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Aviation History
1954
1954 - 2950.PDF
25 October 1954 639 Tht aft interior of the R.A.E. Comet cabin model, 0.22 sec after decom pression. This corresponds to about 0.7 sec full-scale. Leopoldo Lorenzini: "I am a driver ... I suddenly heard a number of explosions and a great roaring noise. I turned in the direction from which the noise was coming and saw a red flame falling into the sea, followed by a wake of smoke in the form of a spiral. . . ." The final affidavits were those of Lt. Col. -Giuseppe Lombardi, officer commanding the Harbour Authority, Porteferraio, Elba, who described his action in arranging for immediate search and rescue operations by both aircraft and shipping, and of Luigi Papi, one of the fishermen who took part in the search. Thursday, October 21st: Third Day.—The evidence of Sir Arnold Hall, Director of the Royal Aircraft Establishment, con cerning the Establishment's comprehensive and thorough investi gations into the Comet accidents, was the main feature of this day's proceedings, amplifying the outline given by Sir Lionel Heald on the opening day. In addition, details were given of the Royal Navy's search for wreckage of G-ALYP, and of the available medical evidence. A full account of the preparation for the naval search and salvage operation off Elba for G-ALYP, and of the results obtained, was given by Cdr. C. G. Forsberg, R.N. [see Flight, October 1st, 1954]. Professor Antonio Fornari, of the Institute of Forensic Medicine at Pisa University, who examined the bodies of 15 of the victims of the Elba accident, was next witness. Speaking through an interpreter, and referring to a report of his findings already published, he said that he found no evidence that the bodies had been subject to any kind of explosion inside the cabin, such as that caused by a bomb, nor had drowning been the cause of death. He considered that the causes of death were violent displace ment of the bodies and sudden impact with parts of the aircraft, explosive decompression, and deceleration. / Dr. R. D. Teare, a Harley Street pathologist, said he had seen Professor Fornari's report, and commented, "I think his observations have been entirely accurate and that the deductions are correct." The burning or scalding of the bodies, he thought, occurred after death, probably through the presence of burning kerosine on the water. Sir Arnold Hall was the next witness. Explaining the R.A.E. approach in the early stages of its Comet investigations, he said, "We clearly wished to satisfy ourselves on the structural integrity of the aeroplane and therefore initiated certain tests on the cabin, tail, and reviewed certain other tests done by the de Havilland Company on other parts of the aircraft. We clearly also wanted to satisfy ourselves about the possibility of explosion in the pressure cabin due to either ignition, for example, of hydraulic fluid, or any other cause. We wanted to satisfy ourselves about the possibility of explosion in the fuel tanks. We needed to satisfy ourselves about any possibilities "of t loss of control due to either aerodynamic cause or other cause. We wished, of course, to satisfy ourselves about the cause of any particular phenomenon we noticed on the wreckage as it came in, and that involved a great deal of metallurgical, chemical and physical testing of parts as they were produced." The sources of information, Sir Arnold said, apart from the wreckage, included Comet G-ALYU which was subjected to fatigue tests, and Comet G-ANAV which was used for flight tests. "Those flight tests in our mind at the beginning were particularly concerned with the possibilities of vibration and flutter, though the aircraft was used subsequently for certain aerodynamic tests and fuel venting tests. . . . We operated it at all times in company with a Canberra aircraft which flew behind it in order to keep general observation on it." The Establishment's report on the Comet investigations, which was before the court, was an account of the results of all the lines of inquiry which had been pursued. The amount of the wreckage which had been recovered (the diagram shown by Sir Arnold to the court is reproduced on page 653) was most useful in checking the original theoretical approaches. Several photographs in the report illustrated the reconstruction of the wreckage—the building-up of the "jig-saw." Following an early examination of the wreckage, Sir Arnold said, "We first formed the impression that the aircraft broke up probably into about six main pieces. There is the nose of the fuselage, the after end of the fuselage, the centre section containing'the power plant, the starboard wing-tip and the port wing-tip and the top of the centre of the fuselage. That was a preliminary judgment, and lined up with the impact damage and also lined up with the plot of the wreckage on the bed of the sea." The sequence in which this break-up took place, already given by Sir Lionel Heald (page 652), was then detailed by Sir Arnold. He went on to give a few examples of the clues which together pointed to this sequence: paint-scratches along the wing as far as the aileron, mdication of downward bending at the wing break-points, fire damage on the centre-section only, and different types of control-cable breakage. Confirmatory evidence that the violent pressure-cabin disruption came nrst was given by the failure of particular electrical circuits and by the appearance of the passenger seats, which had failed in a forward direction (towards the source of the cabin failure) and subsequently backwards (on hitting the sea). Fhe source of the pressure-cabin failure was a point near the rear A.O.F. window: fracture analysis showed that all fractures in the tutelage in this area ran away from this point, and none ran into that ar «. On the actual piece of top-fuselage wreckage, on view in the c° ~.rt, Sir Arnold pointed out also a large compression buckle. A similar oppression buckle and fracture, but occurring at the forward port P i ,l,enSer window, was next shown to the court on part of the fuselage °t ;~omet YU, which had been the subject of tests at Farnborough. *r Lionel Heald: Is it fair to say that there is a very close corre spondence between those two fractures?