Explanation of Jet Liner Crashes Poses New Questions About Metal Fatigue
By DREW MIDDLETON
LONDON - A mystery story in which the villain is a
mathematical ratio and
the detectives a group of somber, humdrum scientists reached its
concluding chapter in
London last week at a Court of Inquiry on the jet Comet. If the
story has a
heroine, it is the Comet, the sleek, swift symbol of Britain’s
leadership in air transport which until this past spring, was the only
jet air liner in
operation on the airways of the world.
In January and again in April Comets crashed and the planes were withdrawn from service by the British Overseas Airways Corporation. The long patient investigation involving the Royal Navy, the Ministry of Supply and, most important, the Royal Aircraft Establishment at Farnborough began.
The findings of the R.A.E.’s investigation are the basis for the Court of Inquiry which sat in somber Church House. It is on this wad of research four and one-half inches thick and the conclusions to be drawn from it that much of the future of British civil aviation rests.
The two Comets that crashed were Mark 1’s. De Havilland, the makers, have already produced bigger and better versions in the Mark 11 and Mark 111. The latter is intended for trans-Atlantic flights. It is hoped that they will restore some of the prestige lost by British aviation through Comet crashes.
The investigation by the R.A.E. which produced the
findings before the
court was the most detailed and exhaustive in the history of
aviation. Ships of the
Royal Navy, scouring the sea bottom off Elba, brought back 70 per cent
of the wreckage
of the Comet that crashed near that Mediterranean island last
January. The wreckage
- ball bearings, tiny scraps of metal, grotesquely twisted seats from
the cabin - was
fitted onto the wooden “ghost” of a Comet at Farnborough.
This mosaic was then immersed in thousands of gallons of water. More water was pumped into its fuselage to simulate the pressure the plane would encounter at all stages of a flight. Other devices imitated the buffeting of the winds. This was the most spectacular aspect of the investigation. But there was much more.
Day after day groups of scientists flew in Comets filled with electric devices for measuring stress and pressure. They took their lives in their hands. For no-one knew what had caused the crashes. There were, in fact, more that sixty hypotheses before the R.A.E. when the investigation began.
But all through spring and summer the planes took off into the upper air over England, flying so high that the North Sea lay bleak and bare to the east and the uneasy Atlantic rolled to the west. Like good detectives, the scientists eliminated suspects.
It wasn’t the crew; the crew were stricken in the execution of their duty. It wasn’t the design; there was some design faults but these were not responsible. It wasn’t the engines; the engines caught fire but only after “it” happened.
It wasn’t sabotage.
One by one the theories were discarded until Sir
director of R.A.E., concluded that there was only one remaining theory
to fit the facts
which reconstruction of the Comet had presented. These facts were that
the Comet G-ALYP
had suddenly burst open, hurling her passengers and crew to their
deaths and then, torn
and broken, had fallen through the clear Mediterranean air into the sea
That was the way Comet “Yoke Peter” (the restored “ghost”) died and because she died that way the “backroom boys” at Farnborough think they know what killed her. The murderer was metal fatigue. Broadly speaking, metal fatigue means that a structure which has an ample reserve of strength when it is new, might fail under its normal working load after a certain length of time.
But the killer is difficult to find. The precise scientific basis of fatigue has not yet been agreed upon. However, the R.A.E. believes that metal fatigue is responsible for the death of the Comet YP and probably for the crash of the second Comet off Naples in April.
In any complex structure there are points where there is a local concentration of stress. If the metal cannot withstand this stress the fracture will occur no matter how strong the rest of the structure.
This apparently is what happened to the Comet. The
stress was about 70 per
cent of the ultimate near the corners of windows in the pressurised
cabin. The R.A.E. is
satisfied that the stress in the cabin became too great near one window
and the fracture
It is still too early to say what can be done to meet the challenge raised by these two accidents. Much of the work done on metal fatigue for aircraft up to the present has been done on movable parts. But with pressurised cabins necessary on high altitude aircraft, the question now arises of metal fatigue in relation to the structure of the cabins.
The question is connected with the aircrafts’ service. Yoke Peter had flown successfully more than a million miles, Yoke Yoke, the second plane to crash, had clocked 2,704 hours in the air. What is the connection between hours in the air, or mileage, and metal fatigue? Is there such a connection? Here, just as in some mysteries where, although the investigating authorities know the cause of the murder, they are stumped about why, the question remains why?
Fact Follows Fiction
Perhaps the oddest part of the inquiry into the two
accidents which cost
fifty-six lives is the manner in which the facts mirror fiction. Long
before the Comets
were in service a writer, Nevil Shute, wrote in “No
Highway” about a
trans-Atlantic plane which falls victim to metal fatigue.
The de Havilland company, which pioneered the development of jet aircraft for civil aviation, has already carried out a large number of modifications on the Comet Mark 11 and Mark 111. But the question of metal fatigue is one which cannot be met by modification of these or any other aircraft, as they are now built. It deals with the composition of the “flesh and bone” of the aircraft, rather than the design.
The investigation has been a race against time, with the de Havilland engineers and Britain’s aviation industry fighting to restore Comets to the commercial airways before jet transports could become available from other manufacturers, particularly those in the United States. No one knows yet whether that race has been won.
Boeing’s long-range jet transport, already test flown at Seattle, Wash., is expected to be ready for commercial use by 1958. Although some American-flag airlines have ordered the new and larger Comets, the delay caused by the crash investigation may make it impossible for deliveries before Boeing’s deliveries.
In the meantime, B.O.A.C. has ordered several of the new American-built DC-7’s to bolster its fleet of fast, long-range airliners.
From: THE NEW YORK TIMES 24th October 1954