ‘light to moderate’ turbulence near Kansas City. It is probably worth mentioning at this point that terms like ‘light’ and ‘moderate’, when used by pilots to describe turbulence, refer to the effects on the aircraft and its occupants of ﬂying through a zone of turbulence, rather than relating to any objective measure of the extent to which the air mass the aircraft is ﬂying through is turbulent. This is a rather subtle, but important distinction (especially to a meteorologist or an aerodynamicist!), much like that between an earthquake intensity scale (measuring damage and the human perception of the oscillations) and an earthquake magnitude scale, measuring the energy released. Of course, knowing that such terms merely describe the human and aeronautical impact of a turbulence encounter makes them no less ambiguous to the layperson, especially because a pilot’s ‘moderate’ may be deﬁned in somewhat stronger terms by a nervous passenger. Table 8.1 could be used as a sort of dictionary aimed at deciphering this aspect of pilot-speak; it might also explain why, after a few further minutes of ‘moderate turbulence’, the crew of Connie 926 requested (and was cleared for) a climb to 31 000 feet in an attempt to ﬁnd calmer conditions (the fuel load on board was still too heavy for further climb). The freighter was just crossing the Colorado state border5 .

4

The fundamental reference on Boulder’s much studied windstorms is the systematic study by Brinkmann [21], from which the ﬁgures cited here originate. 5 The reconstruction of the details of Connie Kalitta 926 and other ﬂights in its vicinity is based on [94].

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Table 8.1 While there is no universally agreed scale of turbulence intensity, the above list should give an indication of the ratings used by most commercial pilots. Description

Flight deck

Cabin

Light chop

Slight bumps, sometimes similar to riding on cobblestones.

Barely noticeable, passengers able to walk around the cabin normally.

Light turbulence

Slight, random up and down movements. Acceleration peaks between about 0.8 G and 1.2 G.

Walking becomes difﬁcult. Flight attendants usually continue cabin service (cautiously). Drinks don’t splash out.

Moderate chop

Sharp jolts, but no signiﬁcant changes in altitude.

Strapped in passengers feel the strain against their belts. Drinks splash out. Cabin service stops.

Moderate turbulence

Strong, intermittent jolts. The aircraft is controllable at all times, but the altitude, the attitude and the speed ﬂuctuate signiﬁcantly. Vertical acceleration peaks between about 0.5 G and 1.5 G

Cabin crew sit down (on the ﬂoor if no seat is available). Walking is very difﬁcult.

Severe turbulence

Abrupt, signiﬁcant changes in altitude. Aircraft out of control at times. Instruments hard to read. Intentional ﬂight into these conditions prohibited.

Strapped in passengers feel intense strain against seat belts. Walking is impossible. Unsecured objects lift off the ﬂoor.

Extreme turbulence

Aircraft impossible to control. Structural damage likely.

Increased risk of injury.

Meanwhile, the weather in Boulder was getting increasingly stormy, with the cold, northwesterly wind now gusting occasionally to as high as 70 knots down the slopes of the Front Range. The effects of the wind blowing over the Rockies were beginning to be felt at high altitudes too, as reports of ‘mountain wave action’ started coming in from the crews of several aircraft crossing Colorado, a few reporting severe turbulence well into the stratosphere, some from as high as 39 000 feet. Mountain waves form as wind hits exposed slopes (the air cooling down during the ascent) and descends as it passes the summit. Even if the surface is completely ﬂat on the leeward (sheltered) side of the mountain, this initial disturbance often triggers several further cycles of air masses rising and sinking downwind, a phenomenon known to air crews as a frequent cause of turbulence. While at high altitude the intensity of the resulting ‘wave action’ hardly ever exceeds moderate levels, in the close proximity of big mountains these invisible up- and downdrafts have actually been known to be savage enough to cause serious damage to large aircraft. In March 1966 a British Overseas Airways Corporation Boeing 707, ﬂying at about 4000 feet above the summit of Mt. Fuji, shortly after departing from Tokyo, hit extremely intense mountain waves on the downwind side of the iconic volcano. 124 passengers and crew perished as the aircraft broke apart mid-air in the violent turbulence6 . The physics of these sometimes destructive and often invisible and 6

Wurtele [136] discusses another fatal mountain wave encounter at a relatively low altitude, the 1964 crash of a Paradise Airlines Lockheed Constellation at Lake Tahoe, California.

8.2 ‘Just so you know...’

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undetectable waves is rather interesting and we shall consider the subject presently, but for the moment let us return to that December morning over Colorado. The crew of Connie 926, now at their new altitude of 31 000 feet, were ﬁnding that conditions were much the same as those they had experienced a kilometer lower down a few minutes before. A FedEx freighter and a TWA airliner crossing the same sector were also ﬁnding ‘light to moderate chop’ in the lower stratosphere, with the FedEx crew eventually requesting descent to 29 000 feet to hunt for calmer air in the troposphere. The TWA ﬂight soon made a similar request, though by now there seemed to be no evidence of smooth rides at any altitude, anywhere near the Front Range. Over the next few minutes the ride reports received by the Denver sector controller were indicating a further worsening of the conditions. The crew of a Boeing 727 that had just departed Denver Stapleton airport was reporting climbing through severe turbulence over the Rockies. A UPS ﬂight, freshly arrived into the sector, was requesting a change of altitude, as it was “getting beat up pretty good” (say again, Captain?). The most urgent sounding request, however, now came from Connie 926: “Sir, we need lower right now.” The clearance arrived promptly, but it now had little relevance to the crew of Connie 926. The DC-8 was violently tossed upwards, nose ﬁrst, by about 500 feet, slowing sharply from 450 to 430 knots, before plummeting back down, rolling 20◦ left and then right and accelerating to 500 knots in the process – all this within the space of ten hair-raising seconds. At the same time the number one engine ﬁre bell sounded, prompting the crew to retard the throttle on that engine and shut it down according to the corresponding emergency checklist, as well as calling air trafﬁc control reporting damage to the aircraft and requesting an immediate emergency landing at Denver. It was 10:10 – they had been airborne for just over two hours, approaching a third of the way to what should have been their destination. The controller issued instructions for what was going to be a tricky approach avoiding densely populated areas into a rather windy Denver Stapleton airport, where the ﬁre brigade was already moving into position to meet the potentially seriously damaged DC-8 arriving with four hours’ worth of fuel on board. Twenty minutes later Connie 926 was on ﬁnal approach, close enough for the Denver airport tower controller to spot the crippled jet and to report to its crew over the radio on the unlikely sight: “Connie 926 heavy, just so you know, the number one engine is missing from the aircraft...” Clearly, the extreme turbulence encounter left the jet spectacularly worse for wear, but there was good news for the crew too: all three landing gears appeared to be down, in spite of a warning light indicating the opposite on the control panel of the DC-8. Eventually, in spite of the aircraft being far above its landing weight (there had been no time to dump any of the fuel carried for the remainder of the aborted California trip) and having sustained signiﬁcant damage, the touchdown and the landing roll were normal and there was no ﬁre – apart from anything, the expected source of ﬁre, engine number one, was in the mountains, several miles away, perhaps never to be found again. As the aircraft taxied off the runway,

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it became clear that a large section of the leading edge of the left wing had also been lost, a long gash revealing some of the internal structure of the wing. The detached engine had also taken its pylon with it as it ‘liberated’ itself from the wing. In short, the crew of Connie 926 had every right to be commended for some remarkable airmanship. It was equally clear though that the conditions they had encountered at 31 000 feet were entirely out of the ordinary and, as we are about to see, the details of the events of 9 December 1992 would keep the meteorologists intrigued for years to come.

Fig. 8.3 Damage to N810CK (ﬂying as Connie 926 on 9 December 1992) after encountering severe turbulence over Colorado.

8.3 Serendipity As we shall see later in this chapter, telltale clouds and precipitation reveal the location of much of Earth’s most turbulent weather, either directly to the standard issue pilot eyeball or through electromagnetic waves they return to a weather radar. Connie 926, however, encountered an instance of the much more insidious clear air turbulence. This eludes direct detection and even when a prelude of light or moderate chop precedes severe episodes

8.3 Serendipity

139

(as in the case of Connie 926), the visually and electronically invisible dry air mass ahead gives no clues for plotting a safe evasive course. While the Colorado encounter thus came without real-time warnings, unusually for clear air turbulence, a rich record of what happened above the Front Range on 9 December 1992 emerged subsequently. This unexpected gift to forensic meteorologists was due, indirectly, to a rather portentous event that had occurred the previous year on the other side of the world. In June 1991 the Philippine volcano Mount Pinatubo erupted, ejecting an estimated 19 million tonnes of sulphur dioxide7 into the stratosphere, which quickly spread around the globe. In all the drama of the natural catastrophe there was one serendipitous effect: weather phenomena normally invisible to electronic ‘eyes’ brieﬂy became detectable by lidar. Lidar8 is a remote sensing technology based on measuring light (normally in the form of powerful laser pulses) backscattered by clouds or aerosols. The presence of large quantities of sulphur dioxide in the post-Pinatubo atmosphere signiﬁcantly enhanced this backscattering effect even in dry air and Colorado meteorologists (among others) took the opportunity to study the airﬂow patterns over the Front Range, attempting to gain an insight into the peculiar windstorms the residents of Denver and Boulder were so familiar with. Fortuitously then, on 9 December 1992 the usual array of weather observation instruments was augmented by a lidar installation sighted a few miles to the north of Boulder, sweeping much of the Colorado section of the Rockies. Thanks to the availability of this wealth of data and the immense scientiﬁc interest generated by the dramatic case of Connie 926 (serious structural damage caused by turbulence at high altitude is by no means mundane), the decade following the incident saw a great deal of analysis and computational modeling of the atmospheric events of 9 December 1992, most notably at the National Center for Atmospheric Research (NCAR). Thanks to their work we now have a fairly clear idea as to how a number of notable elements of the atmospheric conditions may have conspired to form a freak ﬂow pattern that made for Connie 926’s frightening ride above the Front Range on a day that to forecasters and to residents of Boulder seemed to promise little out of the ordinary. The most intriguing aspect of the puzzle was that none of these elements, when viewed in isolation, could account for the ferocity of the turbulence encounter. The ﬁrst component was a general north-westerly ﬂow accompanying the arrival of a weak cold front. Second, a prominent jet streak (fast winds embedded within the jet stream) was observed near and just above the altitude of Connie 926 (Figure 8.4). Finally, as we have already seen, the north-westerly wind hitting the Rockies created mountain waves, far downstream of the Front Range. So far, so commonplace. So what was wrong with this picture? At this point it is probably worth taking a closer look at how a mountain wave actually forms. Consider a parcel of air moving towards a mountain side in a stable atmosphere, that is, an atmosphere whose heavier (denser) layers are nearer the surface. As the slope pushes the parcel upwards, it gradually cools, so that by the time it reaches the summit, it is considerably denser than the surrounding air – it has been displaced from its normal, equilibrium position. Much like a deﬂected pendulum, it then tries to return to this equilibrium, 7 8

See [14] for a geological account of the eruption. Short for LIght Detection And Ranging.

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Fig. 8.4 Winds and temperatures aloft recorded at Denver Stapleton airport on the morning of Connie 926’s (aircraft registration N810CK) encounter with severe turbulence at 31 000 feet. The black arrows indicate wind directions and speeds (the latter proportional to the arrow lengths) at various altitudes (data from the docket attached to [94]). The temperature color bar on the right is positioned against the altitude axis according to the International Standard Atmosphere temperature proﬁle.

ﬂowing down the other side of the mountain and, as it accelerates down the other slope (fast enough to generate windstorms such as those occasionally experienced in Boulder), it overshoots the equilibrium position (once again, like a swinging pendulum would), that is, it sinks until colder air surrounds it. Buoyancy then acts as the restoring lifting force, but the ascent overshoots the equilibrium point again – and several such cycles follow until eventually turbulence and friction forces extinguish the wave. The damping of these oscillations can, however, be weak enough to allow the waves to extend far downstream of the topography that caused them – this can sometimes be seen even with the naked eye when the temperature at the wave crests is low enough for the water in the air parcel to condense and form clouds (see Figure 8.5). Such cloud patterns also show that the oscillations form a standing wave, that is the waves themselves are stationary as the air ﬂows through them. Most pilots are familiar with the typical sensation of ﬂying into such a standing wave – this is the ‘wave action’ many ﬂight crews approaching the Front Range were reporting to air trafﬁc control. However, the crew of Connie 926 encountered something rather more dramatic – after all, few would expect to have an engine ripped clean off a large airliner’s wing merely as a result of ﬂying into mountain waves, especially as high as the upper troposphere or the lower stratosphere. A possible explanation for the unusual ferocity of

8.3 Serendipity

141

Fig. 8.5 Mountain waves generated by a south-westerly wind blowing over a volcanic mountain (Amsterdam Island, Indian Ocean, seen near the bottom left corner) are made visible by clouds on this remarkable image captured by the Terra satellite. Image courtesy of NASA.

this wave encounter was discussed in a 1997 issue of the journal Geophysical Research Letters by Ralph et al., who, examining the lidar data from the Boulder installation, found evidence of wave breaking on 9 December 1992. Nature’s most obvious display of this phenomenon is on the surface waves of oceans: as the wave crests steepen, the smooth water surface suddenly breaks down into a complex, dynamic, turbulent pattern, turning into white froth and spray at the surface. Something akin to this can happen with mountain waves too: as their vertical propagation intensiﬁes, they may suddenly become unstable and break down amidst a release of turbulent energy, often at very high altitude. Flying into such a turbulent wave crest is quite a different matter from encountering a run-of-themill standing wave.

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Unfortunately, the reliable operational prediction of when mountain waves might break is difﬁcult (probably still years away)9 , but expensive and extremely time-consuming subsequent analysis of computer models based on lidar data can reveal evidence of wave breaking – this is what happened in the Colorado case. Further results published in 2000 by scientists at NCAR and the University of Colorado [34] revealed another sting in the tail, suggesting that the high speed jet streak observed in the stratospheric altitude band just above Connie 926’s cruising level also played a part in the almost unprecedented turbulence event. Their paper, published in the American Meteorological Society’s Journal of the Atmospheric Sciences, puts forward the intriguing hypothesis that “an arc-shaped downburst of turbulent kinetic energy appeared to result from constructive interactions between the jet stream undulations and the cross-mountain internal gravity wave ﬂow”. In other words, they have found that the precise juxtaposition of the undulations in the jet stream and the waves generated by the winds hitting the western slopes of the Rockies may have hit a freak optimum, where the two ampliﬁed each other, causing wave breaking and turbulence of extreme magnitude.10 One legacy of the Connie 926 incident is thus a better understanding of this particular mechanism behind the occurrence of clear air turbulence, potentially leading in future years to the development of reliable methods for the routine forecasting of such freak events. Bullet-proof forecasting methods are also lacking, at the moment, when it comes a different phenomenon, suspected by many meteorologists of being the cause of the majority of high altitude clear air turbulence events – this is what we turn to next.

8.4 Billows Consider two ﬂuids moving alongside and in contact with each other, one faster than the other. Over time, the initially straight interface between the ﬂuids begins to deform. First a series of small undulations appear in it, then these grow into larger billows. As the billows increase in amplitude, they begin to curl back onto themselves and eventually they break down and the two ﬂuids are now separated by a thick, turbulent layer. Figure 8.6 shows an example of this so-called Kelvin–Helmholtz instability developing between two ﬂuids. Two different ﬂuids make for a neat visualization, but Kelvin–Helmholtz instability can occur within a single ﬂuid, provided that there is a strong velocity shear, that is, neighboring layers move at different speeds. Take the example of a fast jet stream blowing along the tropopause or perhaps the lower stratosphere, with slower moving air-masses above and below (we can revisit Figure 8.4 for an example) – the conditions may be perfect for Kelvin–Helmholtz billows to develop on the upper or lower boundaries of the jet stream. The reason for the “may be” is that it is not entirely clear what the triggering circumstances are, though there are some telltale markers. 9

In a review of the state of the art in mountain wave modelling, McCann [83] discusses the numerical difﬁculties associated with ﬂow modeling at a high enough resolution that would identify wave breaking, though he notes that there are a number of relatively easily identiﬁable telltale signs frequently associated with the phenomenon. 10 A conclusion reinforced recently by Vollmer [128].

8.4 Billows

143

Fig. 8.6 Laboratory demonstration of a Kelvin–Helmholtz instability. Two layers ﬂowing from left to right join downstream of a thin plate (visible on the left of the top photograph). The upper and faster moving ﬂuid is slightly less dense than the ﬂuid below. Waves ﬁrst turn into billows and later degenerate into turbulence [43].

One such metric is the Richardson number (Ri)11 , a measure of the ratio of the potential and kinetic energy in the system, or, put a different way, the ratio of the density gradient and the velocity gradient. Essentially it conveys the relative importance of buoyancy with respect to inertial forces. Theoretical studies in the early 1960s indicated that low values of Ri, typically around 0.25, would be associated with a high probability of turbulence. Observational studies conﬁrming these predictions began to emerge in the late 1960s and the early 1970s with the ﬁrst radar observations of Kelvin–Helmholtz instabilities in the upper troposphere and the lower stratosphere. These studies also gave an insight into the 11

Named after English mathematician and meteorologist Lewis Fry Richardson (1881–1953), who proposed numerical weather forecasting formulations in the 1920s, three decades before the advent of electronic computer – a true visionary.

