Comet 1 Crashes – The Court of Inquiry 1954
The cause of the accident was the adventurous pioneering spirit of our race …here was a great imaginative project …we were conscious of the dangers that were lurking in the unknown.. of course we gave hostages to fate .. but I cannot believe that this Court, or our country, would censure us because we ventured. Everything in the realm of human knowledge and wisdom was put into this machine .. it is metallurgy not aeronautics that is in the dock.
Lord Brabazon of Tara.1954.
The Court of Inquiry into the loss of Comet’s Yoke Peter and Yoke Yoke commenced at Church House, Westminster, London on Tuesday 19th October 1954. Before it was concluded, on the 24th November, the Court had sat for some 22 days and some 66 witnesses had given evidence.
Principal personnel were as follows –
The Commissioners Lord Cohen & Lord Gardner
Sir Lionel Heald, Q.C. The Attorney General.
Mr. L.G. Scarman A.R.B.
The Rt.Hon.Sir Hartley Shawcross Q.C.,M.P. de Havilland Aircraft Company.
The following all played an important role in the Inquiry –
Sir Arnold Hall, Director of the Royal Aircraft Establishment, Farnborough, Hants.
Capt., J.R. Johnson. Pilot of the Argonaut.
Mr. C.P. Pinfield, Station Superintendent for B.E.A. at Ciampino Airport, Rome.
Mr. Jablonsky, of Jablo Propellers, Croydon, London.
Mr. Boyd Carpenter, Minister of Transport and Civil Aviation.
Sir Frederick Bowhill, of the Board of Trade.
Mr E.L. Ripley, of the Investigation Division of the R.A.E., Farnborough.
Lords Brabazon of Tara, Chairman of the Air Registration Board (A.R.B.)
Mr. P.B. Walker, Head of Structures Dept., R.A.E., Farnborough.
Mr. W. Tye, Chief Technical Officer of the A.R.B.
Mr R.E. Bishop, Chief Designer of de Havilland.
Mr. R.H.B. Harper, Chief Structural Engineer , de Havilland.
Mr H. Povey, Director of Aircraft Production, de Havilland.
In addition there were a number of de Havilland staff, Test Pilots e.g. Capt. Peter Bois, and Engineers that were on hand to give technical advise to Sir Hartley Shawcross.
The Inquiry .
By the time the Inquiry sat all the important conclusions had already been drawn. There was no dispute between the parties as to what the cause of the failures was, and the tragic sequence of events. Being a public inquiry it only remained for the facts and findings to be presented in the public arena and for anyone who had differing theories as to the cause of the Comet losses to be allowed to put their case – and be cross-examined.
The Inquiry opened with a statement made by Sir Lionel Heald. He outlined the Crowns opinion on the circumstances of the loss of both Comets (G-ALYP and G-ALYY) earlier in the year. He said all the evidence led the Crown to the view that metal fatigue leading to the disintegration and break-up of the pressurised cabin was the cause of the loss of the Comet jet airliner, Yoke Peter, near Elba whilst on a flight from Singapore to London on 10th January 1954 when 29 passengers and a crew of six lost their lives.
The Inquiry would also examine the cause of the loss of (Yoke Yoke) which crashed into the Mediterranean with the loss of 14 passengers and seven crew. The aircraft fell into deep water near Naples on a flight from Rome to Cairo on April 8th.
Sir Lionel Heald Q.C.,M.P. who appeared for the Crown, made, inter alia, the following comments in his opening remarks-
regarding what had happened to the Comet which crashed off Elba he felt justified in putting forward a positive explanation. At the moment he knew of no other explanation which fitted the facts. The Comet has undergone an elaborate and searching test. No aircraft in history had ever been subjected to such an examination and, from the investigation which had been made, it was surely reasonable to hope that results would flow that would be of the greatest value not only to the aviation industry but to the this country, and the whole world, by enabling high speed air travel to be developed with increased safety and efficiency. It will perhaps be some consolation to the relatives of those that lost their lives to feel that some good may come out of this evil.
