On this day in 1952, a deHavilland Comet took off from London for Johannesburg, carrying the first paying passengers on the first commercial jet airline flight. The Comet was a sleek looking, futuristic plan that whooshed its passengers about in luxurious jet silence, compared to the Douglas and Convair piston airliners of the time that rattled occupants fillings loose with the roar of multiple radials.
It was something of a shock, therefore, when Comets began falling out of the sky for no adequately explainable reason just over a year later. Exhaustive investigation pointed to a likely culprit: Square windows. The Comet's square-with-rounded-corners windows served to focus stressors at their corners, and when combined with the riveting method used on the skin, stress cracks propagated that caused explosive decompression failures, large chunks of skin to rip away, and the plane to land somewhat sooner and more vertically than anyone, least of all the pilots, had intended.
The defect was rectified in subsequent Comets and it would go on to have a long and distinguished career, but it would now be remembered by posterity not as "The First Jetliner" but as "That Plane That Crashed Because Of Square Windows. Oh, yeah, and The First Jetliner."
It turns out that the Comet was aptly named, since a lot of them had great success doing a fair approximation of their namesake's uncontrolled parabolic flight path.
ReplyDeleteSpecifically, as I recall, it was a pressurisation issue.
ReplyDeleteAccording to an old Materials lecturer of mine, De Havilland had done pressurisation fatigue tests on a Comet airframe. In a perfectly understandable move, to save a little cash they combined it with an overpressure test. Well, it's not like it's going to extend the fatigue life, right? Ummm .... in this case, wrong.
By overpressurising, they'd caused a little plastic deformation of the window frames, a little stress relief around any cracks in those problematic corners. So when they ran fatigue cycles the stresses in those critical locations were lower than they would have been in a pristine, unabused airframe. And it was those unabused airframes that went into service and started breaking up after rather fewer cycles than expected.
This sounds like a bit of mis-information to me. The insinuation being that it was not a design problem, but an error in testing procedure that lead to the crashes.
ReplyDeleteOh, it was a design error alright. The fact that corners lead to stress concentrations is utterly fundamental.
ReplyDeleteI'm willing to cut De Havilland a little slack on the design error though. Square windows were at the time quite common in passenger aircraft, and pressurisation was novel in passenger aircraft at the time.
We know that square windows are a bad idea now, mainly because of the wonderful example the Comet 1 provides, but had it not been for that little error in testing procedure, De Havilland would have caught the problem before it ever resulted in crashes. That is the general idea of testing, after all, to catch the mistakes you don't know you've made.
Any clearer, anonymous?
No, I understand the stress issues, what I mean is it sounds like mis-information to say that the failures were caused by combing the tests. This statement would imply that the failures were caused by puting units into service that had been tested to the point of plasic deformation, not by the stress concentrations caused by the shape o the windows.
ReplyDeleteanon,
ReplyDeleteJon is saying that the overpressurization tests allowed the tested airframes to last LONGER than the untested, non-overpressurized airframes delivered to the customer and subsequently flown.
By overpressurizing the airframe the windows deformed (stretched) and small stress fractures formed which had the effect of relieving future stresses thus prolonging the time before failure.
Has anyone ever seen the Jimmy Stewart movie "No Highway"?
ReplyDeleteI always thought it was about the Comet, but I just noticed the release date: 21 September 1951
I guess I didn't understand as much as I thought, after some further reading I see what you mean about the over-pressure test, but it still sounds like finger pointing and blame shifting to me.
ReplyDeletesome PR guy trying to convince us it wasn't a design error, but a testing one.
I think he's saying that trying to cheap out on the testing procedure failed to uncover the design flaw...
ReplyDeleteAxeman, I would suspect that No Highway is based on the novel of the same name by Nevil Shute.
ReplyDeleteThat would be the same Nevil Shute who under his professional name of Mr Norway happened to be an aeronautical engineer, and who oddly enough had a brief stint with De Havilland at the start of his career thirty years earlier.
Coincidence is a funny thing.
Yeah, I got that. I guess I am just not expressing myself well.
ReplyDelete