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time- and length scales at work. Browning [22], working at the Royal Radar Establishment in 1970, identiﬁed 16 instances of the phenomenon over a ﬁve month period, with durations ranging from a mere two minutes to over four hours (only ﬁve lasted for more than 15 minutes). The maximum crest-to-trough amplitudes of the billows were between 220 and 450 meters. The wavelengths showed signiﬁcant variability, with the shortest measuring 800 meters and the longest 4 kilometers. Similar ﬁgures have since been recorded using more accurate instruments. For example in March 1995 scientists working at the Max Planck Institute in Katlenburg, Germany, managed to capture a series of high altitude Kelvin–Helmholtz billows using VHF domain interferometry [30]. At an altitude of around 30 000 feet they found evidence of billows with a peak-to-trough amplitude of around 230 m, underneath the jet stream. The instability stretched over approximately 27 kilometres, with a wavelength of about 4 kilometers. This line of billows persisted for about ten minutes. Some analysts go as far as arguing that this is how most clear air turbulence happens [22], but this is hard to verify, because – and here is the bad news when it comes to trying to avoid regions with Kelvin–Helmholtz instability – the phenomenon is still hard to observe via radar, especially at high altitudes (not least because of its highly ephemeral nature). Low Richardson numbers, now known to be a necessary, but not sufﬁcient condition for the development of Kelvin–Helmholtz instability, are also proving to be a not entirely reliable turbulence forecasting tool. They are therefore usually viewed in conjunction with other telltale signs and conditions that, through the experience of many millions of ﬂight hours over the last few decades, have become associated with clear air turbulence. For instance, for the jet stream edges (typically the poleward side) to be likely locations for signiﬁcant turbulence, the speed of the core of the jet generally has to exceed 100 knots. A vertical windshear greater than about 4 knots per 1000 feet of altitude change is another warning sign, as is a horizontal windshear of over 20 knots per degree of latitude (the latter would indicate moderate turbulence, 30 knots over the same distance is a predictor of severe clear air turbulence) [134]. A strongly curving jet stream is also likelier to have turbulent edges than a straight one. Recent research has focused on combining these, as well as a host of less intuitive metrics (for example the time-variation of the Richardson number, eddy dissipation rates, etc.) into one predictor of turbulence probability [116], which is, for example, a weighted sum of the individual metrics, where the weightings are chosen such that the combined metric does the best job at predicting a large pool of past turbulence observations, based on the metrics as recorded hours before those observations were made. A similar artiﬁcial intelligence technique can, in principle, be applied ‘in real time’ to the returns generated by the on-board radio frequency pulse Doppler weather radars most passenger airliners are ﬁtted with. The radar can paint a picture of any precipitation in the air mass ahead of the aircraft (typically over a range of up to about 150 miles). The colored returns displayed on the screen also contain information about the estimated speed of the airﬂows the water droplets travel with (these are calculated based on the phase shifts in the backscattered radio waves) – see Figure 8.7 for an illustration. The color spectrum of the return usually ranges from green (possible light rain) to magenta (intense thunderstorm) and it thus contains relatively clear (though sometimes not

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Fig. 8.7 The weather radar display of this Airbus A330 (second screen from the right) paints a clear picture of a cluster of cumulonimbus clouds, which, in this case, can also be seen through the windscreen, towering menacingly to the left of the aircraft. Photograph courtesy of Darren Howie.

entirely accurate12 ) information relating to convective weather, thus allowing pilots to navigate around this most pernicious type of turbulent air mass (of which age-old enemy of aviation more presently). It offers, however, no explicit information on the more insidious variety, the clear air turbulence, which occurs in air masses only marked by much smaller particles, much ﬁner aerosols. There might, nonetheless, still be implicit information concealed in innocuous-looking (perhaps mostly green) returns, which a machine learning algorithm like that outlined above could use, perhaps in conjunction with other data, to forecast clear air turbulence. It is important to underline here that this method merely makes better use of the information provided by the radar on the larger droplets of air it can see. It does not, however, ﬁx the fundamental shortcoming of the standard pulse Doppler weather radar that it cannot detect very small water droplets and other particles in the air, which might visualize turbulent ﬂows even when the moisture content is relatively low (as in the case of most clear air turbulence). The minimum particle size detectable by any electromagnetic detection and ranging device is about the same order of magnitude as the wavelength of the wave

12

There are a series of phenomena that can cause false echoes and other inaccuracies on a pulse Doppler weather radar, with ambiguities typically increasing with range. Additionally, it is generally difﬁcult to paint a picture of the precipitation behind a squall line characterized by heavy rain.

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used – in the case of radar this means that droplets of the size that make up clouds and precipitation are visible, but ‘clear(er) air’ is not. The obvious, though technologically rather challenging solution is therefore to use waves of higher frequency – such as light. Lidar uses a high frequency pulsed laser, which enables the visualization of much ﬁner aerosols. Of particular relevance for aviation is the Doppler Lidar, which, by a process analogous to that of the Doppler weather radar estimates the speed of the airﬂow along its beam from the frequency shift of the backscattered light and it is able to do this even in the absence of precipitation or cloud. This is an extremely useful tool in modern ground-based meteorology and there is ongoing research aimed at developing a practical Lidar small and light enough to be ﬁtted routinely to passenger aircraft [63]. In spite of all these rather impressive advances though, many aspects of the forecasting and detection of clear air turbulence are still widely seen as open research questions, an observation mirroring perhaps the words of Steven Weinberg quoted at the top of this chapter.

8.5 But for a chime “...To release the seat belt, just lift the buckle. Once we’re airborne, the captain will turn off the seat belt sign, but for your safety, we would like you to keep your seat belt fastened when you’re seated [...] If needed, an oxygen mask will appear automatically. When you see the mask, please extinguish your cigarette13 [...] Enjoy your ﬂight and thank you for ﬂying the friendly skies...” United Airlines Flight 826, with 374 passengers and 19 crew on board, began its takeoff roll on the active runway at Tokyo Narita airport shortly after 21:30 on 28 December 1997. Less than half an hour later, after a smooth ascent, the Boeing 747-100 leveled off at 31 000 feet, settling into its approximately six hour cruise to Honolulu. The initial ﬂight plan had called for a 35 000 feet cruise; for trafﬁc reasons United 826 was, eventually, only cleared to this lower altitude. The crew had been, however, granted their choice of track for the crossing (several pre-ordained routes are available between Narita and Hawaii, allocated to ensure separation between aircraft over the large expanse of water not covered by air trafﬁc radar). The captain had opted for Track 12, as this was the only available route not to cross any of several areas over the Paciﬁc where severe turbulence or intense thunderstorm activity had been forecast. Nevertheless, even this track did not promise a completely clear ride, with patches of embedded cumulonimbus cloud (that is, cumulonimbus partially enclosed by other types of cloud) forecast to stretch well into the stratosphere in an area they were going to reach about two hours into the crossing. Clearly, this was going to be an evening of keeping a close eye on the weather for any signs of convective activity up ahead. This being a busy route, several additional pairs of eyes were doing the same in the vicinity: a few minutes after the United ﬂight another 747, Northwest Airlines Flight 22, took off on the same route. From about 30 miles behind 13

O tempora, o mores! The exact wording of the safety announcement and the reconstruction of the rest of the events of that evening are based on [95].

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and 2000 feet above Flight 826, the Northwest pilots could see in the clear, starry night the ﬂashing strobe lights of the United 747 in the distance. Both crews were also in radio contact with Northwest Airlines Flight 10, around half an hour ahead on the same route. Having reached its cruising altitude, United 826 ﬂew through a brief spell of slight wave action, as a result of which, in the light of the pre-ﬂight weather brieﬁng, the captain turned the seatbelts sign on as a precaution. Fifteen minutes later, with no further waves encountered and the neighboring ﬂights also reporting nothing worse than occasional light chop, the captain decided to turn them off again, though he made an announcement to the passengers telling them about the weather forecast he had received and reminding them that turbulence was always possible at that time of the year anyway and they should keep their seat belts fastened when seated. A short time later a ﬂight attendant made a similar announcement in Japanese, after consulting with the purser on what wording would create the least alarm in the cabin. A calm hour later, during which the evening meal service had begun throughout the two decks of the 747, the purser was standing in the forward galley when he saw the seat belt sign come on once more. He continued working, as not much more than the earlier wave action could be felt – for about two minutes. Then something altogether more terrifying happened. The 747 dropped slightly, giving the purser just enough time to grab a nearby countertop. The next thing he knew was that the aircraft shot up, then back down with such amplitude that his body swung around until he was hanging upside down, still holding on to the countertop, with his feet smashing into the ceiling. The jet then pitched up and climbed steeply, ‘as if riding up the front of a wave’14 , before a huge fall, accompanied by the right wing dropping sharply. Another climb followed, though this time at a more moderate rate and the aircraft ﬁnally returned to normal ﬂight. The whole encounter lasted a little over ten seconds, but it left the cabin resembling a war zone. The aisles and the galleys were littered with the remains of hundreds of meals, broken glass, several trolleys, overturned bins and other assorted debris. Large panels were hanging from the ceiling in places, as were a number of oxygen masks, which a few confused passengers were now scrambling for. More worryingly though, several ﬂight attendants and numerous passengers who had not been strapped in, were now coming to terms with serious injuries. A Japanese woman, who had been sitting in 46F, with her seat belt unsecured, was now lying in the aisle, unconscious and bleeding heavily. She was immediately attended to by several ﬂight attendants (many of them nursing injuries themselves), but, in spite of their resuscitation efforts, a shortly arriving doctor (another passenger) could merely pronounce her dead. Thankfully, she was to remain the only fatality of the evening, but many others were in serious condition. All told, 15 passengers and three ﬂight attendants had sustained spine and neck fractures and other serious injuries, with a staggering 161 others having sprains, bruises and other minor complaints to remind them of those ten calamitous seconds. There was little doubt in anyone’s mind that carrying on towards Honolulu that night was not an option, but the captain had to choose between landing as soon as possible – this would have meant a diversion to Midway Island airport – or returning to Tokyo. After 14

In the words of a business class passenger sitting in seat 5B – quoted from [95].

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Aircraft attitude

10o o

4.6 nose up 0o

Yaw Pitch Roll

ï10o

18o right wing down

o

ï20

Normal acceleration 2g

+1.814g

1g

0

ï0.824g

ï1g

Indicated airspeed 350 kts 340 kts 330 kts

1: 07 :1 4 1: 07 :1 5 1: 07 :1 6 1: 07 :1 7 1: 07 :1 8 1: 07 :1 9 1: 07 :2 0 1: 07 :2 1 1: 07 :2 2 1: 07 :2 3 1: 07 :2 4 1: 07 :2 5 1: 07 :2 6 1: 07 :2 7 1: 07 :2 8 1: 07 :2 9

320 kts

Cruise time elapsed [h:mm:ss] Fig. 8.8 Anatomy of a turbulence encounter. One hour, seven minutes and 19 seconds into the cruise United 826 experiences a normal acceleration peak of +1.814 G. Almost immediately the nose begins to pitch up. Six seconds later an even more violent event follows: the aircraft rolls rapidly to the right by 18◦ ; at the same time the normal acceleration drops from 1 G, through weightlessness, to -0.824 G, then jumps up to 1.3 G – all within one terrifying second. The airspeed of the 747 ﬂuctuates by about 30 knots during the episode (data from the [95] docket).

assessing the chaos in the cabin and concluding that, apart from some broken interior trim, the aircraft itself appeared to have weathered the turbulence encounter without signiﬁcant impairments, he chose the latter: Tokyo was likely to offer the better medical facilities. Just under three hours after the high altitude mayhem the 747 touched down safely back at Narita airport, where the injured passengers and ﬂight attendants could be taken to hospital. The true extent of what the aircraft had been through only became clear later, when the National Transportation Safety Board downloaded the data from the Boeing’s

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ﬂight data recorder. The sensors had recorded a peak normal acceleration of 1.814 G in the ﬁrst big jolt of the encounter, though most of the injuries probably occurred six seconds later, when the out of control aircraft rolled by 18◦ and plunged, reaching an extreme ‘negative G’ of –0.824g. All the more remarkable, then, was the robustness of the nearly unscathed jet, especially when considering that the 747 in question (N4723U) had been heading towards imminent retirement: United Airlines had been planning to withdraw it from service early in the new year, a decision only brought forward a few weeks by the stormy trans-Paciﬁc ride of 28 December. The subsequent investigation did, however, reveal one small, but potentially very signiﬁcant technical problem. While many passengers and cabin crew recalled that the seat belt sign had been on at the time the aircraft hit turbulence – all indications were that the captain had switched it on about two minutes earlier – almost nobody could recall hearing the familiar chime that usually accompanies the light coming on (or going off). Indeed, a small photoelectric cell was found to have failed in the chime unit, thus precluding the aural warning. Of course the accounts of witnesses that had been through a harrowing ride is our only evidence for the chime not having sounded – after all, the photoelectric cell could conceivably have failed during the turbulence episode. Nonetheless, we shall forever be left with the suspicion that the failure of a component worth a few dollars may have removed one of the lines of defence that could have prevented some of the injuries. The real villain of the piece, however, was, of course, the weather. But what was the phenomenon that caught out a modern airliner, preceded and followed by other aircraft on the same route and ﬂown by a well-trained, experienced crew provided with detailed forecasts?

8.6 A monstrous cat It is hard to imagine what being inside a cumulonimbus cloud might feel like. The history of stratospheric ﬂight has recorded the name of one man who did not have to imagine it – he experienced it. In the summer of 1959 William Rankin, a United States Marine Corps pilot, was ﬂying at 47 000 feet above North Carolina when a sudden engine failure and an imminent ﬁre forced him to eject from his F8U Corsair ﬁghter jet. He was not wearing a pressure suit... and he fell straight into the top of a thunderstorm. Astonishingly, he lived to tell the tale in The Man who Rode the Thunder [106] : “I was buffeted in all directions – up, down, sideways, clockwise, counterclockwise, over and over; I tumbled, spun and zoomed, straight up, straight down, and I was rattled violently, as though a monstrous cat had caught me by the neck and was determined to shake me until I had gasped my last breath... I know I vomited time after time... At one point, after I had been literally shot up like a bullet leaving a gun, I found myself looking down into a long, black tunnel, a nightmarish corridor in space...” A thunderstorm, such as the one William Rankin fell into, is, essentially, a violent manifestation of atmospheric instability. In an unstable, moist air mass, once air starts rising it will continue to be driven upwards. The resulting convective updraft (sometimes a few miles in diameter) gives rise to cumulus clouds ﬁrst in the so-called development stage of the thunderstorm. The updraft can reach speeds in excess of 4000 feet per minute.

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Fig. 8.9 Vast, anvil-shaped cumulonimbus, stretching into the stratosphere, lit by the rising moon. Photograph courtesy of Captain Dave (ﬂightlevel390.blogspot.com).

Usually about 10–20 minutes from its formation the thunderstorm reaches its mature stage, when the rising water droplets will grow too heavy to be sustained by the updraft and will begin falling – this rain drags large air masses down with it, causing powerful and very cold downdrafts. Meanwhile the cloud turns into a cumulonimbus, often exhibiting the classic anvil shape (Figure 8.9). Flight through cumulonimbus cloud is to be avoided at all costs and the fact that the anvil relatively rarely makes it past 30 000 feet, is one of the major attractions of high altitude ﬂight – thus stratospheric or high tropospheric ﬂight is sometimes viewed as being ‘above the weather’. However, ‘relatively rarely’ does not remove the need for caution at high altitudes: cumulonimbus clouds and their ferocious updrafts do occasionally reach 40 000 feet or, at the tropics, even in excess of 50 000 feet and being caught unaware by the top of a thunderstorm can have devastating consequences. Perhaps the most chilling illustration of this caveat of the ‘above the weather’ principle occurred above the Ukraine on 22 August 2006. A Pulkovo Airlines Tupolev Tu-154M (three-engined, T-tailed airliner) encountered turbulence at around 40 000 feet at the top of

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a thunderstorm that had been forecast to stretch even higher and the intensity of which was causing considerable disruption on the ground. In spite of being very close to the Tu-154’s service ceiling and its cofﬁn corner (where the margin between the stall speed and the critical Mach number vanishes) the crew elected to attempt to out-climb the storm. Amid hail and vertical gusts of up to 1400 feet per minute15 they managed to climb a thousand feet or so, but there the cofﬁn corner ﬁnally closed in on them and the aircraft stalled. Tragically, the subsequent precipitous descent turned into a ﬂat spin, from which they never managed to recover and, around four minutes later, the Tu-154 struck the ground near the city of Donetsk. 170 lives were lost [10]. It is rather important to emphasise here that severe turbulence accompanying an encounter with convective weather at altitude is quite normal – losing an aircraft upon such an encounter, especially when it happens at 40 000 feet, is certainly not. As with most accidents, multiple causes contributed to the tragedy. Most prominent amongst these was the crew’s ill-advised attempt to climb above the cumulonimbus with the aircraft so close to its operating envelope. It is also unclear what prevented them from plotting an alternative route around the thunderstorm in the ﬁrst place. Beyond all this, the stall itself should not have been fatal – recall, for example, the ‘410 club’ CRJ pilots we encountered earlier, who also stalled at 41 000 feet and, in spite of not even following the recommended recovery procedure, they managed to regain controlled ﬂight (only to crash later due to a series of misjudgements leading to them running out of landing options, but that is a different matter). The crew of the Pulkovo ﬂight, however, had a much more malign situation to contend with: the Tu-154, like some other T-tail aircraft, was prone to deep stall, a condition where the turbulent wake of the stalled wing envelops the horizontal stabilizer, thus rendering ineffective the very control surfaces required for normal stall recovery (the elevators). The ensuing ﬂat spin (that is, a spin characterized by a very high angle of attack) is generally irrecoverable. Prototypes of the Tu-154 had been equipped with drogue chutes designed to enable the test pilots to recover from ﬂat spins, but these did not make it onto the production aircraft [9, 10]. So was an unexpectedly high convective system responsible for the United Airlines Flight 826 turbulence encounter? As the subsequent analysis of the weather conditions in the area was to reveal, the synoptic conditions were indeed consistent with the formation of a convective cell underneath the path of United 826: the satellite images indicated the presence of a rapidly developing frontal wave system, that is, a front shaped into a wave by a cyclonic circulation moving along with it. A 117 knot jet stream and mesoscale16 gravity wave structure completed the meteorological picture of the strange night of 28 December 1997. It was a night on which the crew of United 826 had selected the track with the least ‘interesting’ weather forecast and was rewarded with a generally smooth journey punctuated by episodes of slight wave action (perhaps as a result of the mesoscale gravity waves?) and ten seconds of complete mayhem. Given the strongly localized nature of the phenomenon (as we have seen, United 826 traveled within a few minutes of unaffected aircraft) it is unsurprising that the forecast had failed to pinpoint what is now thought to have been a rapidly developing cumulonimbus, the powerful up- and downdrafts of which 15 16

According to the report of the Interstate Aviation Committee. Of the order of kilometers, up to hundreds of kilometers.