He went on-
In coming to an opinion as to the cause of the accidents the crown had taken account of direct evidence form the wreckage and the results of fatigue testing of the cabin along with factors such as the age of the aeroplane. “We have former the opinion that the accident at Elba was caused by structural failure of the pressure cabin, brought about by fatigue”. In analyzing the test results it was clear that in respect to pressure fatigue the Comets cabin had low fatigue resistance and that, combined with the aircraft’s age, would predispose the aircraft to failure of the cabin. Evidence showed the cabin was the first part to fail. The failure noted from the wreckage was similar to that reproduced in testing and all the evidence of failure related to that – meaning that what other faults were detected in structural pressure-testing were not found to have occurred on the crashed aircraft.
Outlining the know facts of the Elba crash he said –
Yoke Peter was the first Comet ever to come into service and it was therefore the first jet airliner to carry passengers on a scheduled service. At the time of the accident it had flown 3681 hours. The Elba Comet was one of the earliest built at Hatfield by Messrs. de Havilland and it was designed for high speed long distance passenger and freight services and to operate at high altitudes. For the physical comfort and wellbeing of the passengers the designer arranged that they should be accommodated in a pressure cabin so that when the Comet was flying at 40,000 ft. a cabin pressure equivalent to 8000 ft was maintained.
The Elba Comet had flown successful for more than a million miles. The accident happened during a regular BOAC passenger flight from Singapore to London on January the 10th. The Comet arrived at Ciampino Airfield, Rome from Beirut 08.30 hours and left at 09.31. Nothing of any significance happened during the stop-over and the takeoff was normal. Leaving Rome the aircraft climbed rapidly in accordance with the flight plan. At 09.50 hours when the last message was received at Ciampino the pilot reported he was at 26,500ft, over the Orbetello beacon, and intended to continue to climb to 36,000 ft. as planned.
The pilot of an Argonaut aircraft that had taken off from Rome earlier had several wireless conversations with the Comet crew and in the last one heard the Comet pilot say, “did you get my …” then it was stopped, or interrupted, by another aircraft. It seemed highly likely now that this was the moment when something happened to the Comet. Several of the inhabitants of Elba had positive indications that an accident had happened. One was a sailor who said in an affidavit that he heard a heavy roaring noise – like thunder – he saw a globe of fire rotating as it came down. He saw it plunge into the sea …he heard no more noise. The time by his wrist-watch was 11.03.
An Italian farmer, in another affidavit, said that his attention was suddenly caught by a roaring sound and he noticed two pieces of an aircraft falling, almost parallel, into the sea. Sir Lionel Heald described the salvage operation – which resulted in the recovery of thousands of pieces of wreckage scattered over the sea bed
The Royal Navy was asked, soon after the accident to recover as much wreckage as possible from depths in the region of 6000ft (1829m). In fact on February 10th 1954 the crew of the Navy’s salvage vessel Sea Salvor located the wreckage and the painstaking, and technically difficult, task of recovery, began. Divers could not operate at anywhere near these depths and the technology to do so was limited. Salvage operations on such a vast scale, and at such great depths – thought to be the greatest salvage attempt ever at the time – are enormously expensive. Without the direct intervention of the then Prime Minister, Winston Churchill, who presided over a hastily convened cabinet meeting with all the responsible departmental Ministers present, it is doubt full whether as much would have been achieved.
To add to the Navy’s problems the salvage was hindered by bad weather but, despite this, some 75 percent of the wreckage was recovered – a remarkable achievement. The recovered wreckage then started its long journey back to Farnborough where the investigation team, under Sir Arnold Hall, were attempting to re-construct as much of Yoke Peter as possible.
Metal fatigue was now suspected and extensive program of testing began. Ex-BOAC G-ALYU was tested in a special pressure water tank measuring 100ft by 20ft by 16ft. During one of these cycles cracks appeared on each side of a rivet close to a cabin window. Was this the cause? Could the relevant piece of Yoke Peter be recovered from the sea to confirm findings? Further examination of G-ALYU revealed cracks at the corner of the Automatic Direction Finder (ADF) aerial. It was thought that failure here would have caused the explosive decompression experienced by Yoke Peter. But did it?
Sir Lionel Heald continued his opening remarks –
All four de Havilland Ghost engines were recovered and these could be given a clean bill of health. There was no question of failure or defect and they played no part in the accident.
On the second Comet crash –
The second Comet crash was on April 8th involving Yoke Yoke which had flown 2704 hours and was flying from London to Johannesburg. It was delayed for 24 hours at Rome but there were no grounds for connecting anything that happened to the aircraft with the cause of the delay. The aircraft arrived in Rome on April 7th at 17.35 hours and took off on April 8th at 18.32 hours. The takeoff was normal and the last message was received at 19.05 hours. It said that the aircraft was climbing to 35,000ft and the pilot was asked to report when he reached this height. No more was heard. A search was organized and six bodies were recovered.
