Comet Questions
14 OLIVER STEWART THE quality which gives importance to the three de Havilland Comet accidents in which passengers have been killed is their mysteriousness. The Comet compares Well with other aircraft in kilometres flown for passengers killed and injured; but these .accidents affect its reputation because no explanation for them has been found. In the report on the Calcutta accident a theory was advanced, but it was not more than a theory, and in the Elba and Naples accidents there has been a complete lack of any soundly based explanation. So it is this .gap in knowledge; this strange,, Unknown event which strikes these aircraft down, which is the ' critical matter. Travellers are ready to accept risks if they know what they lire. Even after the Elba crash, when the British Overseas Airways Corporation withdrew its Comets from service, some of those who had booked passages in Comets waited for them ,to return to service rather than go in other slower machines. .„1311t it is doubtful if this mood would have remained after the Naples crash. As it turned out the Minister of Transport and Civil Aviation, through his agents the Air Registration Board, Withdrew the aircraft's Certificate of Airworthiness so the Matter was not put to the test. Without any such test, however, it is abundantly clear that no aircraft can survive as a successful type if it is subject to accidents, however infrequent, for which no rational explanation can be found. As British transport aviation leans so heavily on the Comet fOF its high reputation and as the Comet is the pioneer jet- driven passenger liner, there is urgency in the investigations. It is , therefore satisfactory to note Mr. John Profumo's words al Parliament on Monday that the inquiries would be regarded as a "major national research project." The circumstances of the three accidents were similar. The Esiircraft had taken off and climbed nearly to operating height. Its captain had been in communication with a ground station _and had given no indication of the slightest defect or anxiety. :ollimunication then ceased; the aircraft became overdue and anally wreckage and bodies were found. No hint came from these machines on the radio telephone or in any other way at the crew was alarmed or expected difficulties of any kind. ,1 0 prevent any cry of alarm or any emergency call from being aeard, the disintegration must have been catastrophic. , Guesses as to the cause have filled the daily newspapers. zxplosive decompression of the fuselage is one, bursting of 2it.e of the turbine discs is mother, fierce air turbulence is a ()Laird, the ignition of an explosive kerosene-air mixture in one i f the tanks is a fourth, fatigue in some part of the structure a fifth and sabotage is a sixth. Not one of these guesses as as yet any solid support from the investigators. m in the Calcutta accident there was reason to suppose, from :it!! type of cloud formations in the region, that the aircraft S.:11 pass into extremely turbulent air. The Comet is built to 14 load factor which should give it ample strength to take the aeaviest bumps without danger; but there was a colourable Porossibility that it had met something extraordinary in the way Ottasts. And an appendix to the report suggested the possi- bauY—it was nothing more—that the aircraft captain might _ ve used his powered controls too sharply in correcting the inovements of the machine and so brought about structural failure. This was why one investigator proposed that artificial feel should be incorporated in the control system so that the pilot could feel on the control column the effects of the air- craft's increased speed just as a pilot of an old-fashioned air- craft with direct control operation used to do.
The Elba and the Naples accidents, however, occurred, it seems, in clear air. It is known that there are clear air gusts. In one of the several journeys I have personally made in a Comet the aircraft met clear air gusts when flying at a height of some twelve thousand metres (about 40,000 ft.). There were no clouds anywhere in the sky and it was a curious experience to feel the machine suddenly running into " cobble- stone " gusts. They took the form of short, sharp bumps and it felt somewhat as if one were being driven over a piece of Belgian pave in a stiffly sprung motorcar. But compared with the kind of bumps all aircraft meet in some kinds of cloud these bumps were mild and, on, their own, could not have endangered the structure.
Moreover it must be borne in mind that large numbers of military aircraft are operating every day in the regions of the upper air where the Comet operates and at speeds com- parable with those of the Comet. If clear air turbulence ever reached the intensity which would endanger aircraft structures, it would seem likely that Service pilots would have had experience of it.
There is a similar difficulty in accepting some of the other suggestions. Turbine blades have often come adrift from centrifugal turbo-jet engines without any disintegration of the engine. There seems little knowledge of what happens if a turbine disc bursts, the reason being that in service aircraft turbine discs have not burst. Nor did they burst in the Calcutta Comet accident. The engine makers say that there is as yet no indication that any turbine disc burst in the Elba accident. Service experience gives no more support to the kerosene-air mixture explosion theory. This theory entails a take-off from a very hot aerodrome and then a rapid climb to 12,000 metres or so with some tanks partly empty. Service aircraft can shed little light on the explosive decompression theory because they normally use lower pressure differentials than civil trans- port aircraft. But experimental decompression in the chamber at Farnborough and other research establishments suggests that the crew would have time to give warning of what had happened and that, if they had oxygen equipment near at hand (which all Cometcrews have), they would be able to bring the aircraft down to a lower level in time to save the passengers. Fatigue is always a possibility and comes more readily to mind perhaps because of the convincing writing in a certain famous novel. But none of .these Comets had done enough flying to make fatigue in any part a likely explanation. If they had had piston engines it would have been proper to exclude fatigue from the possibilities; but with turbo-jets there are always rumours of different vibration forms which might have different effects from those known and studied with piston • engines. But once again Service experience does not support the fatigue theory.
It is not for the aviation specialist to seek to assess the chances of sabotage. All that can be done is to recall the fact that sabotage has been experienced in aircraft operating on the main routes both in Europe and in the United States.
One of the best known attempts was that in an aircraft plying between London and Paris. If it had succeeded and the aircraft had fallen—as it was intended to do—into the sea it would have been difficult for the investigators to demonstrate the cause of the accident. Aircraft do lend themselves to the activities of the saboteur.
There is one other theory of which I am not at the moment at liberty to speak in detail. It has to do with resonance, a kind of correspondence between the natural vibration rate of parts of the structure and the clear air gusts. This has recently been put forward by a consultant and will no doubt be examined by the investigators. It is exceedingly fortunate that the Royal Navy's salvage operations were so successful after the Elba crash for there seems to be no chance of salvaging much of the Naples enable it to be stated categorically that no fundamental and basic danger in turbo-jet transport flying will be found. What- ever may be discovered about the Comet Series I, it will remain one of the great pioneer aircraft. It has started a development which no set-back of this kind will stop. Serious adverse effects may be expected throughout British aviation as a consequence of these tragedies. The whole of our engineering and aerodynamics will be on trial. Bit it is in such conditions that the British engineer and aerodynamicist should thrive. He should overcome these difficulties; discover the hitherto missing facts and set off on further aeronautical 'achievements to match those which now stand in his name.
More depends upon the speedy resolution of the Cornet accident problem than the future of this one aircraft on the air lines of the world.
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