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ultimately led to loss of life and serious injury. The positive note to take-away from this serious incident is that today crews are usually guided safely around this very type of ‘popup’ convective weather by their modern weather radars – a luxury of the digital age, which a near-retirement 747 did not (and was not expected to) possess in 1997. To round up this short foray into the role played by convective weather in high altitude ﬂight, a small disclaimer is in order in the interest of terminological accuracy. It is common to talk about incidents such as the Donetsk disaster or United 826 as ‘turbulence’ encounters, although quite often the aircraft in question may have been struck by relatively ordered convective updrafts or downdrafts inside or near a cumulonimbus. While even these ﬂows may contain chaotic structures at a small scale, these do not, in themselves, have a signiﬁcant effect on the ﬂight – the momentum of the gusts hitting the aircraft penetrating a rapidly rising or descending air mass does. In fact, along the spectrum of length scales noticeable in turbulent atmospheric ﬂows, only the presence of eddies ranging in size from 30 to about 2000 feet [50] causes disturbances that can be genuinely attributed to ‘turbulence’. If only smaller eddies are present in air moving in an otherwise largely ordered, predictable way, these usually average out over the surface of the aircraft and have little effect on the ride. Eddies larger than about 2000 feet may affect the ﬂight trajectory, but not in the abrupt, sharp way a passenger would perceive as ‘turbulence’. On this somewhat punctilious note we move on to ‘genuine’ turbulence of an entirely different origin.

8.7 An artiﬁcial ‘force of nature’ The expansive glaciers and massive, grey peaks of the North Cascades National Park in northern Washington state were still enveloped in the freezing, pre-dawn darkness shortly before 07:00 on 10 January 2008. High above them, from a warm and comfortable 35 000 foot vantage point, the red eyes of the 83 passengers and ﬁve crew of an Air Canada Airbus A319 were just beginning to glimpse the light of the new day on the distant horizon. Bound for Toronto, Ontario, ﬂying on an approximately easterly course at a speed of around 450 knots, the crew of the Airbus had just been cleared by the Vancouver Area Control Center to commence a climb to Flight Level 370 (approximately 37 000 feet), where they were to remain for the rest of the journey [123]. At least, this was the plan. As the A319 began to climb, suddenly three sharp jolts were felt in quick succession (a sensation described later by the crew as being akin to driving over speed bumps), followed by a series of violent rolls to the left and to the right. Two passengers in the aft lavatories were thrown against the ceiling and the walls, one sustaining serious injuries. Two ﬂight attendants, in the process of serving breakfast, were thrown into the air, falling back onto their service trolleys as the right wing dropped to a bank angle of 55◦ . By the time the aircraft gradually returned to normal ﬂight, a mere 20 seconds later, it had lost over a thousand feet of altitude and there was much to take stock of in the cabin. Assorted food items and coffee pots were covering the ﬂoor, but the greatest damage had been done by the two trolleys, which had impacted several ceiling panels during their short, but lively bids for freedom. A passenger’s laptop hit one of the overhead bins with such force that it left a paint mark on it. In the cockpit a heavy manual had ﬂown out of its holder, hit the captain on the head (fortunately not causing serious injury) and scattered

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its pages across the ﬂoor in front of the ﬂight deck door, such that, at ﬁrst, the cabin crew could not open it. Following an uneventful emergency landing in Calgary, nine passengers and ﬂight attendants were taken to hospital. The U.S. National Center for Atmospheric Research quickly concluded that there were no weather conditions in the region that could have been responsible for turbulence of this intensity, which left only one explanation for the incident: the Airbus must have ﬂown into the wake of another aircraft – in this particular case that of a United Airlines Boeing 747-400, Flight 896, in level cruise at the time at Flight Level 370. The ﬂight paths of the two aircraft nearly crossed, with the A319 slightly slower and around ten nautical miles behind the 747. A highly turbulent wake is an unpleasant byproduct of the complex process whereby a wing generates lift. It can stretch behind (and slightly underneath) an aircraft over a surprisingly long distance and can persist for a surprisingly long time. The intensity and dissipation time of the turbulence in the wake depends to some extent on the aerodynamic design features of the aircraft, as well as on the manoeuvres it might be performing, but the single most important factor determining the dangers of ﬂying into a turbulent wake is the weight of (and thus the amount of lift required by) the aircraft that had generated it. This rather intuitive observation is encapsulated in the rule that the word ‘heavy’ is always appended to the radio callsigns of aircraft with a take-off weight exceeding 250 000 pounds (115 tonnes), as an additional warning to users of the same block of airspace (e.g., ‘Speedbird Two Eight Seven Heavy’ could be the callsign of a Boeing 747-400 operating British Airways Flight 287). We shall not launch into a discussion of the uncertainties and challenges of the enormously complex process of estimating wake turbulence intensity as a function of aircraft weight and distance downstream – let us merely consider, instead, the following image, illustrating the rather counter-intuitively persistent nature of wake turbulence. The standard rate turn is a manoeuvre much loved by light aircraft ﬂight instructors and executed ad nauseam by student pilots the world over. It simply involves making a level 360◦ turn at a rate of 3◦ per second. In other words, a two-minute circle is drawn in the sky in a horizontal plane. In practice this is a little more difﬁcult to do well than it sounds (though this might be a personal reﬂection of this author’s abilities), as maintaining perfectly level ﬂight throughout takes a little practice, due to the aircraft’s natural tendency to ‘drop its nose’ into the turn. If one loses a few feet due to insufﬁcient back pressure on the elevator control, on a calm, smooth day, exactly two minutes after commencing the turn, the aircraft will hit its own wake generated at the start of the manoeuvre. In a light aircraft (say, a Piper Cherokee) the sensation is similar to riding over a small pothole in the road. In spite of the lack of earth-shattering drama, it is still rather interesting to contemplate that the two minute old wake of an aircraft weighing well under a tonne can still be remarkably hard-edged. In the case of large passenger airliners, such as the ‘heavy’ 747 of the Air Canada incident, the turbulent wake tends to descend slowly behind the aircraft by about 600– 1000 feet, progressively expanding and weakening. It is generally assumed that the lifetime of the eddies that make up the wake is around two to three minutes, which, in the circumstances of the incident described, would mean that the turbulent ‘tail’ is about ﬁve nautical miles long – the separation the Canadian air trafﬁc controller had created between the 747

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and the Airbus was 10.7 nautical miles. In this case, however, the turbulence persisted long enough for the Canadian ﬂight to climb straight into it. Such wake longevity had not been unheard of before the Canadian incident. Earlier National Research Council ﬂight tests involving intentionally ﬂying Falcon 20 and T-33 aircraft behind large, twin-aisle airliners recorded vortices capable of affecting aircraft roll and yaw control and causing g-loading spikes at wake lengths up to 25 nautical miles. Moreover, severe upsets were experienced at eight to 16 nautical mile wake lengths. Measurable turbulence was detected during the ﬂight test campaign up to 20 nautical miles behind and 1000 feet below a Boeing 747-400 [123]. In fact, there is even some anecdotal evidence that the wake of some ‘heavies’ can cause upsets to other heavy or medium size aircraft as far behind as 30–35 nautical miles [32]. Wake turbulence is one of aviation’s most active research areas, but not because of the comparatively rare high altitude incidents it causes; it imposes a bottleneck on busy airports. It is not uncommon to see 40–50 aircraft queuing along the taxiways of major international hubs at peak times, burning fuel and pumping CO2 into the air – in such circumstances having to wait up to two minutes after each departure is a costly restriction. A better understanding of how weather conditions (crosswinds, in particular) affect wake dissipation time may, in the next few years, contribute to a reduction (or a dynamic, weather-dependent calculation) of departure spacings and these results might also yield revised high altitude spacings. Meanwhile, there is a surprisingly simple potential solution to the problem, at least as far as oceanic crossings are concerned. In oceanic airspace (most of which is not covered by radar) the International Civil Aviation Organization permits pilots to deviate by up to a mile from their designated tracks – this reduces drastically the probability of being directly behind and beneath another aircraft. Strangely though, a survey by the UK’s National Air Trafﬁc Services in 2004 indicated that only around 4% of pilots were ﬂying an offset track in oceanic airspace [31] – most ﬂights stick to the precise centers of the tracks, as identiﬁed by their high accuracy GPS-based navigation systems. Incidentally, offset tracks have been debated in aviation circles ever since the advent of GPS, though the main reason has nothing to do with turbulence. The impetus is that ﬂying to the right of the center of an airway might reduce the risk of mid-air collisions. The error margin of GPS-based air navigation systems is typically less than a wingspan, so there is a very good chance that aircraft ﬂying (say, as a result of an air trafﬁc control error) at the same altitude along the same airway on reciprocal tracks will collide head on. A recent chilling illustration of this strange side effect of high precision navigation was the collision of a Gol Transportes A´ereos Boeing 737 and an Embraer Legacy business jet over Brazil in 2006. They had both been mistakenly assigned the same ﬂight level, a decision that, with both aircraft precisely in the middle of the airway, effectively condemned them to an inevitable collision with a closing speed of well over a thousand miles an hour. The winglet and the tail of the Embraer sheared off the wing of the 737; the latter subsequently came down with the loss of 154 lives. Of course, such mid-air collisions along the airways are extraordinarily rare. It is also difﬁcult to over-emphasize the fundamental fact that high altitude wake vortex encounters are similarly rare and, even when they do happen, their outcomes are rarely as dramatic as in the case of the Canadian A319 incident. In fact, the enquiry of the Transportation

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Safety Board of Canada concluded that severity of the response of the aircraft to the de-stabilizing effect of the 747’s wake was largely the result of inappropriate corrective control inputs by the pilots. The report into the incident also noted that “in the Airbus A318/A319/A320/A321 series, it may be possible for a pilot to apply rudder control inputs that result in aerodynamically generated structural loads in excess of certiﬁcation design limits and approaching ultimate loads” [123]17 .

8.8 A strong recommendation As we have seen over the last few pages, the upper troposphere and the lower stratosphere do not quite live up to their traditional tagline of being ‘above the weather’. Aviators venturing beyond 55–60 000 feet are unlikely to be troubled too often by any turbulence (though this assertion is not backed up by as much data as one would like) – but, ﬂying at more moderate altitudes, just how much of the weather have we left behind? We have NACA to thank for the ﬁrst systematic attempts at answering this question – Figure 8.10 distills the results of their 1953 test campaign. Specially instrumented aircraft and sounding balloons were used to estimate the variation with height of ﬂight time spent in non-thunderstorm turbulence. The proﬁle may serve as a rather compelling argument for stratospheric ﬂight... 60

Altitude [ thousands of feet ]

50

40

30

20

10

Sea Level 0

5

10 15 20 25 30 35 Fraction of time spent in turbulence [%]

40

Fig. 8.10 Percentage of total ﬂight time spent in turbulence (based on data from [84]).

17

Of course, cycling the rudder in ‘just the wrong way’ could probably cause structural failure in many other aircraft.

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What happens though if we raise the bar slightly and only consider turbulence that is perceived by aircrews as being at least moderate? (Table 8.1 could be used here as a reminder of the deﬁnition of ‘moderate turbulence’). Climatologists at the National Center for Atmospheric Research recently reviewed 12 years’ worth of pilot reports (PIREPs) of moderate or more intense turbulence ﬁled over the contiguous United States [135]. Perhaps slightly surprisingly the frequency of such reports peaks at around 1.7 times the background value at an altitude of around 29 500 feet. Separating clear air turbulence reports, these peaked in a band between 29 500 and 36 000 feet18 . Seasonal variations were also considered, with December, January, March and April turning out to be the least auspicious times to ﬂy for those wary of turbulence, at least over the United States. The geographical distribution of areas experiencing more than 50% greater than average included some of the ‘usual suspects’ (the Sierra Nevada, the Rocky Mountains), but it also included southeastern Texas, Florida, the upper midwest and the Paciﬁc northwest. If the climatology of turbulence is difﬁcult to estimate, its impact on aviation is an even less well understood question. We have seen that fatalities directly linked to turbulence are fortunately rare, as are serious injuries. Of course, with the many millions of miles ﬂown by the airlines each year, the impact can still be hard to neglect. Sharman et al. [116] quote a vice president for an unnamed major airline mentioning a ﬁgure of the order of tens of millions of dollars per year for passenger injury payouts, and a loss of about 7000 employee days in turbulence-related injuries (these numbers relate to events cumulated across all altitudes). Finally, to spare a thought for the ﬂying machines themselves, the millions of gust load peaks encountered by an airframe throughout its lifetime contribute signiﬁcantly towards its tally of fatigue cycles. This is especially true of the wings, but other components can have complex dynamic responses to turbulence too (see, for example, the vibration mode shapes of the fuselage of a North American XB-70 Valkyrie aircraft, Figure 8.11). Active gust load alleviation techniques, employing control surfaces deﬂected in response to gusts at the appropriate moments in a way that momentarily reduces the loads on a wing, are a promising technology, but their widespread application is not quite here yet. Perhaps the best way to conclude this chapter dedicated to turbulence at high altitudes is by an excerpt from United Airlines’ new safety brieﬁng, introduced after the unfortunate Flight 826 of 28 December 1997: “To release the seat belt, just lift the buckle. Once we’re airborne, the captain will turn off the seatbelt sign, but we strongly recommend that you keep your seat belt fastened while you are seated...”

18

Though the authors of the study went to great lengths to normalize all these numbers, the variable density of the air trafﬁc along the vertical will introduce certain biases.

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Fig. 8.11 Calculated fuselage deﬂections for the ﬁrst three mode shapes of the XB-70 Valkyrie aircraft [132] (see Figure 5.7 for a photograph of the aircraft on its maiden ﬂight).

9. A gray area

“All four engines have stopped.” Capt. Eric Moody Passenger announcement on ﬂight BA009, 24 June 1982

9.1 Descent into Anchorage 15 December 1989 was never going to be an entirely ordinary day for Captain Karl van der Elst and the rest of the crew of a KLM Royal Dutch Airlines Boeing 747, operating Flight 867 from Amsterdam to Tokyo, via a fuel stop in Anchorage, Alaska. The reason for the peculiarity of the ﬂight lay about 100 miles to the southwest of Anchorage, in the shape of 10 197 feet high volcano Mount Redoubt, which had been erupting for the last 36 hours or so. A number of airlines operating in the area had already decided that discretion was the better part of valor and had suspended their ﬂights [133], but others, including the KLM crew, were proceeding on the assumption that, if necessary, they would be able to alter their approach routes into Anchorage such that they would avoid the plume emerging from the Alaskan volcano. As their ﬂight plan envisaged a broad daylight arrival to their stopover destination, they were expecting to be able to see the ash cloud and take evasive action accordingly, also guided by any radar-based updates received from air trafﬁc control. News of a further eruption was received en-route by the KLM crew and, shortly after commencing the descent into Anchorage from their cruising altitude of 39 000 feet, they were beginning to make out what may have looked like an ash cloud. “It’s just a little browner than the normal cloud...” – the ﬁrst ofﬁcer remarked. Very soon, however, as they were descending through 26 000 feet, another, much more uncomfortable warning of the presence of the ﬁne volcanic ash aerosol demanded immediate evasive action. “We have to go left now... it’s smoky in the cockpit at the moment, sir....” [38] The air trafﬁc controller in Anchorage Center promptly responded to the request by clearing them to change heading and altitude at their discretion. By now, a black cloud was enveloping the 747 and the crew, who had donned their oxygen masks, were becoming

A. Sóbester, Stratospheric Flight: Aeronautics at the Limit, Springer Praxis Books, DOI 10.1007/978-1-4419-9458-5_9, © Springer Science+Business Media, LLC 2011

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extremely concerned. A full power climb was initiated in an attempt to climb out of the ash plume and the Boeing duly began to ascend – for about a minute. Then the situation suddenly turned critical. As it is often the case with such technical emergencies, several crises erupted at once. All four engines spooled down rapidly to less than a third of the commanded power, a ﬁre warning alarm for the forward cargo area sounded, the airspeed indicators stopped working and there were interruptions in the electrical power supply. The ailing engines could no longer sustain the climb and, a few seconds later, not even level ﬂight. The crew immediately attempted to restart the engines, without success. The powerless jet was descending through the dark clouds, gliding towards the 10 000 feet high mountain tops beneath. The next attempt at a restart failed too... and the next... and the next... “KLM 867 heavy, we are descending now... we are in a fall!” – came the desperate sounding call from the Dutch airliner. With the mountain summits only around 2000 feet away the four General Electric CF6 engines of the 747 ﬁnally restarted – on the eighth or ninth attempt. After an uneventful approach KLM 867 landed in Anchorage. Its 231 passengers and 14 occupants were shaken by the experience, but otherwise unharmed. This could not be said of the barely three month old, 900 hour ﬂying time Boeing 747-400: the external surfaces of the aircraft had suffered heavy abrasion as a result of the encounter with the ash cloud and the high pressure turbines of all four engines were damaged – all told, the repairs to the 747 cost a staggering 80 million dollars [27, 38].

9.2 A modern menace The KLM encounter was not the ﬁrst case of a jet aircraft coming close to disaster as a result of inadvertently ﬂying into a cloud of airborne volcanic dust particles. Only seven years earlier, in what was to become a legendary incident, a British Airways Boeing 747 ﬂew into the ash cloud generated by the eruption of Mount Galunggung, Indonesia. Similarly, all four engines were lost – for over a quarter of an hour – and, following a harrowing glide, a partial engine restart, another engine failure followed by another restart, and some severe comprehension difﬁculties in the local air trafﬁc control center, Captain Eric Moody and his crew performed a splendid display of airmanship, bringing the aircraft to a safe landing 1 . The major difference between the two events was that the British Airways crew had received no prior warning from the Indonesian authorities and therefore had no idea what was causing the multiple engine ﬂame-outs. Moreover, the BA ﬂight took place at night, giving them no chance to see the ash plume (and, incidentally, providing the added spectacle of a strange glow around the airframe, as well as sparks of St Elmo’s ﬁre in front of the windscreen).

1

Moody’s announcement to the passengers of BA009, made through his oxygen mask, has become permanently engraved into aviation lore: “Ladies and gentlemen, this is your captain speaking. We have a small problem. All four engines have stopped. We are doing our damnedest to get them going again. I trust you are not in too much distress.”