Referring to the accident to both aircraft Sir Lionel Heald emphasized first –
that there were no grounds for the slightest reflection on the captain or any members of the crew. From the circumstantial evidence it was clear that some sudden disaster came upon these men and women while they were doing their duty and secondly – there was no reason to suppose any kind of violent weather prevailed. Because of the similarity of the accidents BOAC had suspended Comet services and the aircraft’s Certificate of Airworthiness had been suspended. That both accidents befell aircraft leaving the same airfield had led to speculation that sabotage could have been the cause – that theory had been discounted.
Sir Lionel Heald believed –
The work of Sir Arnold Hall, Director of the RAE, Farnborough, and his team on the finding of the wreckage, its piecing together, and his conclusions would be generally regarded as one of the most remarkable pieces of scientific detective work ever done. He also believed that the main conclusion would command acceptance. Referring to the possibility of sabotage being responsible for the crash he said that, “this was a matter on which the public were entitled to the fullest assurance. It is always difficult to prove a negative but it is not impossible. In the case of the Elba aircraft we are much better off than usual in this respect. Not only is there a complete absence of evidence supporting the suggestion of sabotage, when experience shows that such evidence would be expected to be present. Far from there being any trace of sabotage being the cause there is the positive evidence of damage in the aircraft which is consistent only with destruction by means other than by sabotage.”
The experts say that an explosion in the cabin would almost certainly have been proved by chemical and physical evidence either on the structure itself, or, on the bodies in the cabin. That an explosion elsewhere in the pressure cabin could not have produced these results will be seen from the wreckage
On sabotage Sir Lionel Heald noted –
In the Elba case there was only one hour at Rome and there seemed, from the evidence, to have been no real practical opportunity for anyone to do anything to the Aircraft in that hour.
He had paid careful attention to the evidence from the best sources available to him and he could say that these gave no support at all to the suspicion that there was sabotage at Rome on either of these two occasions.
Sir Lionel Heald looked at another matter of public concern –
Why were the Comets were allowed to go on flying after the first accident?. On this he would repeat the obvious tag – that it was easy to be wise after the event. Sir Arnold Hall had informed them that the aircraft must have broken up at a height of 30,000ft and come down in a definite series of parts. The medical examination of bodies gave remarkable indication of the conditions which must have existed when the disaster happened. It was quite clear there had been an almost instantaneous and tremendously powerful forward force generated inside the cabin which had thrown most of the passengers in their seats forwards and upwards against the roof. There was evidence that this force must also have driven some of the passengers right out of the aircraft.
It was clear from general principles, and it could be demonstrated by experiment, that if a fracture of any substantial size, or a mere perforation, occurred in a wall of a tube or vessel under 8 lb. pressure the hole would immediately open up and the tube would resemble a compressed air gun. The blast of air would force everything out of the hole and would throw and aircraft into various contortions as well as tear it to pieces. To be added to this was the initial motion of the aircraft in flight. Huge forces were acting in different directions and with the violent decompression there would have been a sudden and general break-up of the fuselage. When enough of the wreckage was obtained Sir Arnold Hall was able to verify not only that this had happened, but how it had happened.
Sir Lionel Heald referred on a model of the Comet aircraft –
to the location of the ADF aerial. There were two windows. The first thing that happened was that there was a violent disruption of the centre part of the pressure cabin. Part of the fuselage, the nose and the outer part of the port wing fell away .. there was a tearing off of these parts. This may have happened when the aircraft was in the vertical rather than the horizontal plain. The main part of the wing separated and caught fire. Next the fuselage, aft of the rear spar with the tail and wings still attached, fell into the sea with the open end first. Last of all the main part of the wing which was on fire hit the water in an inverted position. The fact was that this could only be accounted for by the sudden disruption of the pressure cabin and this was something which was a ‘priori’ the most likely of all when the aircraft was reaching its maximum height as it was in this case.
This was the time when –
the greatest strain and stresses were being imposed upon the cabin pressure. After eliminating several possible causes Sir Lionel Heald said that Sir Arnold Hall would say that there was only one thing left – there was one factor known as metal fatigue and everything in Sir Arnold Hall’s investigation supported the theory that this was the cause of the failure of the cabin. There was nothing in the evidence or findings to contradict it.
