Testing the Comet

It is now known that the previous static testing had relieved the stress concentrations around the various cut-outs in the cabin

Alec Collins is unfair to the de Havilland design team when he criticises their apparent “inability to design a simple pressure vessel”.

The Comet fuselage was fatigue tested beyond the requirements of the regulations as they existed at the time. The regulations were in a state of flux while the regulators tried to keep apace of this new field and so ensure that a civil aircraft pressure cabin would be able to demonstrate the capability to withstand large numbers of pressure cycles – a step into the unknown.

The designers were well aware of the possibility of static failure of the cabin. Indeed the accident report states: “So much importance did they attach to this that they made many tests of window panes to very high pressures. In addition they applied pressures of between P and 2P some 30 times to the test section of the front part of the pressure cabin together with a series of 2,000 pressurisations to rather over P.”

Owing to their concerns regarding fatigue failure, they went on to apply 16,000 cycles of working pressure, critically to the same front fuselage test item, and were therefore satisfied that the pressure cabin was sound statically and would not fail due to fatigue.

Unfortunately it is now known that the previous static testing had relieved the stress concentrations around the various cut-outs in the cabin, and this resulted in the subsequent fatigue testing being unrepresentative of the actual loading. Hence the disparity between the 16,000-plus flight cycle test life, the crashes at 1,290 cycles and 900 cycles, and the post-accident test of a fuselage taken out of service after 1,230 cycles which failed after a further 1,830 test cycles, a total of 3,060 cycles.

This is the reason why the test programme in support of modern aircraft designs consists of two test items, one for static loading and one for fatigue loading.

Les Bent, Ayr

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