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In spite of a handful of such events in recent decades, the history of jet airliner ash encounters is a short one – another reason why Captain Moody and his crew did not recognise the cause of the crisis they had been caught up in: they had clearly not heard of similar incidents (it was only upon landing when the ﬂight engineer examined the strange gritty, sooty, sulphurous smelling dust that had settled in the cockpit that they realised the cause of their memorable evening [118] 2 ). This is largely because, somewhat counterintuitively, modern gas turbine engines are less robust to the effects of volcanic ash than their predecessors. To see why this is, it is worth considering what happens when volcanic ash enters a turbofan engine. The ash particles ﬁrst meet the fan, which acts as a centrifugal separator (a rather useful function when it comes to ingesting any foreign objects, such as birds or ice); that, is, it forces the larger particles radially outwards and thus they ﬂy out harmlessly through the bypass duct. Only the ﬁner dust makes it into the core of the engine, where it is sucked through the several rows of increasingly high pressure compressor blades. Things start to get interesting at the high pressure (HP) stage – by the time the air reaches the downstream end of the compressor its pressure can be as high as 50 times the ambient air pressure. Efﬁcient compression is only possible if the clearance between the tips of the compressor blades and the casing is very small – one way of achieving this is via an abradable casing lining, which the blade tips themselves erode to the correct radius. Volcanic ash has the potential to erode this lining further, causing a signiﬁcant loss of compression performance. It may erode the blades themselves too, which further reduces the pressure the HP compressor is able to produce. The ﬂow path now takes the ash into the combustors, where the temperature can be as high as 2000◦ C (3632◦ F). This is far above the melting point of the ash – generally around 1200–1300◦ C (∼2200–2400◦ F) [75]. As the air carrying the molten ash exits the combustor, it enters the ﬁrst row of static nozzle guide vanes (NGVs). The NGVs form convergent passages (nozzles), through which the air pressure drops, but its velocity increases, as it is directed onto the ﬁrst row of turbines. The staggeringly complicated process that keeps the NGVs below 700◦ C (∼1290◦ F) in such conditions is probably worth a book in its own right (they would melt at the temperatures produced by the combustor) – here we merely note that this signiﬁcant drop in temperature brings the ash to a state where it sticks to the vanes, gradually reducing their nozzle area. This area reduction causes a pressure rise upstream, to the extent that it can cause a reversal of the ﬂow along the turbomachinery, especially if the HP compressor has already suffered erosion damage. In such conditions the engine will almost inevitably surge with a loud bang. The resulting drop in temperature (very hot air is no longer arriving from the combustor) will allow the ash to solidify into a glassy mass, which may eventually fall off the NGVs, allowing normal engine operation to resume – only for the entire process to repeat if the ash content of incoming air is still high. The above is merely a summary review of the headline effects of ash ingestion – others include cooling hole blockage and damage to the cooling system itself (the cooling air will also contain ash), abrasion of the turbine blades, damage to various transmission components caused by the ash getting into their lubricant, etc. Moreover, engine damage may 2

Six other aircraft suffered varying degrees of damage that night [55].

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occur even before the aircraft reaches the air masses containing the ash aerosol. When a volcano erupts through a glacier, as, for instance, the Icelandic Eyjafjallaj¨okull (more on which later) did in 2010, water may rush into the crater and react with the lava, producing large quantities of hot sulphates. Typical examples of the latter are sulphur dioxide (SO2 ), sulphuric acid (H2 SO4 ) and hydrogen sulphide (H2 S), which all have the potential to cause corrosion damage to engines exposed to them [55]. In fact, this is a slightly more insidious effect of volcanoes: the sulphidation caused by these compounds may only manifest itself later. So why is all of the above of particular concern to operators of aircraft equipped with new engines? Simply because the modern day drive towards low environmental impact (low emissions, low fuel burn) demands high temperatures – more speciﬁcally, modern engines have considerably higher turbine entry temperatures (TETs) than the turbofans and turbojets powering previous generations of airliners and are therefore more likely to melt the ash particles they hoover up. Given the comparative rarity of volcano eruptions, this is, of course, a very small price to pay for the spectacular reductions fuel burn seen over the last two decades or so...3 The strong correlation between turbine entry temperature and the probability of ash damage also points to the most important step on the ‘volcanic ash encounter’ checklist of most airlines: reduce thrust. Closing the throttles will cool down the engines, so the safest way of escaping an ash cloud is not to attempt to power out of it (like the crew of the 1989 Mount Redoubt encounter did), but to descend into clear air. Incidentally, here is a generic list of further actions that aircrews are generally advised to take in case of an ash encounter [27]: turn off the auto-throttles (automatic thrust adjustments might cause further surges), exit the ash cloud as quickly as possible (a 180◦ descending turn is likely to be the most effective), turn on engine and wing anti-ice devices and all air-conditioning packs (such measures designed to increase bleed air ﬂow will improve the surge margins of the engine), start the auxiliary power unit (this will be useful when trying to restart the engines, as well as providing power for the electrical systems of the aircraft), don crew oxygen masks at the 100% setting (in case the aerosol has entered the cockpit), turn on the continuous ignition system of the engines (for a restart attempt), monitor engine exhaust gas temperatures (if too high, as a result of ash buildup, stop the engine and attempt another restart), ﬂy on the assumption that the airspeed readings may be unreliable (ash may have entered the Pitot tubes of the airspeed measurement system).

9.3 Too much of a bad thing The regulations of the International Civil Aviation Organization (ICAO), introduced after the KLM near-disaster in Anchorage, used to be clear: if there is ash in the atmosphere, do not ﬂy. In other words, the ash tolerance threshold was set to zero. Then, on 14 April 2010 the Eyjafjallaj¨okull volcano erupted from under one of Iceland’s glaciers, ejecting ash to 3

The rate of progress in terms of fuel burn has been around a percentage point per year for the last couple of decades [75].

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altitudes greater than 30 000 feet, and high altitude air trafﬁc over most of Europe ground to a halt. With 104 000 cancelled ﬂights and 10 million passengers unable to make their planned journeys [115] and a frustratingly unfavorable north easterly wind continuing to blow the ash cloud around Europe’s airspace (actually making a full circle in the space of less than a week), it was clear that the line in the sand had to be redrawn – but where? The concentration of the Mount Redoubt cloud that nearly brought down KLM 867 had been estimated at around 2 g/m3 – ﬂying through such a dense aerosol, a gas turbine engine could potentially be drawing between one and two tonnes of ash per hour through its core. Subsequent examination of the engines of the KLM jet found around 80 kg of ash in each of its turbines [113]. On the evidence of that incident, this value is clearly far beyond what is acceptable. As it turns out, the current view shared by most in the airline industry is that the acceptable concentration is actually three orders of magnitude less – that is, 2 × 10−3 g/m3 , which results in an engine core throughput of about 1 kg/hour (see Figure 9.1). Given an accurate map of ash concentrations, the contour lines around the ‘no ﬂy zone’ can therefore be drawn at this level – the only difﬁculty is that an accurate map of ash concentrations is, at the moment, not given. To account for the error margins of density forecasting (which, by some estimates, at the time of the 2010 Eyjafjallaj¨okull eruption were as great as plus or minus one and a half orders of magnitude!), a trafﬁc light system was put into operation in April 2010. This has the following colors: white (forecast ash concentration under 2 × 10−4 g/m3 , no danger), red (forecast ash concentration between 2 × 10−4 g/m3 and 2 × 10−3 g/m3 , ﬂying possible, subject to maintenance restrictions), gray (forecast

Ash ingested into engine core [ g / hour ]

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Fig. 9.1 How much ash is ingested by a gas turbine engine at various ash concentrations? This is represented by two lines on this logarithmic scale plot: the blue and the green lines correspond to 100% and 50% core ﬂow through the engine respectively (data from [75]).

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ash concentration between 2 × 10−3 g/m3 and 4 × 10−3 g/m3 , ﬂying only possible with the permission of the aircraft manufacturer) and black (forecast ash concentration above 4 × 10−3 g/m3 , do not ﬂy under any circumstances). It was also made clear that, regardless of any forecasts, there was to be no ﬂight into visible ash. The chart in Figure 9.1 is color-coded according to this new semaphor system. Consider the widths of the various bands. Logarithmic scales are needed to even attempt to make sense of them – even so, their respective sizes are not immediately obvious. What such charts do highlight though is the importance of an accurate forecast.

9.4 The ash forecast Why is it so difﬁcult to forecast the ash concentration at high altitudes following a volcano eruption? Where do the greatest uncertainties arise? The modelling of the high altitude winds is certainly not the guilty party here. As we have already seen in our discussion of the jet stream, these are forecast with staggering accuracy. Following the Eyjafjallaj¨okull event the movements of the ash-bearing air masses were tracked and forecast with precision, triggering the airspace closures around Europe. The next building block of effective ash cloud modelling – and this is were uncertainties begin to creep in – is the tracking of virtual ash particles ‘thrown into’ the numerical computer model of the current weather systems. Research over the past decades into the modelling of the spreading of pollutants or a potential nuclear fallout is of great assistance here, but there are many complex processes to consider: ash particles come in a range of shapes and sizes, they gradually fall out of the atmosphere as a result of gravity, some of the plume may be ‘washed out’ of the atmosphere by precipitation, some particles are absorbed by water droplets, small scale turbulence causes a gradual dispersion. Still, accounting for all of the effects listed above is possible – reasonably close approximations of these phenomena are available. This leaves us with the ﬁnal element of the problem, where the uncertainties are by far the biggest: the understanding of what happens at the source, the dynamics of the eruption itself. The question here is, what is pumped into the atmosphere and at what rate? Whether the eruption is of the violent hydromagmatic (resulting from the magma encountering water) or the dry type, determining the distribution of particle sizes at the source is extremely challenging. The rate at which the particles leave the volcano is even more difﬁcult to estimate, mostly because of the sheer intensity of such processes. Even Eyjafjallaj¨okull’s 2010 eruption, which falls ﬁrmly in the ‘moderate’ catergory, ejected around 750 tonnes of tephra (the generic name for all expelled volcanic material) into the air – every second [55]. The ash then rises with the powerful convective current generated by the heat of the magma to an altitude where the ash column density equals that of the ambient air. Its excess momentum then carries it a little further – at this stage the plume begins to ﬂatten into the familiar umbrella shape (sometimes distorted by the wind) [24]. A good indicator of the magma ejection rate is the height at which this happens, but, as far as surrogate metrics go, this is a somewhat problematic one: it is not that easy to measure either and the ejection rate is proportional to the fourth power of the plume height. As a result, relatively small plume height measurement errors can lead to vast over- or under-estimates of the

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source strength – these errors then propagate into the dispersion model (suffering from its own, though much less signiﬁcant errors). The task of the various national meteorological ofﬁces, the World Area Forecast Center (WAFC) as well as of the nine Volcanic Ash Advisory Centers (VAACs) is not an easy one and much research is still needed to reduce these error margins.

9.5 Clear of cloud The recorded geological history of Iceland catalogs 205 volcanic eruptions to date. Somewhat unsettlingly, there appears to be a certain periodicity in the frequency of these eruptions and the last 50 years or so have been relatively quiet (April–May 2010 notwithstanding). Are we due for a spell of increased activity? What about the ‘usual suspects’ of Montserrat, Pinatubo, Mount Merapi and the Aleutian Range (of which Mt Redoubt is one) – are they likely to be the sources of the next major disruption to air transport around the world? Eyjafjallaj¨okull 2010 was a painful illustration of the potential economic impact of such events. The airline industry is estimated to have lost over 2.2 billion dollars over those two weeks in the spring of 2010, with the aggregate loss to global GDP approaching 5 billion dollars (not including long term effects). Long after the crisis had fallen off the radars of the media, the ‘long tail’ of the disruption was still causing damage to airlines already struggling with the impact of the recession, with tactical re-routings around the Iceland area adding to fuel costs and causing resource allocation problems, with aircraft operating routes at the limits of their ranges having to be substituted with longer range models (for instance Boeing 767s for 757s) on trans-Atlantic routes [115]. If ash clouds come with a silver lining, the one statistic that regulators, forecasters, scientists, aircraft manufacturers and operators can rejoice in is that no human life has ever been lost in a volcanic ash encounter throughout the history of aviation. Nevertheless, the numerous close calls and the staggering economic cost point at the importance of more research in this area. There is relatively little that can be done in terms of ash-prooﬁng gas turbine engines, so all that remains is to improve our awareness of the position and the density of future ash clouds, whether through sensors carried on board aircraft (trials of special infrared ash detection cameras are underway at the time of writing) or through more accurate forecasting models – it will then be possible to ﬂy around, or, more to the point, it will be possible to ﬂee into the safety of the stratosphere from the abrasive menace of the volcanoes.

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Fig. 9.2 16 May 2010 – a second wave of airspace closures is caused by the ash cloud from Eyjafjallaj¨okull drifting over Europe. This image, taken by NASA’s Terra satellite, shows the plume just to the south of Iceland, with the color overlay on the bottom panel showing plume height estimates based on stereoscopic images acquired by the spacecraft. Close to the source (the volcano is just outside the ﬁeld of view in the top left corner, the plume reaches altitudes exceeding 8 km (26 247 feet), descending to about 6 km (19 685 feet) about 250 km (155 miles) downwind. Image courtesy of NASA/GSFC/LaRC/JPL.

Part IV

Where next?

10. Higher still

“The pale blue of the sky that we’re used to seeing from Earth was still visible in a thick band along the horizon, but if I let my eyes drift up, I could see the sky gradually darken from dark blue to indigo to an almost indescribable black. It was the blackest black I had ever seen. Blacker than ink. And it was morning!” Joseph W. Kittinger (1928–) describing his stratospheric ascent in the Manhigh I capsule in 1957 [72]∗

10.1 96 863 feet The aircraft that lined up with the US Navy Paciﬁc Missile Range Facility runway on the island of Kaua‘i, Hawaii, on 13 August 2001 was nothing short of extraordinary. Powered by 14 brushless electric motors, it had a wingspan of 75 m (over twice that of a Boeing 737NG), with no fewer than 72 trailing edge elevators providing ﬂight control. Five streamlined pods supported the enormous wing – these also contained the payload, the instrumentation and the landing gear of the rather unusual looking unmanned aircraft. The AeroVironment NASA Helios Prototype HP01, for this was the name painted on the pods, had been constructed mostly out of carbon ﬁber tubing, Kevlar and styrofoam, weighing in at a mere 725 kg. These and every other feature of this remarkable aircraft spoke of the designers’ overarching goal that it had to be able to climb high – very, very high; and on that August test ﬂight the HP01 did fulﬁl these ambitions, reaching an altitude of 96 863 feet. More impressively still, this was not the result of a ‘zoom climb’ that converted the aircraft’s momentum into altitude (in fact, the light and rather slow HP01 – designed for airspeeds under 30mph at sea level – had little momentum to convert anyway); the large, fragile looking craft actually cruised at altitudes exceeding 96 000 feet for 40 minutes. To date, this is the highest any winged aircraft has ever ﬂown. It successfully battled its way through the more turbulent troposphere and avoided its cofﬁn corner as it climbed to ∗

Reproduced by permission of UNM Press.

A. Sóbester, Stratospheric Flight: Aeronautics at the Limit, Springer Praxis Books, DOI 10.1007/978-1-4419-9458-5_10, © Springer Science+Business Media, LLC 2011

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a stratospheric altitude where the ambient pressure of the air is a mere 12mbar (1% of the sea level value!), before descending gracefully and landing on the Kaua‘i runway. Of course, this inspiring technical achievement did not come cheap. In fact, it was the result of two and a half decades of high altitude ﬂight research at NASA and elsewhere. As early as the 1970s the need had become evident for a low cost platform capable of carrying instruments to high altitudes at the fraction of the cost of the U-2 and the SR-71 Blackbird. A light, easily deployable, cheap to buy and cheap to operate unmanned aircraft was sought that could carry out research tasks (such as the monitoring of atmospheric parameters, radiation levels, etc.), as well as becoming a kind of ‘atmospheric satellite’, serving as a stable, long endurance platform for communications and surveillance equipment.

Fig. 10.1 The early days of high altitude unmanned aircraft research: the Mini Sniffer III ran on hydrazine and therefore had to be approached with extreme caution. Image courtesy of NASA.

The ﬁrst in a long line of prototypes designed towards this ultimate goal was the Mini Sniffer. Its brief was pollution monitoring at altitudes up to 70 000 feet. A series of increasingly odd design iterations culminated in the Mini Sniffer III, an 89 kg unmanned drone with the somewhat unappealing feature that it could only be approached by specially trained personnel wearing full body protective suits (Figure 10.1). The reason was that, uniquely amongst aircraft of this kind, its engine ran on hydrazine (N2 H4 ), an extremely toxic compound more frequently encountered in spacecraft propulsion systems (for instance, the Space Shuttle’s auxiliary power units run on hydrazine). This was needed because the Sniffer’s small piston engine simply could not draw enough oxygen from the atmosphere near its ambitious target altitude and the solution NASA’s engineers came up

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with was to use a fuel that could expand in the cylinders and generate heat without needing any air. Hydrazine does just that when coming into contact with an iridium catalyst [73]. Eventually the Mini Sniffer project was canceled before the little drone could get anywhere near its target altitude, but it set a benchmark for a string of light (though ever heavier and larger) unmanned stratospheric ﬂyers. The solar-powered AeroVironment Pathﬁnder, developed throughout the 1980s and 1990s, had a span of 29.5 m and reached the Mini Sniffer’s original target altitude of 70 000 feet in 1997. A longer span (36.3 m) updated version, Pathﬁnder Plus, made it to 80 000 feet a year later. The enormous (61.8 m span) Centurion followed in 1998, with the name leaving little doubt as to its altitude ambitions. In the event, it only ﬂew a series of low altitude missions, before AeroVironment and NASA turned their attention to the crowning achievement of the program, the 75 m span Helios. A long series of experiments and test ﬂights later Helios did not quite make it past the magic barrier at 100 000 feet, but, as we have seen, its performance of 96 863 feet is still yet to be equaled by any other winged aircraft. A second goal of the Helios project was to push the limits of the endurance of the record-breaking prototype, by exploiting its ability to climb into very thin air. This required some modiﬁcations, including the removal of some of the motors and the addition of a set of hydrogen fuel cells to generate enough power to keep the motors running overnight when the solar panels were ineffective. These and other changes pushed the weight of the aircraft up by about 32%, but it still only weighed just over a tonne. The new Helios prototype was ready to ﬂy by the spring of 2003 and it had its maiden ﬂight on 7 June 2003. The daytime portion of the ﬂight was described by mission managers as ‘ﬂawless’, but a failure of the fuel cell system meant that it had to land prematurely – about halfway into the ﬂight that had been intended to last for 30 hours. The second test ﬂight was planned for 26 June 2003, with the objective of climbing to 60 000 feet and testing the fuel cell that had curtailed the previous mission. The two halves of the giant aircraft (it could not be handled and stored in its entirety) were carefully wheeled out of the hangar the previous evening. Working through the night, the technicians completed the assembly and the pre-ﬂight preparations by 10:00 the following morning. The result of the customary pre-ﬂight go/no-go review was ‘go’, though the weather forecaster was only willing to venture a ‘very marginal go’. The cause of the concern was some low level turbulence on the shear line between the island’s wind shadow and its ambient trade wind ﬂow; additionally, billows in the cirrus clouds suggested the probability of turbulence around the jet stream too. All this was slightly more pronounced than on previous ﬂights, but the Helios Prototype still took off from Kaua‘i, about an hour and a half after the planned time. As expected, around 13 minutes into the ﬂight the aircraft hit turbulence, responding with mild pitch oscillations and an increase in the dihedral (upward slope) of the wings. Pressing on, Helios continued to climb at a rate of about 100 feet per minute, approaching the shear line. Half an hour into the ﬂight the dihedral continued to increase, accompanied by ﬂuctuations in the airspeed and pitching oscillations. Moreover, by this point, the oscillations began to diverge, their intensity roughly doubling at every cycle. The delicate, light structure of the prototype aircraft had not been designed for the air loads associated with this violent motion and, to the dismay of the crew of the nearby chase helicopter, began to

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Fig. 10.2 AeroVironment’s solar-powered Helios aircraft being prepared for launch (top), in ﬂight above the Paciﬁc (middle) and crashing into it (bottom). Images courtesy of NASA.