Explaining to the Inquiry he said that broadly metal fatigue meant –
that a structure which had an ample reserve of strength when it was new would fail under its normal working load after a certain length of time. The precise scientific basis of fatigue was not by any means agreed even yet.
If a load were repeatedly applied to a structure, or alternated, the structure might fail eventually … one might say that it would fail. That would not matter so much if one could tell by periodic examination when the critical stage was approached. With metal fatigue, that could not be done as a general rule.
One might know nothing about it until a disastrous fracture occurred. Unfortunately in any complex structure, however perfectly manufactured, there were bound to be points where there were local concentrations of stress. If at one of these points the concentration was such as to exceed the ability of the metal to withstand a stress of this nature then – when it is repeatedly applied – the structure would fracture sooner or later no matter how strong the rest of it might be. A crack might develop form a notch, a curve, an edge or a screw thread.
To some extent these things can be measured. But there was bound to be “unknown territory” and a safety factor was required to cover it. The danger of fatigue in the design of an aircraft in this particular connection had been appreciated by designers for a long time. But it was only in the past five years, or so, that much progress has appeared to be made in tackling the problem on strictly scientific lines. He believed that until comparatively recently the problem had been dealt with, both here and in the United States, on very broad lines indeed. Putting it in the roughest way it appeared to have been thought that a safety factor of two to one would take care of it.
The Air Registration Board, in 1952 when the Comet received its Certificate of Airworthiness, thought that provided it could be shown that parts of the pressure cabin could be proved to have withstood twice the working pressure that would give an ample safety margin in relation to fatigue.
By the end of 1952 it appeared to have been appreciated that fatigue was more likely to be a serious factor in the case of aircraft relatively highly pressurized – such as the Comet. The ARB. governing aspects of the subject appeared to have been satisfied at the time that – on the information they then had available to them – the maximum concentration caused by pressure loading, under operational conditions, would be only be about 40 to 50% of the ultimate stress strength of the material – therefore there was still a big enough load factor for safety.
Sir Lionel Heald then referred to a point which had not been appreciated previously –
it appeared from the RAE. investigation, and their tests on the other Comet airframe, that in the pressure cabin there could be, and were, points where there was a loading of as much as 70% of the ultimate stress. That stress had occurred near corners of windows .. it was not suggested for a moment that anyone in de Havilland, or the ARB could have known this in 1951. He thought that it would be agreed that looking back with present knowledge that the stress concentration was under-rated. It was no-ones fault. de Havilland made the most of their extensive tests and they had what they believed to be a comparable specimen of the pressure cabin under test which broke down only after 54,000 hours. It was possible that the cabins subjected to this test were not really reliable for the test for without anyone appreciating it – it could in fact be strengthened by having heavy pressure (water) applied to it before the test was carried out. In other words the pre-stressing of the structure by the application of a static load (water pressure) could have improved fatigue life in the test specimen.
The Farnborough tests on G-ALYU, which had already been in service, showed that the cabin had failed at 5,546 pressurization cycles. Translated into structural hours it was equivalent to 9,000 hours. In terms of ‘fatigue life’ the failure of the test specimen was not unreasonable compared to the hours accumulated by the lost aircraft – Elba Comet 3,681 and Naples Comet 2,701 hours.
Returning to the series of RAE tests Lord Cohen said –
that the more important question was whether tests could prevent a recurrence in future. Sir Lionel Heald said it would be essential to know what kind of test could be made and how far they could be relied upon. It would be disastrous if any wrong impression remained that this was something that could not be tested. He did not think that anyone would say that. Sir Lionel Heald quoted some conclusions reached by Sir Arnold Hall in his report on the other accident and said that the same explanation could be applicable to the Naples accident.
The Royal Aircraft Establishment
Sir Arnold Hall described the RAE testing procedures-
and explained that during the course of the investigation his team went, “from the laboratory back to the wreckage and from the wreckage back to the laboratory.” There had been no preconceived ideas as to the cause. G-ALYU had been used for fatigue tests and G-ANAV had been used, un-pressurized, for flight tests logging over 100 hours. On all these flights a Canberra had accompanied the Comet so that observations could be made.