—Yes, I think that is correct. The time of descent, from the extent of fire damage, appeared to be araut three minutes, implying that the height of the aircraft was J ^OjOOOft. Tests with dynamically similar models had given an approximate time of descent, in the broken-up state assumed, of 2i minutes, and had given a wreckage pattern similar to that found on the sea-bed. The medical evidence also, Sir Arnold thought, was consistent with a breach of the fuselage. Another section of the Farnborough report described tests on a one-tenth scale Perspex model of a Comet cabin, fitted with seats and dummy passengers (scaled to the correct size and strength as accurately as possible). This had been placed in a pressure chamber evacuated in effect to 40,000ft, with the cabin at 8J Ib/sq in differential pressure, and had been caused to burst near the point at which Comet YP was believed to have failed. A number of photographs (example above) showed the effect of the sudden decompression: after the equivalent of 0.03 sec, the seat-backs in the aft end of the cabin were moving forward; after 0.1 sec, complete chaos had developed; after 0.7 sec, seats were flying about in all directions and one dummy passenger was hitting the roof with considerable violence. Although the timing could not be considered exact, these pictures gave a qualitative idea of what probably happened and again gave agreement with conclusions from all other sources. Having concluded that the occasion of the Elba accident was a failure of the pressure cabin at the rear A.D.F. window, a number of possible causes had been considered, in turn, Sir Arnold said, before what he believed to be clear evidence was found on G-ALYP. Sir Lionel Heald: Can you say that in spite of the most careful examination you have been able to find no evidence of a deliberate explosion anywhere in the wreckage?—That is so, yes. Concerning a possible accidental explosion, Sir Arnold said that although electrical and hydraulic components were together located in the equipment bay, they did not constitute an obvious hazard, and such an explosion would not in any event produce the results actually found. An explosion due to hydraulic mist, or in the fuel tanks, was also unlikely. Only continued and extreme negligence by the flight engineer in his control of the pressurization could cause abnormally high cabin pressure leading to an explosion and this possibility, too, was discounted. Also discounted after thorough investigation as the cause of the cabin failure were abnormally high tail-loads produced by failure of the auto pilot or the power-operated control system; in addition, the tailplane itself was shown to be structurally sound. Over-control by the pilot was possible, but improbable under normal gust conditions; dangerously abnormal gusts in this particular case were themselves extremely improbable. Two other factors were discovered during the R.A.E. investigations; neither was apparently a cause of the Elba accident but they were mentioned in the repon. The first was that it was possible for fuel escaping from the underwing vents to reach a position less than one inch from the jet-pipe shrouds. This had been traced using radio-active fuel and Geiger counters. As a normal shroud temperature was 350 deg C and the minimum ignition temperature of kerosine was 250 deg C, this was "slightly worrying," although the ventilation at that position was high. It had also been found that there was a possibility of the integral wing tanks being damaged during pressure-refuelling "on remote occasions." It had been established however, that even if the tanks were damaged the loss of strength to the aeroplane was relatively little. Friday, October 22nd: Fourth Day.—Most of the day was taken up with discussions on fatigue, to which the cabin failure of YP was attributed. Sir Arnold Hall and Dr. P. B. Walker, head of the Aircraft Structures department at Farnborough, both gave evidence. The existence of cracks in G-ALYP, formed during manufacture and drilled to prevent their propagation, was revealed. Sir Arnold Hall first gave his conclusion that the probable cause of the primary failure in the pressure cabin was "a phenomenon known as fatigue." "The essence of the phenomenon," he said, "is that whereas the structure will stand one application of the load quite satisfactorily, it may not stand many hundreds, thousands or millions of applications of the load satisfactorily, and may in the end fail under a load which it is well capable of bearing when new. . . ." Sir Lionel Heald: Could you enlarge just a little on the point how far it is possible to know what is going on prior to the failure actually taking place?—So far as I am aware it is not possible to know what is going on. I know of no physical test one can apply to a piece of material which will tell one how much it is fatigued. . . . To be continued) (Sir Lionel Heald's opening: page 652)
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