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disintegrate. 90 seconds after the start of the turbulence encounter Helios broke up and fell into the Paciﬁc (Figure 10.2) [92]. It is possible that Helios’s altitude record will, in due course, be broken by another aircraft. It is also possible that this aircraft will surpass the 100 000 feet barrier1 . What the Helios mishap shows beyond doubt, however, is that beyond 90 000 feet any winged aircraft will inevitably be pushed to several of its limits simultaneously. If the thrust of its power plant, severely limited by the low density of the air, is to be sufﬁcient to lift it into these upper layers of the stratosphere, its weight has to be very low. At the same time, pushing its cofﬁn corner higher will require increasing its aerodynamic efﬁciency, which almost inevitably means long, slender wings. However, long, slender wings either tend to be heavy or very ﬂexible and therefore prone to instabilities of the type encountered by Helios on its way up through the lower atmosphere. Thus close in the feasibility boundaries of high altitude ﬂight on any aircraft with 100 000-foot aspirations, putting it, almost inevitably, into a very precarious equilibrium. The Helios case illustrates that wing-generated lift ceases to be a practical means of ﬂight near the upper reaches of the stratosphere. This leaves us having to resort to another type of force to carry us higher still, one that started humankind’s aeronautical endeavors back in the 18th century: buoyancy.

10.2 Manhigh If the US space program ever had an unsung hero, it must be Lieutenant Colonel John Paul Stapp. In charge in the 1950s of the US Air Force Aerospace Medical Laboratory at Alamagordo, New Mexico, Stapp was conducting ground-breaking research into the human factors of space ﬂight nearly a decade before project Mercury put the ﬁrst American into space – indeed, long before the possibility of manned space exploration became a widely held notion. Stapp’s research facility was the ﬁrst in the US to give serious consideration to the effects of zero-G, beginning with experiments involving cats sitting in the laps of test pilots ﬂying parabolic arcs; the goal of these tests was to see whether a free-ﬂoating cat, if rolled upside down, could re-orient itself while still weightless (it could). On the negative side of the acceleration spectrum, Stapp, a man always keen to lead from the front, had himself strapped onto a rocket sled, which accelerated to over 600 miles an hour, before ‘slamming the brakes on’, just to see if he would survive the resulting deceleration, on one occasion peaking at –41 G (he did – just). The Aero Med Lab’s most lasting legacy, however, is due to project Manhigh, the ﬁrst attempt at developing the technology necessary to take humans into the upper stratosphere, far into the space equivalent zone. As we saw in the Prologue, mankind’s ﬁrst stratospheric venture was onboard a balloonborne gondola. Colonel Stapp also saw a balloon as the only practical means of reaching 1

It is also not entirely impossible that a sailplane (glider) will be the ﬁrst aircraft to get there. The current altitude record held by the late Steve Fossett, millionaire adventurer and aviator, and former NASA test pilot Einar Enevoldson, is 50 724 feet. Supported by NASA, Enevoldson’s sights are now set on soaring past the 100 000 feet mark in his pressurized glider, encouraged by radiosonde measurements indicating the presence of mountain waves as high up in the stratosphere as 105 000 feet [90]. One man’s high altitude mountain wave turbulence (page 135) is another’s precious updraft.

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the altitude necessary for testing the life support technology that would later ensure the safety of the ﬁrst manned space ﬂights. Piccard and Kipfer reached an altitude of 51 775 feet in 1931 – Manhigh was aiming for nearly twice that. The Air Force had good reason to be conﬁdent in being able to meet that target. In 1947 an unmanned polyethylene gas balloon launched by aeronautical engineer Otto Winzen reached 100 000 feet above St Cloud, Minnesota, setting a world altitude record. Having subsequently founded his own ballooning research company, Winzen received the contract to design and produce a series of helium balloons for Manhigh. Three of these balloons would carry single-seat capsules designed to take a brave volunteer test pilot into the stratosphere. After a series of unmanned test ﬂights Captain Joseph “Joe” Kittinger was the ﬁrst to don a partial pressure suit and board one of the phone booth-like pressurized capsules (see Figure 10.3) on 2 June 1957. A 14 kg cap of dry ice was placed on top of the capsule to keep the temperatures inside at an acceptable level, after the early unmanned ﬂights indicated a possible repeat of the overheating that Piccard and Kipfer suffered from on their pioneering ﬂight two decades earlier. At the launch site above Manhigh I (as this ﬁrst capsule was called) ﬂoated the long, jellyﬁsh-like Winzen Research balloon, which was to swell to a volume of 56 600 m3 and a diameter of over 50 m at its peak altitude.

Fig. 10.3 Project Manhigh capsule. Image courtesy of NASA.

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Not unusually for a ﬁrst-of-its-kind, 1950s experimental vehicle, things did not go completely as planned. First, the communication system was only working in one direction – Kittinger could hear ground control, but could only respond in Morse code. Then, an hour into the ﬂight, it became clear that the oxygen supply was depleting at a higher rate than originally thought and that Kittinger was going to have to be very economical with it. At an altitude of around 45 000 feet, as forecast, the jet stream struck the balloon, producing an effect that Kittinger later described as ‘not subtle’ 2 . By this he meant that the ‘train’ was nearly horizontal – that is, the capsule was inclined by 90◦ as the balloon was dragged into the over 100 miles per hour stream of wind. This was to be the harshest test of Winzen’s work and the balloon emerged intact on the top of the jet stream. As Manhigh I reached an altitude of 96 000 feet, mission control on the ground calculated that if the oxygen was to be enough for the return leg of his journey, the pilot had to begin his descent so they ordered him to start venting helium. In his autobiography Kittinger recalls not being able to resist tapping out in Morse code the message ‘c-o-m-e u-p a-n-d g-e-t m-e’; by then, however, having made the same oxygen calculations, he was already on his way down [72]. Manhigh II was preceded by a simulated ﬂight in a pressure chamber. Major David Simons, the pilot designated for the actual ﬂight, climbed into the capsule on 26 July. Once again, there were problems with the oxygen pressurization systems and the test had to be halted several times (a luxury the pilot would not have during the actual ﬂight!). Temperature control proved to be a serious problem too, with Simons sweating profusely inside his pressure suit at ﬁrst, then feeling extremely cold as a freezer was turned on to simulate stratospheric temperatures. Eventually a rather mundane combination of diarrhoea and the pressure suit not having provision for eliminating solid waste led to the termination of the ‘ﬂight’ [71]. Nevertheless, Manhigh II was ready to launch and, after numerous weather delays, Simons, who had had to climb into the capsule the previous night at the Winzen factory, was ready to go at the launch point on 19 August 1957. The 425 foot deep Portsmouth Mine near Crosby, Minnesota had been selected for this launch in order to shield the balloon from the surface winds during inﬂation. The steep rock faces did shield the launch site, but, as the capsule was released and the wind caught the top of the rising balloon, the same rock face presented a menacing obstacle – the gondola swung to within a meter of one of the cliffs. A steep ascent followed, with the balloon rising at times at rates exceeding 1200 feet per minute, as Simons was climbing through the stratosphere. Finally Manhigh II levelled off at an altitude of 102 000 feet, where Simons spent the day taking photographs and making sky brightness measurements. The sunset brought a drop in the temperature of the helium and thus a descent, which the pilot countered by shedding ballast. Thunderstorms were forming far beneath the capsule as Simons settled down for the night. He awoke shortly after 04:00, ﬁnding himself buffeted by the top of the storm cell, which the balloon had descended into overnight – a quick release of two spent batteries allowed him to climb out of danger and by sunrise the balloon was at 100 000 feet. 2

For a more detailed, vivid description of what it feels like to ascend on a gas balloon into a strong jet stream, the reader may wish to pick up a copy of Richard Branson’s account of experiencing just that on his historic ﬁrst crossing of the Paciﬁc with Per Lindstrand [18].

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Just before 11:00, for the ﬁrst time during the ﬂight, the messages coming from Manhigh II began to give mission control serious cause for concern. Simons’ breathing rate had increased to about three times the normal value, he began to get confused about simple tasks and, as a clear indication of the possible cause of all this, he reported a cabin CO2 level of 4%. It was time to bring him back. Unfortunately, large helium balloons warmed by intense sunshine do not respond very readily to control inputs and it took several hours of helium venting before he could begin his descent. Eventually, after a 32 hour ﬂight and a rather hard landing, Simons stood in an alfalfa ﬁeld near Frederick, South Dakota, waiting for his recovery team [71]. First Lieutenant Clifton McClure was assigned to pilot the ﬁnal Manhigh capsule and he lifted off at 06:51 in the morning of 8 October 1958. Three hours and ten minutes later the balloon stabilized at a rather frustrating 99 700 feet above the Sacramento Mountains in New Mexico. By lunchtime, the high altitude balloonist’s classic problem began to set in: the capsule was overheating. By 14:00 McClure’s body temperature was a rather alarming 39.7◦ C (103.4◦ F). The decision was made to terminate the ﬂight, but, as on Simons’s ﬂight, descending during the daytime was not an easy operation and, with the slow descent, McClure’s temperature continued to rise, reaching 40.9◦ C (105.6◦ F). After a physically exhausting descent he ﬁnally touched down in the desert, not far from the launch point. His pulse rate was 180 and his temperature a rather remarkable 42.5◦ C (108.5◦ F); but he was alive. Stapp’s Aero Medical Laboratory had demonstrated that it was possible (if not easy!) to take humans into near-vacuum conditions and, more importantly, the technology was ready for bringing them back safely too.

10.3 The highest step in the world The end of Manhigh did not mean an end to manned stratospheric balloon ﬂights at the Aero Medical Laboratory. Project Excelsior was devised and run by Joe Kittinger, the pilot of Manhigh I, to solve one of the Air Force’s most pressing problems in the mid-1950s: the safety of high altitude bail-outs. The question was, was it possible to escape safely from aircraft (or, as project Mercury was by then in full swing, space capsules) at high stratospheric altitudes? Two possible procedures were known at the time. First, the pilot could deploy his parachute immediately after ejecting from the aircraft. This had several drawbacks, mainly that the pilot would need to be provided with very substantial life support systems for the long descent through the thin, freezing air of the stratosphere (a previous chief of the Aero Medical Lab had nearly lost his life after leaping from a B-17 bomber at 40 200 feet and deploying his parachute straightaway – the opening shock knocked him unconscious and he nearly froze on the way down [72]). The alternative was to free-fall to a safe altitude and only open the parachute late in the dive. Experiments conducted on dummies highlighted a serious danger here: the pilot could go into a very fast, ﬂat spin, lose consciousness and die. A former Navy parachute rigger and engineer, Francis Beaupre, devised a solution that would later bear his name: the Beaupre Multi-Stage Parachute (BMSP). It allowed the pilot a 16 second free fall, after which a 45cm parachute would pull out a 1.5m stabilization

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chute from the backpack. This was not meant to reduce the speed, merely to prevent violent spins and tumbles. A secondary role of the stabilization chute was to begin to pull out the main chute, held closed at this point by a nylon web. Finally, the web would release at a pre-set altitude, allowing the main chute to open. After testing the BMSP at lower altitudes (Kittinger jumped out of Excelsior I from 76 400 feet and out of Excelsior II from 74 400 feet), the ‘highest step in the world’ (the Excelsior III capsule bore this label) was ready for the ‘big one’: a 100 000 foot ascent and bailout test. On 16 August 1960 Kittinger suited up for the historic ﬂight. In spite of being about to step out into the harshest conditions ever experienced by a parachutist, he chose not to wear a full pressure suit, in order to keep the experiment as similar as possible to a ‘normal’ bailout from an Air Force high altitude jet. He thus donned a standard partial pressure suit with winter ﬂying coveralls on top. In addition to the parachute he also carried a box containing an oxygen bottle and a number of instruments. With all this kit on he weighed in at 145 kg, nearly twice his body weight [71]. The ﬁrst few minutes of the ﬂight went according to plan. As the balloon ascended through the troposphere, the suit gradually pressurized and at 40 000 feet Kittinger decided to check, as per the procedures, that every part of the suit was working as planned. The exercise had a chilling result: the right glove had failed to pressurize. Kittinger later recalled weighing up the technical, physiological and political aspects of his predicament. If he had informed ground control of the malfunction, he would have been ordered to abort the ﬂight. He also knew too well that Excelsior was on the limit of its budget and it was likely that this ﬂight was going to be the last. What would happen to his unpressurized hand though, if he were to keep quiet and continue? His target altitude was seven miles above the Armstrong line, the level where, as we saw in Chapter 1, the blood boils at body temperature. The experience was clearly going to be excruciating, but would it have a lasting impact? Hoping that the glove would be tight enough to contain the inevitable swelling to some extent, he elected to continue. The balloon reached its equilibrium height at 102 800 feet. Having to wait 11 minutes there until Excelsior III drifted over the landing target, Kittinger had time to admire the scenery and recall an insight Colonel Stapp had once shared with him: jumping out at that altitude was like being enveloped in cyanide. If the suit were to depressurize, there would be no contingency – he would be dead in seconds. With this comforting thought in his mind and a hand feeling ‘like a chunk of ice’ Kittinger ﬁnally told ground control about the problem with his glove and, without listening to the answer, he stood up in the capsule and stepped out (Figure 10.4) [72]. After his 16 second free fall the stabilization chute opened – and it worked perfectly. As expected, it did not do much to his rate of descent though: he was passing 90 000 feet at 614 miles per hour, close to Mach 1. It was not until he entered the troposphere that his speed began to decay signiﬁcantly, reducing to about 250 miles per hour, as he was passing 30 000 feet, three and a half minutes after leaving the Excelsior III gondola. The main parachute ﬁnally opened, as planned, just over four and a half minutes into the descent. Less than ten more minutes later Kittinger was on the ground, where doctors could attend to his hand. As it turned out, in spite of being exposed for several minutes in the space equivalent zone, it suffered no lasting damage. Nor was his health otherwise affected after this, his fourth high altitude balloon ﬂight and third high altitude jump. In

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Fig. 10.4 102 800 feet above Earth Joe Kittinger steps out of Excelsior III. Image courtesy of the US Air Force.

fact, he would later step into a high altitude gondola again as part of project Stargazer, which had the rather ambitious goal of raising a telescope, complete with an astronomer, high above the light-distorting troposphere; this it successfully did on 13 December 1962 (Figure 10.5). Half a century has passed since Excelsior III and, perhaps surprisingly, Kittinger’s jump remains the highest and longest in history. Moreover, the ﬁrst part of his jump is still the longest free fall in history. These are unofﬁcial records, as, for a variety of tedious reasons, the F´ed´eration A´eronautique Internationale never sanctioned them. The Colonel himself insists that they were merely trying to develop a safe escape system (a quest in which they succeeded, developing a system for counter-acting the potentially fatal ﬂat spin) and

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Fig. 10.5 Project Stargazer. Colonel Kittinger and astronomer William White hovered for 13 hours at 81 500 feet, making telescope observations. Image courtesy of the US Air Force.

record breaking was never one of Excelsior’s goals. This cannot be said, however, about a string of would-be record breakers who have, over the last ﬁve decades, attempted to better Kittinger’s performance. Perhaps the earliest determined contender was an adventurer named Nicholas Piantanida, described by Craig Ryan in his excellent account of the history of high altitude manned ballooning [112], as a ‘dark-eyed loner’, a former pet store owner, soldier, boxer, diamond prospector, animal collector, truck driver and parachutist. He developed an obsession for breaking the jump altitude record and began working towards it with a great deal of enthusiasm and with an attitude that Kittinger would later describe as ‘cavalier’ [72]. After a failed ﬁrst attempt, his Strato Jump II vehicle reached an eye-watering altitude of 123 500 feet on 2 February 1966, but he was unable to disconnect himself from the

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Table 10.1 Selected milestones in high altitude ballooning (the † symbol denotes fatality resulting from the ﬂight). Data from [71, 105, 112]. Date

Vehicle

Pilot(s) / Designer

15 April 1875

Le Zenith

J. Crocce-Spinelli † T. Sivel † G. Tissandier H. C. Gray † A. Piccard, P. Kipfer unmanned / O Winzen unmanned J. W. Kittinger D. G. Simons C. McClure J. W. Kittinger M. D. Ross V. G. Prather † J. W. Kittinger W. White N. Piantanida N. Piantanida † unmanned / NASA 690 kg payload unmanned / JAXA 10 kg payload

4 November 1927 27 May 1931 25 September 1947 9 February 1956 2 June 1957 19–20 August 1957 8 October 1958 16 August 1960 4 May 1961

FNRS Manhigh Manhigh I Manhigh II Manhigh III Excelsior III Stratolab High V

13 December 1962

Stargazer

2 February 1966 1 May 1966 25 August 2002

Strato Jump II Strato Jump III Big 60

2003

Altitude [feet] 30 000

42 470 51 775 100 000 85 000 97 000 102 400 99 700 102 800 113 740 81 500 123 500 57 600 162 000 173 900

gondola, so he had to descend with it. On 1 May 1966 he took to the skies once more, on board Strato Jump III, in Sioux Falls, South Dakota (see Figure 10.6 for a picture of the gondola). At an altitude of 57 600 feet the ground crew suddenly heard a hissing sound over the radio – Piantanida’s suit was depressurizing. The gondola was cut loose, letting it parachute back to the ground. Piantanida was still alive when rescuers found Strato Jump III, but he never regained consciousness and died in hospital a few months later [112]. Today, there are at least six teams around the globe working towards fresh attempts at Kittinger’s record. In spite of having a 50-year technological advantage over the Excelsior team, they are encountering tremendous difﬁculties and this speaks volumes about the ingenuity of Kittinger, Stapp, Beaupre and the others. The fact that a small team, working half a century ago on a small budget on the fringes of the US space program managed to set an altitude benchmark that still stands, remains one of the greatest achievements in the history of stratospheric ﬂight and, indeed, the history of aeronautics in general. Jumping out of a stratospheric balloon is clearly not an exercise to be undertaken lightly. Traveling on board one, however, for no other purpose than to admire the stunning views, may soon become possible to anyone with sufﬁciently deep pockets. A slightly lower cost alternative to the suborbital space ﬂights offered by the burgeoning space tourism industry, high altitude ﬂights in a comfortable, pressurized cabin may prove popular in the future. At least one company, Spanish-based bloon, are proposing such a service, with prototype testing of a smaller scale version of their vehicle already underway (see Figure 10.7).