When the RAE investigation had begun they had no way of knowing how much, if indeed any, wreckage would be recovered. The team, had therefore, set out to find the causes by their own test procedures. When wreckage was recovered that saved an enormous amount of time because the team could compare test results directly with the evidence before them.
The RAE had concluded that the first part of the aircraft to fail was the pressure cabin. Wreckage indicated, furthermore, that this was so. The wings were marked laterally with gold and blue paint and these markings extended across a wing fracture – indicating that the wing was whole at the time of break-up and that it was the pressure cabin that failed first. Similarly markings on the wreckage of the rear fuselage indicted that this was not a primary site of structural failure but followed the disruption of the cabin. Furthermore the RAE were able to demonstrate that the tail section broke away before the nose – this evidence was contained in the type of fracture found in the control cables.
The evidence suggested that the aircraft fractured along the top of the centre line and such was the violence of the event that the structure was thrown sideways and out scoring the wings as it went. Indications were that the primary failure was at the rear of the ADF hatch in the top of the cabin. In tests at Farnborough similar failures had been reproduced. Further a Perspex model of the Comets fuselage, stressed to scale, had been equipped with seats and dummy passengers. Pressurized to 8.25 psi. (40,000ft) the model was ruptured along the top centre line. In 1/10th second there was complete chaos and after 0.5 sec. dummy passengers could be seen hitting the roof with considerable violence.
Why the Comets flew on.
Giving evidence about the decision to allow flying to continue again after the loss of Yoke Peter Sir Frederick Bowhill (Board of Trade) Air Chief Marshal and Chairman of the Board said –
there had been a meeting on March 2nd and the Board had concluded that everything humanly possible had been done to ensure that the desired standard of safety had been maintained and that there appeared to be no justification for placing special restrictions on Comet aircraft. Having considered all the evidence there was nothing to indicate that there was something inherent in the design of the Comet that caused the Elba crash – so having taken every point into consideration they could not have envisaged a similar disaster happening again.
At the meeting they had had the assistance of the ARB on the results of their investigations and the modifications proposed and there were also talks with the de Havilland Company, BOAC and outside experts. The minute of this meeting stated the while it was realized that no cause had been found that would account satisfactorily for the Elba disaster, and that while the Calcutta disaster was accounted for if as was supposed the aircraft was hit by a gust of great severity which would have broken up any aircraft, they could not eliminate the possibility that the accident might have been due to some other cause, which was possibly common to the other disasters – but having said that, at the time, the possibility looked remote. A return to service after modification and test flying was therefore recommended.
Mr J.M. Shaw, acting on behalf of some of the relatives of victims said –
he was perturbed about the Comet being allowed to fly again.
Answering Sir Frederick said –
the board had considered both metal fatigue and failure of the cabin pressure in their review of the Elba accident. No modifications were introduced for the pressure cabin because at that time they did not consider that this was a possible cause. The board believed that the modifications would have covered all the points humanly possible to prevent further accidents. He confirmed that Sir Arnold Hall was a member of the Board.
Sir Hartley Shawcross, Q.C. for de Havilland said that –
the best expert advice in the country had been available to them. He agreed that the ARB had laid down certain requirements to guard against the risk of damage to the pressure cabin – following the practice of the International Civil Aviation Organization. Indeed the American Civil Aeronautics Board had laid down standards that were substantially the same. Neither bodies requirements had differed significantly in this regard from that of the Civil Aviation Authority. Up to the time of the two Comet disasters it had never been suggested that these requirements had not been sufficient to ensure safety.
Sir Hartley Shawcross asked about the future of the Comet.
Sir Arnold Hall said that –
in his opinion there was no reason why Comet aircraft should not go back into service after modifications and have a very successful career provided a thinker gauge skin was used and there was some strengthening of the fuselage and wings.
He thought there was no inherent defect in the design of the Comet which would prevent this type flying again. So provided steps were taken along the lines described the aircraft should go back into service. He agreed with counsel that subject to the conclusions concerning the Elba accident that the Comet was a safe aircraft, well designed and well made. Although knowledge of high level fatigue had increased greatly in the past years he doubted if we knew everything yet but he was quite certain that we now know enough to make aviation safer.
