Cobra Effect · Engineering failures
The cracks nobody could see
How investigators found the hidden cracks that brought down the first jet airliner.
7 cards, read aloud in 2:55, with a test and sources.
In May 1952, the Comet began carrying passengers as the world’s first jet airliner.
Built by the British firm de Havilland, it flew higher and faster than the propeller airliners of its day. Flying so high meant its cabin had to hold far more pressure than earlier airliners, around double. On every flight the cabin swelled very slightly, and relaxed again on landing.
In January 1954, a Comet broke up in the air soon after leaving Rome.
It fell into the sea near the island of Elba, and all thirty five people aboard died. The Comets were grounded, modified against several possible causes, and returned to service in March. Then, in April 1954, another Comet was lost near Naples.
The Royal Navy raised much of the first wrecked Comet from the seabed.
The pieces were taken to the Royal Aircraft Establishment at Farnborough. There, investigators fitted them onto a frame, rebuilding the aircraft piece by piece. They were looking for the place where the break had begun.
Meanwhile, they put a whole Comet inside a huge tank of water.
They pumped water into the cabin and let it out again, over and over, to copy flight after flight. Water was used because a cabin failing under air pressure would have burst like a bomb. After about three thousand cycles, counting the flights it had really made, the cabin split open.
The culprit was metal fatigue, where tiny cracks grow a little with every cycle of stress.
In the tank, a crack had grown from a rivet hole near an escape hatch. Around cutouts in the skin, the stress was more than three times higher than elsewhere in the cabin. The official inquiry, reported in 1955, blamed fatigue failure of the pressure cabin.
The story is often told as a tale of square passenger windows.
But on the recovered wreck, the crack began at a bolt hole beside a cutout for a navigation aerial in the roof. The holes for rivets and bolts near sharp corners mattered as much as the corners themselves. And de Havilland had tested the cabin, but on a section first pressurised far beyond normal, which probably hid its weakness.
So when a test says something is safe, ask whether it tested the thing that actually flies.
The Comet’s early tests were real, but the order and method of testing changed what they showed. Engineers learned to test whole structures, in the way they would really be used. A test can only find the weakness it is built to look for.
Sources
- Comet Misconceptions, Aerossurance. Common myths about the Comet, including the windows, checked against the evidence.
- The Comet 1 inquiry, de Havilland Aircraft Museum. The inquiry and the water tank tests.
- Comet Aircraft Accidents, Report of Court of Inquiry, Hansard, House of Commons, 1955. The report presented to Parliament.
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