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Fig. 10.6 The Strato Jump III gondola did bring Nicholas Piantanida back to the ground, but it was too late – he never recovered from what had probably been the most severe stratospheric decompression in the history of aviation.

Fig. 10.7 Stratospheric passenger balloon test gondola. Image courtesy of inbloon.com.

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So where are the physical altitude limits of stratospheric – or, indeed, atmospheric – ﬂight? As we saw earlier, the evidence of the AeroVironment Helios unmanned vehicle suggests that it is unlikely that we will ever see a truly practical winged aircraft pass the 100 000 feet mark. This is approximately the halfway point of the stratosphere and it is clear that wing lift, as a sole vertical force, will never take us far beyond it. It is harder to estimate the practical altitude limits of buoyancy as the lifting force. As the example of a 2003 unmanned balloon ﬂight managed by the Japanese Aerospace Exploration Agency shows (see Table 10.1), even routine ﬂights into the mesosphere (beginning at around 165 000 feet) may be possible one day. What is certain, however, is that those seeking the thrill of winged or ﬂoating ﬂight at substantially higher altitudes need to take their ﬂying machines to planets with denser atmospheres; and that is precisely what an international team of engineers, led by the Soviet space agency, set out to do in 1984.

10.4 Above alien worlds By all accounts, Venus is an extremely inhospitable place. The ‘Earth-like’ planet is a textbook illustration of the results of runaway greenhouse effect. The average surface temperature is around 460◦ C (860◦ F) and the atmosphere is almost pure CO2 , except for the clouds, which are made up of sulphuric, hydrochloric and hydroﬂuoric acids. Perhaps most unpleasantly though, the mean surface pressure is about 91 times greater than on Earth – this is the pressure we would experience 910 meters under water on Earth. Although Venus is slightly smaller than Earth, its atmosphere is about 93 times heavier: a great environment for high altitude ﬂight! In spite of its comparative proximity, much of Venus’s geology, climatology and meteorology is poorly understood. It is almost certain that it once had abundant water, but it is unclear how and when its oceans disappeared. Its surface appears to be relatively new, yet there is no evidence (yet) of volcanic activity. Perhaps most intriguing though is the super-rotation of its atmosphere: it rotates in the same direction as the planet, but much faster. With the atmosphere moving about 50 times faster than the surface, Venus’s is the most extreme case of super-rotation in the Solar system. What maintains this large speed discrepancy is one of the great riddles of planetary meteorology [65, 119]. With such enduring mysteries limiting our knowledge of our nearest neighbor in the Solar system, Venus has long been the subject of constant interest from astronomers and this resulted in a string of orbiters and landers sent there to investigate geological and atmospherical phenomena. The backbone of this effort to collect more observational data was, over three decades, the highly successful Soviet Venera program, consisting of no fewer than 16 probes launched between 1961 and 1983. A follow-on international project, the Vega program, provided the opportunity for mankind’s ﬁrst attempt at ﬂight in another planet’s atmosphere. Vega-1 and Vega-2, the two twin spacecraft making up the Vega mission, carried an atmospheric science package each, which were to be deployed above Venus under automatically inﬂated helium balloons. Vegas 1 and 2 were launched on 15 and 21 December 1984 respectively. Vega-1 arrived at Venus on 11 June 1985. Following atmospheric entry at an altitude of about 410 000 feet, the main parachute of the spacecraft opened at an altitude of 210 000 feet. 55 seconds later,

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at 200 000 feet, the balloon package was extracted from the spacecraft by the parachute. 200 seconds after entry, at an altitude of approximately 180 000 feet, a second parachute extracted the furled balloon, triggering the inﬂation process. This was completed in about 100 seconds, at 177 000 feet, at which point the parachute and the inﬂation system were jettisoned, the balloon and its payload being ﬁnally free at an altitude of around 165 000 feet. 15-20 minutes after the high speed entry sequence the balloon ﬁnally reached its stable ﬂoating height between about 173 000 and 177 000 feet above the surface of Venus (four days later Vega-2 would arrive in similar fashion) [16]. The 3.54m diameter balloon (see Figure 10.8) was of a spherical, closed, superpressure design (that is, different from standard latex weather balloons in that it had a constant volume), made from a Teﬂon plastic cloth matrix, coated with Teﬂon, in order to withstand exposure to the sulphuric acid aerosols found in the Venusian clouds at the ﬂoat altitude. The total weight of the probe was 21 kg, with the balloon itself weighing 14 kg (including the 2 kg of helium inside) and the payload making up the remaining 7 kg. Experiments had shown that the helium would diffuse through the fabric at a rate that would allow it to maintain the required pressure and therefore its target altitude, for about 5 days [74]. This target altitude had, incidentally, been chosen such that the operating environment would be relatively benign, with the pressure at 175 000 feet being about the same as at 16 000 feet above Earth’s surface and the average temperature around 30◦ C (86◦ F) [47, 56]. This also placed it into the most active middle layer of Venus’s three-tiered cloud system [16]. A global array of 20 radio observatories tracked the two balloons on their journeys through Venus’s atmosphere using very long baseline interferometry. This is how we know that Vega-1 and Vega-2 both travelled over 11 000 km over the next two Earth days, carried by Venus’s super-rotating atmosphere at an average speed of around 150 miles per hour [103]. During this time they collected data from their thermometers, anemometers, photodetectors (designed to detect lightning) and nephelometers (cloud density sensors). The data was held for periods of 30 minutes in an on-board memory unit (during which time the 4.5 W transmitter of the sonde beamed back the signals required for the interferometrybased tracking), before being transmitted back to Earth at a rather patience-trying rate of approximately 4 bits per second [137]. It is interesting to speculate what may have happened to the balloons after contact with them was lost. How far did they ﬂoat silently on Venus’s rapid ‘jet stream’ before sinking into the dense CO2 ocean of its troposphere for the batteries and sealed compartments of their payloads to be crushed, still in ﬂight, like tin cans ran over by a road roller? What is beyond all speculation, however, is the technological achievement that made the Vega ﬂights possible and demonstrated that ﬂight in the atmospheres of alien worlds was not only possible, but scientiﬁcally valuable. Of course, the Vega balloons were dropped into the almost Earth-like ‘sweet spot’ of Venus and a better understanding of how the planet’s strange atmosphere works will require ﬂights at a range of altitudes. They were also only equipped with light payloads, severely limiting the life of their batteries, as well as the range of instruments they could carry. Unfortunately, neither of these shortcomings appear to be easy to solve. The strength to weight ratio of the balloon materials used on the Vega mission was rather low and would therefore scale poorly to larger balloons, not to mention the fabric’s inability to cope with higher temperatures [56]. Moreover, both Vega balloons leaked helium and longer dura-

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Fig. 10.8 Balloon-borne atmospheric exploration vehicle of the type deployed by the Vega space probes on Venus in 1985 (the actual vehicles had a 13 m long tether).

tion missions would have to consider other lifting gases (water has been suggested as a substitute, as it can only exist in vapor form on Venus below an altitude of about 140 000 feet) [65]. What about atmospheric ﬂights above other planets and moons? Table 10.2 lists the principal candidates. Titan, Saturn’s largest moon, is perhaps the most interesting case. The surface atmospheric pressure is 1.5 times that of Earth’s. While on Earth the 100mbar level is at 50 000 feet or so, on Titan the atmosphere reaches the same pressure at 50 km (164 000 feet) [15]. At the same time, its gravity is relatively weak. This is a rather tantalizing

Table 10.2 Physical properties of Earth-like celestial bodies. Name

Grativational acceleration [m/s2 ]

Atmosphere

Mean surface pressure [mbar]

Mean surface temperature

Earth Venus Mars Titan

9.78 8.87 3.72 1.35

N2 , O2 CO2 CO2 N2

1013 92 100 5.6 1500

15◦ C (59◦ F) 460◦ C (860◦ F) –63◦ C (–81◦ F) –180◦ C (–292◦ F)

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185

combination from the point of view of ﬂight, whether on wings or under a balloon. It has been estimated that an 8 tonne aircraft would only need a wing area of 4m2 to ﬂy at 100 miles per hour; moreover, human-powered ﬂight might be a practical proposition [141]. Incidentally, like Venus, Titan also appears to be a super-rotator, which might make ﬂight in certain directions and in certain layers of the atmosphere far more appealing than in others.

Fig. 10.9 The strongly layered, hazy atmosphere of Titan on a photograph taken by the Cassini spacecraft. Image courtesy of NASA, JPL and the Space Science Institute.

Alas, much more accessible (at least by unmanned probes) Mars has a rather thin atmosphere and this makes Martian aviation a greater technological challenge. The surface pressure on the red planet’s surface is about the same as 100 000 feet above Earth’s surface and, in spite of the rather low gravitational acceleration, this makes only the lowest layers of the Martian atmosphere suitable for any kind of ﬂight. Nevertheless, aircraft might still play an important role in the running of a future Mars colony and the technologies developed for high altitude ﬂight on Earth might ﬁnd applications in low altitude operations above the Martian surface.

Epilogue

At 22:29 UTC on 31 May 2009 Air France ﬂight AF447 took off from Rio de Janeiro with 216 passengers, 12 crew and 70.4 tonnes of fuel on board. The Airbus A330-203 was scheduled to arrive at Paris Charles de Gaulle airport at 09:00 the following morning. Just under two and a half hours into the ﬂight the aircraft passed over the Brazilian coast near the city of Natal and began its long overnight Atlantic crossing at an altitude of 35 000 feet. Half an hour later AF447 was approaching the handover point between Brazilian and Senegalese air trafﬁc control and, following a request from the Brazilian controller, the crew radioed them their estimated arrival times at a sequence of waypoints that lay ahead on the route. The Brazilian controller had a further question, but no answer came from AF447. In fact, the crew of the Paris ﬂight was not heard from again. To this date it is unclear what happened to the Airbus. With the last transmission received from far beyond the coverage of the Brazilian radars, even the time and the location of whatever calamitous event brought down the airliner are uncertain. Some of the wreckage has been located and recovered, but ocean currents may have carried it a long way from the area of impact. The shapes of the mangled pieces of aluminium lifted out the Atlantic speak of an impact with high vertical speed and a slight nose high attitude. In the absence of the two most important pieces of the wreckage – the ﬂight data recorder and the cockpit voice recorder are still deep under the ocean – it is hard to infer much more about the last seconds of AF447. There is, however, a very intriguing piece of evidence that may play a crucial role in determining the causes of the disaster. While the crew were not heard from again after that exchange with air trafﬁc control three hours into the ﬂight, the Airbus itself was. About 40 minutes after this radio conversation, over a period of about ﬁve minutes, the Air France maintenance center in Paris received a burst of 24 system messages from the A330’s computers. A typical use of this automated message relay system (which uses a satellite data link or VHF radio) is to enable the aircraft operator to plan maintenance tasks on its aircraft while they are still airborne; it is also used sometimes to help the airline’s dispatchers by providing regular position updates (once every ten minutes in the case of Air France). On this occasion, however, the 24 messages contained a very odd sequence of faults, some of which will have been displayed in the cockpit too, along with the corresponding aural warnings.

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188

Epilogue

At ﬁrst glance, they make little sense; in fact, even deeper analysis has failed, to date, to come up with a plausible scenario to explain all of them. Here is a small selection. The autopilot disconnected. The trafﬁc collision warning system was inactive. The ﬂight computer has switched to an alternate ﬂight control law. The auto-thrust disconnected. The rudder deﬂection limitation calculation function was unavailable. One of the primary and one of the secondary ﬂight control computers stopped functioning. The cabin altitude varied by more than 1800 feet over the space of ﬁve seconds... Most telling, perhaps, is an entire sequence of messages related to failures in the calculation of the airspeed of the Airbus, indicating potentially erroneous data from at least some of the Pitot-static (airspeed measurement) systems [13]. Perhaps the ﬂight data recorders will be found and we will learn more about what happened after AF447’s last radio transmission. We might learn whether it ﬂew into one of the many thunderstorm cells observed in the area that night, whether its Pitot tubes failed due to the accumulation of ice or for some other reason or, indeed, a different scenario might come to light. In any case, AF447 is yet another reminder that the encounter of a highly sophisticated and immensely complex ﬂying machine with the cold, thin and sometimes turbulent air of the stratosphere is an event that we may understand very well, but not completely. Whenever accidents like AF447 expose any gaps that may still exist in our knowledge, these are almost inevitably magniﬁed by the delicacy of the equilibrium that must be achieved for high altitude ﬂight to be possible. Unknown unknowns can almost be guaranteed to emerge from time to time as we make increasing use of the higher layers of our atmosphere. Meanwhile, there are many known unknowns for scientists and engineers to focus on, some of an academic nature (not to be neglected – these are the ones that may reveal some of the unknown unknowns!), others of an immediate importance. Here is a far from exhaustive list. There are still numerous unanswered questions about our physiological responses to low pressure environments. The impact of high altitude contrails on our climate is poorly understood and the efﬁciency and the effectiveness of proposed contrail reduction methods are uncertain. Our ability to forecast the concentration of ash clouds generated by volcano eruptions, as well as their reliable in-ﬂight detection need improvement. Much is still unknown about high altitude clear air turbulence and, once again, its reliable in-ﬂight detection is not a mature technology yet. On-board weather radars have seen spectacular developments over recent years, but they are not foolproof – in certain circumstances they may not pick up severe convective activity obscured by lesser cumulonimbus cells. We are still relatively uncertain about the altitude limits of winged or balloon ﬂight; in fact, it is unclear whether practical ﬂight will ever be possible beyond the stratosphere. Will the reader ever hold a volume similar to this, bearing the title Mesospheric Flight? All these questions are worth investigating. The answers might cost many millions and many years’ worth of effort, but the payoffs are tremendous. High altitude ﬂight already places us within a day’s travel of any point on the globe, but economic pressures demand continued technological development if the airline industry is to survive. The stratosphere is a potential stepping stone towards higher destinations too. Launching a spacecraft from a 40 000 foot stratospheric platform is a more economical proposition than launching it from the ground. Small satellites are already routinely launched into low

Epilogue

189

Earth orbit in this way and the new space tourism industry has embraced the idea too; time will tell whether this technology will be scalable to heavier launch vehicles. The upper atmosphere is set to become an increasingly useful environment for scientiﬁc facilities too. NASA’s Stratospheric Observatory for Infrared Astronomy may be the ﬁrst of many alternatives to space telescopes. High altitude balloons can now carry unmanned scientiﬁc laboratories to the edge of the stratosphere; developments in balloon materials and vehicle design are likely to further improve their payload and altitude capabilities. Over the 80 years since Piccard and Kipfer’s stratospheric balloon came to a safe landing on an Austrian glacier, high altitude ﬂight has come a very long way. The ‘space-like’ environment of the upper atmosphere, however, is no less harsh today than it was then and those who venture up there without due engineering and scientiﬁc diligence will almost inevitably be reminded of this. In spite of the technological leaps of the last eight decades, stratospheric ﬂight is still aviation at the limit.

Part V

Appendices

11. Unit conversions

0.95

14

28

13 12

24

0.75

11

22

10

20

0.6 0.55 0.5 0.45 0.4 0.35

Pounds per square inch ( psi )

Atmospheres ( atm )

0.65

9 8 7 6

Inches of Mercury ( in Hg )

0.8

18 16 14 12

650 600 550 500 450 400 350 300

5

10

4

8

200

3

6

150

250

0.3 0.25 0.2 0.15

2

4

100

1

2

50

0

0

0

0.1 0.05 0

1013

0

950

2

900

26

0.85

0.7

700

Millimeters of Mercury ( mm Hg )

0.9

760

4

850 800 750 700 650 600 550 500 450 400 350 300 250 200 150 100 50 0

Pressure Altitude based on the ISA ( thousands of feet )

29.92

2

14.79

Millibars ( mb ) = Hectopascals ( hPa ) = 100 N / m = 100 Pa

1

6 8 10 12 15

20

25 30 35 40 45 50

60 70 100

Fig. 11.1 Pressure conversion chart, aligned with the pressure model of the International Standard Atmosphere.

A. Sóbester, Stratospheric Flight: Aeronautics at the Limit, Springer Praxis Books, DOI 10.1007/978-1-4419-9458-5_11, © Springer Science+Business Media, LLC 2011

193

11. Unit conversions

120 110 100 90 80 70 60 50 40 30 20 10 0 ï10 ï20 ï30 ï40 ï50 ï60 ï70 ï80 ï90 ï100 ï110 ï120 ï130 ï140

Kelvin ( K )

50 45 40 35 30 25 20 15 10 5 0 ï5 ï10 ï15 ï20 ï25 ï30 ï35 ï40 ï45 ï50 ï55 ï60 ï65 ï70 ï75 ï80 ï85 ï90 ï95 ï100

Degrees Fahrenheit ( oF )

o

Degrees Celsius ( C )

194

320 315 310 305 300 295 290 285 280 275 270 265 260 255 250 245 240 235 230 225 220 215 210 205 200 195 190 185 180

Fig. 11.2 Temperature conversion chart.