Lord Brabazon of Tara also gave evidence as Chairman of the ARB –
he saw nothing in the Elba accident which justified the grounding of the aircraft and pointed out that modifications were made to cover every imaginable cause of the disaster. “The Elba accident was completely unexplained to us.” The Comet was a machine that was being talked about all over the world, being as it was, the most remarkable machine in the world and if we grounded the type of every aircraft that had unexplained accidents you would scarcely have a machine in the air. The cause of the accident was the adventurous pioneering spirit of our race …here was a great imaginative project …we were conscious of the dangers that were lurking in the unknown.. of course we gave hostages to fate .. but I cannot believe that this Court, or our country, would censure us because we ventured. Everything in the realm of human knowledge and wisdom was put into this machine .. it is metallurgy not aeronautics that is in the dock.
Addressing the Commissioner he said-
“First of all I know you my Lord, are charged with this duty of trying to find the physical reason for these disasters, and you may even have to try and find a scapegoat. I am quite prepared to be that scapegoat, because it was true to say, is it not, that the Air Registration Board, of which I am Chairman, could have forbidden the Comet to fly at all. I do not mind what other people think; the only thing is whether one thinks one has done the right thing, and as I look back at it all, there is nothing that I did, with the knowledge I had at the time, which now I would have done otherwise.”
de Havilland statement
It was disclosed to the Court as the Inquiry moved into the third week that –
The de Havilland Company proposed to issue a statement as soon as the Inquiry was complete outlining their plans for the two later versions of the airliner the Comet Mk.II and the Mk.III. When work was suspended on them pending the outcome of the Inquiry de Havilland had completed six or seven Comet 2s and construction was well advanced on another 13 or 14. A total of over a million pounds had been spent on these 20 aircraft.
The Company was now faced with the decision as to whether to strengthen the existing fuselages or to dismantle them and rebuild them completely. A total of 32 Comet aircraft were on order – 12 by BOAC and the remainder by Air France, U.A.T., the national airlines of Venezuela, Japan, Canada and Brazil. So far only one prototype Comet III had been built but de Havilland had orders for eleven: 5 for BOAC, 3 for Pan American Airways and two for Air India International and a prototype for the Ministry of Supply.
It was probable that the Comet I was not likely to return to airline service. One of the important decisions facing the manufacturers was whether, in the interest of good business relations, some form of financial or other form of adjustment should be made to the purchasers of these airliners who were no longer able to use them.
The two French companies, Air France and U.A.T. which each bought three Comet Mk.Is had also ordered the Mk.II. The Royal Canadian Airforce had bought two Mk.IAs. BOAC had a total of ten Comet Is of which four were lost in accidents. Three of the remainder were at the RAE, Farnborough, Hants.; one of which has been employed for flight tests, one for general examination and the other for water tank tests. Another BOAC Comet was at the de Havilland works at Hatfield and two were in storage at London Airport.
de Havilland also stressed that an early decision with regard to future plans would be facilitated if the Court of Inquiry could make an Interim Statement at the close of the hearing on its views on the RAE technical findings as to the cause of the accidents. Soon after the Courts verdict was know however de Havilland expected to make known their future intentions. It was intended that this statement would also give delivery dates for the various types of aircraft.
R. E. Bishop, designer of the Comet, told the Inquiry
when the Comet was being designed the de Havilland Company were fully alive to the consequences of any failure in the cabin pressure at 40,000ft. He added that a great deal of thought and testing was done to make the cabin safe. We obviously missed the main line. At the end of the war nearly all commercial aircraft were American. The de Havilland Company had had experience with jet fighters and felt that they should be able to produce a useful civil aeroplane which would be a step ahead of conventional piston engined aircraft. Mr Bishop said that to his knowledge no aircraft had been tested as much as the Comet but there had been no rush into production.
He saw no evidence from the many tests, or from the wreckage, to suggest that Redux had been a contributory factor to the failure of the pressure cabin. He felt satisfied in continuing to use Redux on aircraft. During production of Dove and Comets the company had made 180,000 test pieces and the integrity of the joints had been repeatedly tested. There had been 425 separate tests for that purpose.
Referring to protection of the pressure cabin Mr Bishop said that the windows had been specifically tested to a high safety factor. At the time there had been many instances of windows blowing out of civil aircraft owning to pressurization. Even after the Naples accident it had been thought that the cause of failure was more probably due to deterioration of the cabin structure than to fatigue of the fuselage. It was a great surprise to him when the fuselage blew up on the Farnborough experimental tank after 9000 hours ..they thought it had a very much longer life than that.