900 880 860 840 820 800 780 760 740 720 700 680 660 640 620 600 580 560 540 520 500 480 460 440 420 400 380 360 340 320 300 280 260 240 220 200 180 160 140 120 100 80 60 40 20 0

880 860 840 820 800 780 760 740 720 700 680 660 640 620 600 580 560 540 520 500 480 460 440 420 400 380 360 340 320 300 280 260 240 220 200 180 160 140 120 100 80 60 40 20 0

Airspeed at 100,000 feet ( 30,480 m ) ( m/s )

980 960 940 920 900 880 860 840 820 800 780 760 740 720 700 680 660 640 620 600 580 560 540 520 500 480 460 440 420 400 380 360 340 320 300 280 260 240 220 200 180 160 140 120 100 80 60 40 20 0

Airspeed at 36,000 feet ( 10,973 m ) ( m/s )

1020 1000 980 960 940 920 900 880 860 840 820 800 780 760 740 720 700 680 660 640 620 600 580 560 540 520 500 480 460 440 420 400 380 360 340 320 300 280 260 240 220 200 180 160 140 120 100 80 60 40 20 0

Airspeed at 32,000 feet ( 9,754 m ) ( m/s )

195

Airspeed at 10,000 feet ( 3,048 m ) ( m/s )

3 2.9 2.8 2.7 2.6 2.5 2.4 2.3 2.2 2.1 2 1.9 1.8 1.7 1.6 1.5 1.4 1.3 1.2 1.1 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

Airspeed at sea level ( m/s )

Mach number (M)

11. Unit conversions

900 880 860 840 820 800 780 760 740 720 700 680 660 640 620 600 580 560 540 520 500 480 460 440 420 400 380 360 340 320 300 280 260 240 220 200 180 160 140 120 100 80 60 40 20 0

Fig. 11.3 The relationship (as per the International Standard Atmosphere model) between Mach number and airspeed at different altitudes (airspeed measured in meters per second).

1700 1650 1600 1550 1500 1450 1400 1350 1300 1250 1200 1150 1100 1050 1000 950 900 850 800 750 700 650 600 550 500 450 400 350 300 250 200 150 100 50 0

Airspeed at 100,000 feet ( 30,480 m ) ( knots )

1750 1700 1650 1600 1550 1500 1450 1400 1350 1300 1250 1200 1150 1100 1050 1000 950 900 850 800 750 700 650 600 550 500 450 400 350 300 250 200 150 100 50 0

Airspeed at 36,000 feet ( 10,973 m ) ( knots )

1900 1850 1800 1750 1700 1650 1600 1550 1500 1450 1400 1350 1300 1250 1200 1150 1100 1050 1000 950 900 850 800 750 700 650 600 550 500 450 400 350 300 250 200 150 100 50 0

Airspeed at 32,000 feet ( 9,754 m ) ( knots )

1950 1900 1850 1800 1750 1700 1650 1600 1550 1500 1450 1400 1350 1300 1250 1200 1150 1100 1050 1000 950 900 850 800 750 700 650 600 550 500 450 400 350 300 250 200 150 100 50 0

Airspeed at 10,000 feet ( 3,048 m ) ( knots )

3 2.9 2.8 2.7 2.6 2.5 2.4 2.3 2.2 2.1 2 1.9 1.8 1.7 1.6 1.5 1.4 1.3 1.2 1.1 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

Airspeed at sea level ( knots )

11. Unit conversions

Mach number (M)

196

1750 1700 1650 1600 1550 1500 1450 1400 1350 1300 1250 1200 1150 1100 1050 1000 950 900 850 800 750 700 650 600 550 500 450 400 350 300 250 200 150 100 50 0

Fig. 11.4 The relationship (as per the International Standard Atmosphere model) between Mach number and airspeed at different altitudes (airspeed measured in knots).

2000 1950 1900 1850 1800 1750 1700 1650 1600 1550 1500 1450 1400 1350 1300 1250 1200 1150 1100 1050 1000 950 900 850 800 750 700 650 600 550 500 450 400 350 300 250 200 150 100 50 0

1950 1900 1850 1800 1750 1700 1650 1600 1550 1500 1450 1400 1350 1300 1250 1200 1150 1100 1050 1000 950 900 850 800 750 700 650 600 550 500 450 400 350 300 250 200 150 100 50 0

Airspeed at 100,000 feet ( 30,480 m ) ( MPH )

2200 2150 2100 2050 2000 1950 1900 1850 1800 1750 1700 1650 1600 1550 1500 1450 1400 1350 1300 1250 1200 1150 1100 1050 1000 950 900 850 800 750 700 650 600 550 500 450 400 350 300 250 200 150 100 50 0

Airspeed at 36,000 feet ( 10,973 m ) ( MPH )

2250 2200 2150 2100 2050 2000 1950 1900 1850 1800 1750 1700 1650 1600 1550 1500 1450 1400 1350 1300 1250 1200 1150 1100 1050 1000 950 900 850 800 750 700 650 600 550 500 450 400 350 300 250 200 150 100 50 0

Airspeed at 32,000 feet ( 9,754 m ) ( MPH )

197

Airspeed at 10,000 feet ( 3,048 m ) ( MPH )

3 2.9 2.8 2.7 2.6 2.5 2.4 2.3 2.2 2.1 2 1.9 1.8 1.7 1.6 1.5 1.4 1.3 1.2 1.1 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

Airspeed at sea level ( MPH )

Mach number (M)

11. Unit conversions

2000 1950 1900 1850 1800 1750 1700 1650 1600 1550 1500 1450 1400 1350 1300 1250 1200 1150 1100 1050 1000 950 900 850 800 750 700 650 600 550 500 450 400 350 300 250 200 150 100 50 0

Fig. 11.5 The relationship (as per the International Standard Atmosphere model) between Mach number and airspeed at different altitudes (airspeed measured in miles per hour).

3100 3000 2900 2800 2700 2600 2500 2400 2300 2200 2100 2000 1900 1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800 700 600 500 400 300 200 100 0

Airspeed at 100,000 feet ( 30,480 m ) ( km/h )

3200 3100 3000 2900 2800 2700 2600 2500 2400 2300 2200 2100 2000 1900 1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800 700 600 500 400 300 200 100 0

Airspeed at 36,000 feet ( 10,973 m ) ( km/h )

3500 3400 3300 3200 3100 3000 2900 2800 2700 2600 2500 2400 2300 2200 2100 2000 1900 1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800 700 600 500 400 300 200 100 0

Airspeed at 32,000 feet ( 9,754 m ) ( km/h )

3600 3500 3400 3300 3200 3100 3000 2900 2800 2700 2600 2500 2400 2300 2200 2100 2000 1900 1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800 700 600 500 400 300 200 100 0

Airspeed at 10,000 feet ( 3,048 m ) ( km/h )

3 2.9 2.8 2.7 2.6 2.5 2.4 2.3 2.2 2.1 2 1.9 1.8 1.7 1.6 1.5 1.4 1.3 1.2 1.1 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

Airspeed at sea level ( km/h )

11. Unit conversions

Mach number (M)

198

3200 3100 3000 2900 2800 2700 2600 2500 2400 2300 2200 2100 2000 1900 1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800 700 600 500 400 300 200 100 0

Fig. 11.6 The relationship (as per the International Standard Atmosphere model) between Mach number and airspeed at different altitudes (airspeed measured in kilometres per hour).

12. Temperature proﬁles around the globe

Table 12.1 Temperatures aloft above McMurdo Station, Antarctica, Lat.: 78o S. The ‘Low’ and ‘High’ values deﬁne the interdecile range: 80% of the ordered values collected over a ten year period fall between them. At these latitudes the stratosphere is generally warmer than predicted by the ISA model. Altitude [ft]

ISA Pressure [mbar]

ISA

4000 6000 8000 10000 12000 14000 16000 18000 20000 22000 24000 26000 28000 30000 31000 32000 33000 34000 35000 36000 37000 38000 39000 40000 42000 44000 46000 48000 50000 52000 54000 56000 58000 60000

875 812 753 697 644 595 549 506 466 428 393 360 329 301 287 274 262 250 238 227 217 206 197 188 170 155 141 128 116 105 96 87 79 72

7 3 –1 –5 –9 –13 –17 –21 –25 –29 –33 –37 –40 –44 –46 –48 –50 –52 –54 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56

Temperature [o C] Mean Low –19 –22 –24 –27 –30 –33 –37 –41 –45 –49 –53 –55 –57 –58 –58 –58 –58 –58 –57 –57 –58 –58 –58 –58 –58 –57 –57 –56 –55 –54 –52 –51 –50 –48

–29 –30 –32 –33 –36 –40 –44 –48 –52 –56 –59 –63 –65 –68 –69 –70 –71 –72 –73 –73 –74 –74 –75 –75 –76 –76 –77 –76 –75 –74 –72 –70 –68 –66

High

ISA

–10 –14 –17 –20 –23 –26 –30 –34 –38 –42 –46 –48 –48 –46 –45 –45 –44 –44 –43 –43 –43 –43 –43 –43 –42 –42 –42 –42 –41 –41 –40 –39 –38 –38

45 38 30 23 16 9 2 –5 –12 –19 –27 –34 –41 –48 –52 –55 –59 –62 –66 –69 –70 –70 –70 –70 –70 –70 –70 –70 –70 –70 –70 –70 –70 –70

Temperature [o F] Mean Low –2 –7 –12 –16 –21 –28 –35 –42 –49 –56 –63 –68 –71 –72 –72 –72 –72 –72 –71 –71 –72 –72 –72 –72 –72 –71 –70 –69 –67 –65 –62 –60 –57 –54

A. Sóbester, Stratospheric Flight: Aeronautics at the Limit, Springer Praxis Books, DOI 10.1007/978-1-4419-9458-5_12, © Springer Science+Business Media, LLC 2011

–19 –22 –25 –28 –33 –40 –48 –55 –61 –68 –75 –81 –86 –91 –93 –95 –96 –97 –99 –100 –101 –101 –102 –103 –104 –106 –106 –105 –103 –102 –98 –94 –91 –87

High 14 7 1 –4 –10 –16 –22 –29 –36 –44 –50 –54 –54 –51 –49 –48 –47 –47 –46 –46 –45 –45 –45 –45 –44 –44 –44 –43 –42 –41 –40 –39 –37 –36

199

200

12. Temperature proﬁles around the globe

Table 12.2 Temperatures aloft above Anchorage, Alaska, United States, Lat.: 61o N. The ‘Low’ and ‘High’ values deﬁne the interdecile range: 80% of the ordered values collected over a ten year period fall between them. At these latitudes the stratosphere is generally warmer than predicted by the ISA model. Altitude [ft]

ISA Pressure [mbar]

ISA

4000 6000 8000 10000 12000 14000 16000 18000 20000 22000 24000 26000 28000 30000 31000 32000 33000 34000 35000 36000 37000 38000 39000 40000 42000 44000 46000 48000 50000 52000 54000 56000 58000 60000

875 812 753 697 644 595 549 506 466 428 393 360 329 301 287 274 262 250 238 227 217 206 197 188 170 155 141 128 116 105 96 87 79 72

7 3 –1 –5 –9 –13 –17 –21 –25 –29 –33 –37 –40 –44 –46 –48 –50 –52 –54 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56

Temperature [o C] Mean Low –2 –5 –9 –12 –16 –20 –24 –28 –32 –36 –40 –44 –47 –50 –51 –52 –52 –52 –52 –52 –51 –51 –51 –50 –50 –50 –50 –50 –50 –51 –51 –51 –51 –51

–11 –14 –18 –22 –26 –30 –34 –39 –43 –47 –50 –53 –55 –57 –57 –58 –58 –59 –59 –59 –59 –59 –58 –57 –56 –55 –55 –55 –55 –55 –55 –55 –56 –56

High

ISA

8 4 1 –3 –6 –9 –13 –17 –21 –25 –29 –34 –38 –43 –44 –46 –46 –46 –46 –45 –45 –45 –45 –45 –45 –45 –46 –46 –46 –46 –47 –47 –47 –47

45 38 30 23 16 9 2 –5 –12 –19 –27 –34 –41 –48 –52 –55 –59 –62 –66 –69 –70 –70 –70 –70 –70 –70 –70 –70 –70 –70 –70 –70 –70 –70

Temperature [o F] Mean Low 29 23 16 10 3 –3 –10 –18 –25 –33 –40 –47 –53 –58 –60 –61 –62 –62 –62 –61 –61 –60 –59 –59 –58 –58 –58 –58 –59 –59 –59 –60 –60 –60

12 6 0 –7 –14 –22 –30 –38 –45 –52 –58 –63 –67 –70 –71 –72 –73 –73 –74 –74 –74 –74 –73 –71 –68 –67 –67 –67 –67 –67 –68 –68 –68 –69

High 46 39 34 27 21 15 9 2 –5 –13 –21 –29 –37 –45 –48 –50 –51 –51 –50 –50 –49 –49 –49 –49 –49 –50 –50 –51 –51 –52 –52 –52 –52 –53

12. Temperature proﬁles around the globe

201

Table 12.3 Temperatures aloft above Nottingham, United Kingdom, Lat.: 53o N. The ‘Low’ and ‘High’ values deﬁne the interdecile range: 80% of the ordered values collected over a ten year period fall between them. At these latitudes the ISA is a good approximation of the average temperatures. Altitude [ft]

ISA Pressure [mbar]

ISA

4000 6000 8000 10000 12000 14000 16000 18000 20000 22000 24000 26000 28000 30000 31000 32000 33000 34000 35000 36000 37000 38000 39000 40000 42000 44000 46000 48000 50000 52000 54000 56000 58000 60000

875 812 753 697 644 595 549 506 466 428 393 360 329 301 287 274 262 250 238 227 217 206 197 188 170 155 141 128 116 105 96 87 79 72

7 3 –1 –5 –9 –13 –17 –21 –25 –29 –33 –37 –40 –44 –46 –48 –50 –52 –54 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56

Temperature [o C] Mean Low 4 1 –2 –5 –9 –12 –16 –20 –25 –29 –33 –38 –42 –46 –48 –50 –51 –53 –54 –55 –55 –56 –56 –56 –56 –55 –56 –56 –56 –57 –57 –57 –58 –58

–3 –7 –10 –14 –17 –21 –26 –30 –34 –39 –43 –47 –50 –53 –55 –57 –58 –60 –61 –63 –64 –65 –65 –65 –64 –63 –63 –63 –63 –63 –64 –64 –64 –65

High

ISA

11 8 6 2 –1 –4 –8 –12 –16 –20 –24 –29 –34 –38 –41 –43 –45 –46 –47 –47 –47 –47 –47 –47 –48 –48 –49 –50 –50 –51 –51 –51 –52 –52

45 38 30 23 16 9 2 –5 –12 –19 –27 –34 –41 –48 –52 –55 –59 –62 –66 –69 –70 –70 –70 –70 –70 –70 –70 –70 –70 –70 –70 –70 –70 –70

Temperature [o F] Mean Low 39 34 29 23 17 10 3 –5 –12 –20 –28 –36 –44 –51 –55 –58 –61 –63 –65 –67 –68 –68 –69 –69 –68 –68 –68 –69 –69 –70 –71 –71 –72 –72

26 20 14 8 1 –7 –14 –22 –30 –38 –45 –52 –58 –64 –67 –70 –73 –76 –78 –81 –83 –85 –86 –86 –84 –82 –81 –81 –81 –82 –82 –83 –84 –85

High 51 47 42 36 30 24 17 11 3 –4 –12 –20 –29 –37 –41 –45 –48 –51 –52 –52 –52 –52 –52 –53 –54 –55 –56 –57 –58 –59 –60 –61 –61 –61

202

12. Temperature proﬁles around the globe

Table 12.4 Temperatures aloft above Budapest, Hungary, Lat.: 47o N. The ‘Low’ and ‘High’ values deﬁne the interdecile range: 80% of the ordered values collected over a ten year period fall between them. At these latitudes the ISA is a good approximation of the average temperatures. Altitude [ft]

ISA Pressure [mbar]

ISA

4000 6000 8000 10000 12000 14000 16000 18000 20000 22000 24000 26000 28000 30000 31000 32000 33000 34000 35000 36000 37000 38000 39000 40000 42000 44000 46000 48000 50000 52000 54000 56000 58000 60000

875 812 753 697 644 595 549 506 466 428 393 360 329 301 287 274 262 250 238 227 217 206 197 188 170 155 141 128 116 105 96 87 79 72

7 3 –1 –5 –9 –13 –17 –21 –25 –29 –33 –37 –40 –44 –46 –48 –50 –52 –54 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56

Temperature [o C] Mean Low 6 3 –1 –4 –8 –11 –15 –19 –23 –28 –32 –37 –42 –46 –48 –50 –52 –53 –54 –55 –56 –56 –56 –56 –56 –56 –56 –57 –57 –58 –58 –58 –58 –59

–5 –8 –11 –14 –17 –21 –25 –29 –34 –38 –43 –47 –51 –54 –56 –57 –59 –60 –61 –62 –63 –63 –64 –64 –63 –62 –62 –62 –62 –63 –63 –63 –64 –64

High

ISA

16 12 8 4 1 –3 –6 –10 –14 –18 –23 –27 –32 –37 –39 –42 –44 –46 –48 –49 –50 –50 –49 –49 –50 –50 –51 –52 –52 –53 –54 –54 –54 –54

45 38 30 23 16 9 2 –5 –12 –19 –27 –34 –41 –48 –52 –55 –59 –62 –66 –69 –70 –70 –70 –70 –70 –70 –70 –70 –70 –70 –70 –70 –70 –70

Temperature [o F] Mean Low 43 37 31 25 18 12 5 –2 –10 –18 –26 –35 –43 –51 –54 –58 –61 –64 –66 –68 –69 –69 –70 –70 –69 –69 –69 –70 –71 –72 –72 –73 –73 –74

23 18 13 7 1 –6 –13 –21 –29 –37 –45 –53 –60 –66 –69 –71 –74 –76 –78 –80 –81 –82 –83 –83 –82 –80 –80 –80 –80 –81 –81 –82 –82 –83

High 61 53 46 40 33 27 20 14 7 –1 –9 –17 –26 –35 –39 –43 –47 –51 –54 –56 –57 –57 –57 –57 –58 –59 –60 –61 –62 –63 –64 –65 –66 –66

12. Temperature proﬁles around the globe

203

Table 12.5 Temperatures aloft above Port Elizabeth, South Africa, Lat.: 34o S. The ‘Low’ and ‘High’ values deﬁne the interdecile range: 80% of the ordered values collected over a ten year period fall between them. Note the large deviations between the ISA and the actual values at higher altitudes. Altitude [ft]

ISA Pressure [mbar]