R. E. Bishop continued his evidence
now the problem of fatigue in pressure cabins was appreciated nobody need fear that the trouble could not be put right. He had complete confidence in the pressure cabin of the Comet and that it could be made safe and there was no reason why they could not again go ahead with the Comet at full speed.
For the final week of the Inquiry proceedings were transferred to the Great Hall of the Institute of Civil Engineers.
Sir Lionel Heald outlined some decisions taken by the ARB to ensure greater safety in aircraft.
He explained that from the beginning of the Inquiry he had hoped to put forward suggestions for improving the safety of civil aircraft. It would have been no value to do so without the assurances that those suggestions had the support of all those concerned. He had therefore consulted the Ministry of Transport and Civil Aviation and the ARB and was able to inform the Inquiry of certain practical steps they proposed to take.
The Board had already indicated their intention that complete cabins of pressurized aircraft should be submitted to tank tests similar to those used at Farnborough at any rate until knowledge of the fatigue problem was more exact. Before the Certification of certain prototypes could be contemplated a provision would now be required for fatigue testing of entire components. In the Boards view at least two airframes of each type would have to be made available .. one for static testing and one for fatigue testing.
The whole of the countries technical resources could be made available to the board. In view of the difficult problems facing aircraft designers, at the Board at the present time, careful consideration was being given to including in the Counsel of the Board one or more additional members who would be selected for their expertise in the scientific field. Referring to other measures under consideration he mentioned the value of extensive test flights on proposed routes using fully-instrumented aircraft and the problem of fatigue in high grade alloys .. the subject being continuously before the appropriate committees of the Inter-services Metallurgical Research Council. Although the subject was a difficult one, and rapid results could not be expected, it was fully recognized that the best brains and resources should be directed to it. The Ministry of Supply had agreed to give the fullest co-operation to this work.
As the Inquiry drew to a close Sir Hartley Shawcross lashed out at certain press coverage. He said –
“Had de Havilland not thought that the aircraft was safe as human skill and testing could make them they would not have allowed them to fly. It turned out that they and other aviation authorities were wrong and the question was why? The Comet had been subjected to testing and scrutiny the like of which had never been brought to any aircraft in the history of aeronautics.
“To the surprise of everybody it had been found to possess one vital fault – that proneness to fatigue in the pressure fuselage. Someday Comet aircraft would be in the air again with the very hallmark of safety.”
Sir Hartley Shawcross criticized a leading article in the Daily Telegraph –
“about which everyone in this court feels extremely indignant” he said, “I suppose there is no technical question about contempt of Court but this newspaper is apparently oblivious of the fact that the Court was still sitting and, obviously oblivious of any findings in the matter, permitted itself to say that what had emerged in the course of the Inquiry would not make foreign buyers anxious to buy British aircraft. If the writer of the article had remembered the evidence on day nine he would have known about the most famous American aircraft – certainly three and possible four had fallen into the Ocean for quite unexplained reasons, and two of these since the first Comet, and three had subsequently been found to be in a critical condition – apparently caused by fatigue of the fuselage.”
Sir Hartley referred also to a statement in a leading article saying –
that dangers of metal fatigue had been long foreseen. The writer of the article had said that as far back as 1952, in an article, it was said that aircraft designers and the Ministry of Supply were expecting that metal fatigue would cause failure after 9000 hours. “And he goes on to point with complacent satisfaction of the most distasteful kind – even if it were justified – that the failure of Yoke Uncle in the tank at Farnborough justified that figure. It was a most disgraceful thing to say. It reflects the ignorance and confusion that existed in the mind of this air correspondent between low level fatigue in relation to wings which was fully recognized as a problem and high level fatigue in pressure cabins that was totally different.”
On the last but one day of the hearing Sir Hartley Shawcross outlined precautions de Havilland were to take in the future –
to guard against metal fatigue, accepting that cracks had nothing to do with the loss of the Comet Yoke Peter, “we shall seek to construct this aircraft in a way that shall ensure that no crack could jeopardize its safety. We shall do that by both using thicker gauge materials for the skin and incorporating such redesign as may be indicated .. a general stress level at which this kind of accidental stress raise would not raise the stress to a danger point. We shall exercise every possible precaution in manufacture to guard against cracks but if, in the life of an aircraft, cracks that do occur would be dealt with on their merits. The principle being that any repair scheme which is sanctioned will leave the aircraft at least as strong as it was before the crack appeared.