ISA

4000 6000 8000 10000 12000 14000 16000 18000 20000 22000 24000 26000 28000 30000 31000 32000 33000 34000 35000 36000 37000 38000 39000 40000 42000 44000 46000 48000 50000 52000 54000 56000 58000 60000

875 812 753 697 644 595 549 506 466 428 393 360 329 301 287 274 262 250 238 227 217 206 197 188 170 155 141 128 116 105 96 87 79 72

7 3 –1 –5 –9 –13 –17 –21 –25 –29 –33 –37 –40 –44 –46 –48 –50 –52 –54 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56

Temperature [o C] Mean Low 13 11 8 4 1 –3 –7 –11 –15 –19 –24 –28 –33 –38 –40 –42 –44 –46 –48 –50 –52 –53 –55 –56 –57 –59 –60 –62 –63 –65 –66 –66 –66 –66

7 3 0 –3 –6 –9 –13 –17 –21 –26 –30 –35 –40 –44 –46 –48 –50 –52 –54 –56 –57 –59 –60 –61 –63 –64 –65 –67 –69 –70 –71 –72 –72 –71

High

ISA

21 18 14 10 6 2 –2 –6 –9 –14 –18 –22 –27 –32 –34 –36 –39 –41 –43 –45 –47 –48 –49 –51 –52 –54 –55 –57 –58 –59 –60 –60 –60 –60

45 38 30 23 16 9 2 –5 –12 –19 –27 –34 –41 –48 –52 –55 –59 –62 –66 –69 –70 –70 –70 –70 –70 –70 –70 –70 –70 –70 –70 –70 –70 –70

Temperature [o F] Mean Low 56 51 46 40 33 26 19 12 5 –3 –11 –19 –27 –36 –40 –44 –48 –51 –55 –58 –61 –64 –66 –68 –71 –74 –76 –79 –82 –84 –86 –87 –87 –86

44 38 32 27 22 15 9 1 –7 –15 –23 –31 –39 –47 –51 –55 –58 –62 –65 –68 –71 –74 –76 –78 –81 –83 –85 –88 –92 –94 –96 –98 –97 –96

High 70 64 57 50 43 36 29 22 15 8 0 –8 –16 –25 –29 –33 –37 –41 –45 –49 –52 –55 –57 –59 –62 –65 –68 –70 –73 –75 –76 –77 –77 –77

204

12. Temperature proﬁles around the globe

Table 12.6 Temperatures aloft above Tucson, Arizona, United States, Lat.: 32o N. The ‘Low’ and ‘High’ values deﬁne the interdecile range: 80% of the ordered values collected over a ten year period fall between them. Note the large deviations between the ISA and the actual values at higher altitudes. Altitude [ft]

ISA Pressure [mbar]

ISA

4000 6000 8000 10000 12000 14000 16000 18000 20000 22000 24000 26000 28000 30000 31000 32000 33000 34000 35000 36000 37000 38000 39000 40000 42000 44000 46000 48000 50000 52000 54000 56000 58000 60000

875 812 753 697 644 595 549 506 466 428 393 360 329 301 287 274 262 250 238 227 217 206 197 188 170 155 141 128 116 105 96 87 79 72

7 3 –1 –5 –9 –13 –17 –21 –25 –29 –33 –37 –40 –44 –46 –48 –50 –52 –54 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56

Temperature [o C] Mean Low 20 16 11 6 2 –2 –6 –10 –14 –18 –22 –27 –31 –36 –38 –41 –43 –45 –47 –49 –51 –53 –54 –55 –58 –60 –62 –64 –66 –67 –68 –68 –68 –67

9 5 1 –2 –6 –9 –13 –17 –22 –26 –30 –35 –39 –44 –46 –48 –50 –52 –54 –56 –58 –59 –61 –62 –63 –64 –66 –69 –71 –73 –73 –73 –72 –71

High

ISA

30 25 19 14 9 4 0 –4 –7 –11 –15 –19 –23 –28 –30 –32 –34 –37 –39 –41 –44 –46 –48 –50 –53 –55 –57 –58 –60 –62 –63 –63 –64 –63

45 38 30 23 16 9 2 –5 –12 –19 –27 –34 –41 –48 –52 –55 –59 –62 –66 –69 –70 –70 –70 –70 –70 –70 –70 –70 –70 –70 –70 –70 –70 –70

Temperature [o F] Mean Low 68 60 52 44 36 29 22 14 7 –1 –8 –17 –25 –33 –37 –41 –45 –49 –53 –56 –60 –63 –65 –68 –72 –76 –79 –83 –87 –89 –91 –91 –91 –89

49 41 34 28 22 15 8 1 –7 –15 –23 –31 –39 –47 –51 –55 –58 –62 –66 –69 –72 –75 –77 –79 –82 –84 –87 –92 –97 –100 –100 –100 –98 –96

High 86 76 67 57 48 40 32 25 19 12 5 –2 –10 –18 –22 –26 –30 –34 –39 –43 –47 –51 –55 –58 –64 –67 –70 –73 –76 –79 –81 –82 –83 –82

12. Temperature proﬁles around the globe

205

Table 12.7 Temperatures aloft above Hyderabad, India, Lat.: 17o N. The ‘Low’ and ‘High’ values deﬁne the interdecile range: 80% of the ordered values collected over a ten year period fall between them. Note the enormous deviations between the ISA and the actual values. Altitude [ft]

ISA Pressure [mbar]

ISA

4000 6000 8000 10000 12000 14000 16000 18000 20000 22000 24000 26000 28000 30000 31000 32000 33000 34000 35000 36000 37000 38000 39000 40000 42000 44000 46000 48000 50000 52000 54000 56000 58000 60000

875 812 753 697 644 595 549 506 466 428 393 360 329 301 287 274 262 250 238 227 217 206 197 188 170 155 141 128 116 105 96 87 79 72

7 3 –1 –5 –9 –13 –17 –21 –25 –29 –33 –37 –40 –44 –46 –48 –50 –52 –54 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56

Temperature [o C] Mean Low 19 17 14 11 7 4 0 –3 –6 –10 –13 –17 –21 –26 –28 –31 –33 –36 –38 –41 –43 –46 –48 –51 –56 –61 –66 –71 –76 –79 –81 –82 –80 –77

18 15 13 10 6 3 –1 –4 –7 –11 –15 –19 –23 –27 –30 –32 –35 –37 –40 –42 –45 –47 –50 –52 –57 –63 –68 –72 –77 –81 –85 –87 –87 –83

High

ISA

21 18 15 12 9 5 2 –2 –5 –8 –12 –16 –20 –24 –27 –29 –31 –34 –36 –39 –41 –44 –47 –49 –55 –60 –65 –70 –74 –77 –77 –75 –73 –70

45 38 30 23 16 9 2 –5 –12 –19 –27 –34 –41 –48 –52 –55 –59 –62 –66 –69 –70 –70 –70 –70 –70 –70 –70 –70 –70 –70 –70 –70 –70 –70

Temperature [o F] Mean Low 67 62 57 51 45 39 33 27 21 15 8 1 –7 –14 –19 –23 –27 –32 –36 –41 –46 –50 –55 –60 –69 –78 –87 –96 –104 –111 –115 –115 –112 –106

65 60 55 49 43 37 31 25 19 12 6 –2 –9 –17 –22 –26 –30 –35 –39 –44 –48 –53 –58 –62 –71 –81 –90 –98 –107 –114 –120 –125 –125 –117

High 69 65 60 54 47 41 35 29 23 17 11 4 –4 –12 –16 –20 –25 –29 –33 –38 –43 –47 –52 –57 –66 –76 –85 –94 –101 –106 –107 –103 –99 –94

206

12. Temperature proﬁles around the globe

Table 12.8 Temperatures aloft above Nauru, Micronesia, Lat.: 0o . The ‘Low’ and ‘High’ values deﬁne the interdecile range: 80% of the ordered values collected over a ten year period fall between them. Note the enormous deviations between the ISA and the actual values. Altitude [ft]

ISA Pressure [mbar]

ISA

4000 6000 8000 10000 12000 14000 16000 18000 20000 22000 24000 26000 28000 30000 31000 32000 33000 34000 35000 36000 37000 38000 39000 40000 42000 44000 46000 48000 50000 52000 54000 56000 58000 60000

875 812 753 697 644 595 549 506 466 428 393 360 329 301 287 274 262 250 238 227 217 206 197 188 170 155 141 128 116 105 96 87 79 72

7 3 –1 –5 –9 –13 –17 –21 –25 –29 –33 –37 –40 –44 –46 –48 –50 –52 –54 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56 –56

Temperature [o C] Mean Low 19 17 14 11 7 4 0 –3 –6 –10 –13 –17 –21 –26 –28 –31 –33 –36 –38 –41 –43 –46 –48 –51 –56 –61 –66 –71 –76 –79 –81 –82 –80 –77

18 15 13 10 6 3 –1 –4 –7 –11 –15 –19 –23 –27 –30 –32 –35 –37 –40 –42 –45 –47 –50 –52 –57 –63 –68 –72 –77 –81 –85 –87 –87 –83

High

ISA

21 18 15 12 9 5 2 –2 –5 –8 –12 –16 –20 –24 –27 –29 –31 –34 –36 –39 –41 –44 –47 –49 –55 –60 –65 –70 –74 –77 –77 –75 –73 –70

45 38 30 23 16 9 2 –5 –12 –19 –27 –34 –41 –48 –52 –55 –59 –62 –66 –69 –70 –70 –70 –70 –70 –70 –70 –70 –70 –70 –70 –70 –70 –70

Temperature [o F] Mean Low 67 62 57 51 45 39 33 27 21 15 8 1 –7 –14 –19 –23 –27 –32 –36 –41 –46 –50 –55 –60 –69 –78 –87 –96 –104 –111 –115 –115 –112 –106

65 60 55 49 43 37 31 25 19 12 6 –2 –9 –17 –22 –26 –30 –35 –39 –44 –48 –53 –58 –62 –71 –81 –90 –98 –107 –114 –120 –125 –125 –117

High 69 65 60 54 47 41 35 29 23 17 11 4 –4 –12 –16 –20 –25 –29 –33 –38 –43 –47 –52 –57 –66 –76 –85 –94 –101 –106 –107 –103 –99 –94

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Index

aerodynamic heating see kinetic heating AeroVironment Helios 170, 172, 182 Pathﬁnder 171 Airbus 44–45 319 152–155 320 130 330 145, 187–188 Anchorage, AK XX, 116, 159–160, 162 Armstrong line (or point) XVIII, 11, 177 ash (volcanic) 159–166, 188 atmosphere international standard (ISA) XVII–XX, 5, 77, 96–97, 140, 193, 195–206 balloon gas XX, XXI, XXII, 6–7, 123–129, 173–185, 188–189 paper 124 weather XVIII, 125–129, 155 barodontalgia 16 Beaufort scale XVII Beaupre, F. 176, 180 Bell 99 47 101 UH-1 101 X-1 99–100 bends (the) 14, 24, 40 (see also decompression sickness) Blackbird see Lockheed bleed air 23, 29–30, 105, 162 Boeing 17, 33, 45, 72, 87–88, 118 707 88, 101, 136 727 137 737 18, 27, 28, 33, 34, 35, 37, 41, 42, 47, 72–74, 154, 169

747 49, 50 52 146–149, 150, 153–155, 159, 160 757 165 767 30, 165 777 43, 113–115, 117–119 787 18, 29, 45 B-29 Superfortress 99, 123 B-52 Stratofortress 38–41, 118 Business Jet (BBJ) 18 Dash-80 56, 88 MD-11 87 Sonic Cruiser 97 Boulder, CO 135, 136, 139, 140, 141 British Overseas Airways Corporation (BOAC) 56–60, 62, 66–67 cabin altitude see pressure cabin pressure see pressure cirrus 171 cofﬁn corner 85–87, 93, 97, 151, 169, 173 Comet (de Havilland) 55–73, 88, 105, 115 Mk4 87–88 Concorde XVIII, 18, 21, 29, 44, 96, 101–106, 108, 128 Connie 926 135–140, 142 cumulonimbous 119, 121, 145–146, 149–152, 188 DCS see decompression sickness decompression sickness 12–15, 24, 45, 49–50 decompression see depressurization de Havilland, G. Jr. 99 depressurization 9, 15, 30, 35, 37–38, 42–52, 60, 72–73, 105, 181 explosive 38, 42, 50, 60, 66, 67, 72 rapid see explosive

214 descent XXII, 11, 23, 25, 32, 35, 41, 59, 66, 85, 87, 91, 115, 117–119, 121, 126, 137, 151, 159, 175, 177 emergency 34, 47–50, 52, 108 rate (or angle) of 15, 32, 114, 177 drag (aerodynamic) XIX, 47, 63, 69, 78, 81–82, 85–86, 96–98, 114 ebullism 11–12 eight-thousanders 4–5 emergency descent see descent Enevoldson, E. 173 Everest XVIII–XIX, 4–6, 9, 11, 31, 56 Excelsior (project and aircraft) XVIII, 176–180 Eyjafjallaj¨okull (volcano) 162–166 fail safe (design method) 72 fatigue (human) 9–10, 14, 17, 23 fatigue (metal) 16, 18, 42, 64–69, 71–73, 156 fuel see also jet fuel burn XIX, 16, 43, 45, 88–89, 97–98, 102, 130, 162 G-ALYP (Comet) 57, 58–61, 63–64, 66–71 G-ALYU (Comet) 67, 68 G-YMMM (B777) 113, 114–115, 117, 118 ‘Glamorous Glennis’ 99, 101 glider see sailplane gravity wave see turbulence Gray, H. C. XX, XXII, 180 hail 103, 104, 151 Hall, A. 62–64, 67, 68, 69, 72–73 Herzog, M. 4 Hibbard, H. 90 Hillary, E. 5, 56 Hives (Lord) 88 ice / icing 36, 59, 118–121, 161–162, 188 hydrazine 170–171 hypoxia (hypobaric) 3, 7–9, 11–13, 15, 31, 33–35, 43, 45, 48–50 stages of 8–9 International Standard Atmosphere (ISA) see atmosphere jet fuel (behavior at low temperatures) jet A XVII, XVIII jet A-1 XVIII, 114–115, 117 jet stream 97, 99, 124, 125–126, 129–130, 139, 142, 144, 151, 164, 171, 175, 183, Johnson, K. 90–91, 93, 106 K´arm´an, T. 100

Index Kelvin–Helmholtz instability 142–144 kinetic heating 105–106, 114 Kittinger, J. W. XVIII, 169, 174–180 Kotcher, E. 95, 100 lightning (atmospheric phenomenon) 183 Lachenal, L. 4 Lightning (English Electric aircraft) 3, 19 Llullaillaco (mountain) 4 Lockheed 91 Constellation 136 P-2 Neptune 90 Skunk Works 90 SR-71 Blackbird XVIII, 11, 106–109, 128, 170 U-2 Dragon Lady XVIII, 13–15, 19, 90–93, 170 Mallory, G. XIX Manhigh (project) 169, 173–176, 180 McClure, C. 176, 180 mesosphere 182, 188 Messner, R. 5 Mini Sniffer 170–171 mountain wave see turbulence myringotomy 25 NASA 10–13, 15, 42, 90–91, 107–108, 126, 132, 141, 166, 169, 170–174, 180, 185, 189 Navier–Stokes equations 131, 133–134 Norgay, T. 5, 56 North American XB-70 (Valkyrie) 106–107, 156–157 Paper Jet 56 Peach Air incident 41–42, 46, 48 Piantanida, N. 179–181 Piccard, A. XVIII, XX–XXII, 174, 180, 189 Powers, F. G. 93 pressure ambient XVII–XVIII, XXI, XXIII–XXIV, 3,7, 9–12, 14–16, 25, 29–31, 62, 72, 81–82, 91, 108, 126, 133, 161, 170, 182, 184–185 breathing 9–11 cabin (pressure cabin/cabin pressure) 11–14, 16–18, 28–33, 38, 42–43, 46–48, 50, 52, 66–67, 72–73, 79, 87, 105, 118 chamber 12, 14, 24, 175 distortion 81, 83, 104 jerkin 10 partial 3, 7, 8–9, 34, regulator 9, 46, 49 suit (full or partial) 10–13, 15, 106, 149, 174–175, 177 vessel XXI, 69

Index Quantas 49–52 radiation (ionizing / cosmic) 19–22, 170 radiosonde 126, 128–129, 173 rate of descent see descent Rolls–Royce 56, 88, 114, 118 safe life (design method) 69–70, 72 sailplane 173 Skunk Works 13, 90, 93, 106 space equivalent zone 11, 173, 177 Space Shuttle 101, 126, 170 SR-71 see Lockheed stall (aerodynamic) 63, 77–84, 86–87, 114, 120–121, 139, 141, 151 Stapp, J. P. 173, 176–177, 180 Strato Jump (project and vehicle) 179–181 stratosphere / stratospheric XVII–XXII, 5, 6, 10–13, 16, 18–20, 25, 28–31, 37–38, 42–44, 55–57, 59, 66, 72–73, 77, 79, 82, 84, 87, 95–96, 98–101, 102, 103–106, 109, 114–117, 119–120, 123, 125–127, 129, 136–137, 139–140, 142–143, 146, 149–150, 155, 165, 169–171, 173–176, 180–182, 188–189 supersonic 19, 86, 95–96, 99–100, 102–104, 108–109 thunderstorm 62, 98, 103, 120–121, 144, 146, 149–151, 155, 175, 188 Tibet 5, 48 Tissandier, G. 6–7, 180

215 Titan (moon) 184–185 titanium (use of in high altitude aircraft) 105–107 total air temperature (TAT) 96, 97, 115, 117 tropopause XIX, 125–126, 129, 142 troposphere XVII–XVIII, 6, 96, 116, 120, 126–127, 129, 137, 140, 143, 155, 170, 177–178, 183 Tupolev Tu-154 150 turbulence clear air 138–139, 142, 144–146, 156, 188 Kelvin–Helmholtz 142–144 length scales 132, 134–135 wake 83, 151, 153–155 wave (gravity / mountain) 136–137, 139–143, 147, 151, 173 U-2 see Lockheed United 826 146–148, 151–152 United Airlines 56, 146–149, 151–153 Vega (probe) 182–183 Venera 182 Venus (planet) 182–185 volcano 136, 139, 159, 162, 164–166, 188 (see also ash) wake see turbulence White, W. 179 XB-70 see North American XB-70 (Valkyrie)

Stratospheric Flight: Aeronautics at the Limit

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