Sir Hartley Shawcross clarified the position –
if a manufacturing crack was properly drilled out it was no more of a stress raiser than a rivet. There was not the slightest evidence that a manufacturing crack or an incipient crack had formed the focus of fatigue. But de Havilland would not now grant concessions for cracks. It was remarkable that in an aircraft in which over a million parts were fabricated together the most careful microscopic examination had found only four errors of workmanship; three cracks and only a handful of rivets out of place but without any deleterious effect on the structure of the aircraft as a whole. It was four mistakes too many but the errors did not justify the sensational criticism being made in some quarters.
On November the 23rd the last day of the Inquiry, de Havilland proposed –
that the fuselage of the Comets now under construction should be rebuilt. Thicker gauge materials would be used in the cabin pressure area and windows and the cut-outs would be redesigned and strengthened. Sir Hartley Shawcross said it was doubtful whether it would be a practical expedient if the proposed modifications were to be carried out on the Comet 1 fuselages so that they could be returned to airline service. Modifications which it was proposed to make in consultation with the ARB had been suggested by the Farnborough findings on a high level of fatigue in pressure cabins. Sir Hartley said that the modifications were designed to avoid high level stress and would be incorporated in the Comet Mk.IIs and Mk.IIIs and these would have little effect on the guaranteed performance of the aircraft.
After modifications the aircraft would be submitted to repeated loading and other stress tests as necessary to prove the complete safety of the new fuselage and the rest of the structure. The possibility of modifying the existing airframes would be determined in consultation with the operators and the ARB. It might definitely be stated, subject to the Courts conclusions and recommendations, that the Comet can -and will – again fly, this time with the insurance of the unique background and testing which has taken place. Although the burden of the enquiry had fallen, properly, on de Havilland it was not theirs alone, they concerned the whole of the modern aircraft industry in every country. The lessons learned would make flying safer throughout the world – de Havilland were entitled to hope confidently not only that the Comet would remain for sometime by demonstration the safest aircraft.
De Havilland announced to the Inquiry that –
they were redesigning parts of the Comet wing which were prone to failure fatigue to reduce the overall stress level. Modifications had been devised to the fuel venting system to prevent venting of fuel during takeoff and climb. The Company were also devising a method of preventing damage from pressure-refuelling systems and would consider a modification to the power control system to suite the convenience, and comfort, of the pilot. Investigations would be continued into the use of non-flammable hydraulic fluid and damage from buffeting from the jet efflux would be further reduced by the thickening of the fuselage skin. Door hatches liable to damage from passengers and freight would be reinforced .
Sir Hartley Shawcross said that –
de Havilland made their stress calculations in 1946. Aircraft designers were not alive to the risk fatigue in pressure cabins and, looking back now, the Company entitled to say that not only had their calculations and tests been above the standard set by every authority but they had demonstrated that there could not have been stress in the pressure cabin of more than 50%. It had therefore been reasonable to assume that the structure was statically strong.
A twist in the tale?
The Inquiry closed and that seemed to be it. But by chance an Italian fishing vessel trawled up one of the cabin windows. Examination showed that this was probably where the primary fatigue failure had occurred not around the ADF hatch. Clearly from what has been said either site could have suffered fatigue failure. As it was Sir Arnold Halls’ Inquiry had undoubtedly come to the right conclusion as to the cause. The Inquiry was not re-opened because little would be gained.
In any case the lessons were learned. The skin of later Comets was to be some 80 percent thicker than on the Comet 1 (19 gauge not 22 gauge sheet). In fact it could be said that out of the Comet experience all the modern approaches to fatigue prevention and routine systematic examination were devised. The principle was that all structures, wherever possible, should be fail-safe.
I asked Sir Hartley, now Lord Shawcross for his recollections of the later discovery and its significance. He could not, unfortunately, throw any light on the matters. In any case it was somewhat academic since the primary findings of the Commission had been correct.
Lord Shawcross did however tell me that he had sympathy for the de Havilland Company at the time of the Inquiry and a great admiration for the work of Sir Arnold Hall of the RAE. “a man of very great technical and personal ability.” He said too that, “the reconstruction of the aeroplane, the parts that had been salvaged from the sea, and the evidence he was able to give as a result, was of the utmost importance.”