The de Havilland Comet entered service in 1952 as the world's first jet airliner, and for a moment Britain led commercial aviation outright. Within two years three had broken up in flight. The investigation that followed is one of the most consequential pieces of engineering detective work in the history of transport.
The accidents
BOAC flight 781 broke up shortly after leaving Rome in January 1954. The fleet was grounded, modifications were made on the assumption of an engine or fire cause, and service resumed. Weeks later, South African Airways flight 201 broke up in similar circumstances, also after departing Rome. The Comet's certificate of airworthiness was withdrawn.
The investigation
Wreckage from the first crash was recovered from the Mediterranean seabed — itself a considerable feat for 1954 — and reassembled at Farnborough. In parallel, investigators did something that had not been done before at that scale: they took a complete Comet fuselage, put it in a water tank, and repeatedly pressurised and depressurised it to simulate flight cycles.
Water was used deliberately. Pressurising a structure with air stores enormous energy, and a failure would destroy the evidence; water is nearly incompressible, so when the structure failed it simply stopped, leaving the fracture intact for examination.
After the equivalent of a few thousand flights, the test fuselage failed. The crack originated at the corner of an opening in the skin.
What they found
The Comet's failure was metal fatigue driven by stress concentration at the corners of its window and hatch cutouts. A square or sharply-cornered opening concentrates stress at the corner far more than a rounded one; repeated pressurisation cycles grow a crack from that point until the structure fails suddenly.
The understanding of fatigue existing at the time significantly underestimated how quickly this would happen in a pressurised fuselage cycling to high altitude several times a day. The Comet was not badly built. It was built to the state of knowledge, and the state of knowledge was wrong.
What changed, permanently
Almost everything about how aircraft structures are designed and certified today traces back to this investigation.
- Rounded windows. The oval or rounded-corner cabin window is a direct product of the Comet. Every airliner since has them, for this reason.
- Full-scale fatigue testing. Pressure-cycling a complete airframe in a water tank to several times its intended service life became a certification requirement rather than an experiment.
- Damage tolerance. Design philosophy shifted from assuming a structure would not crack to assuming it would, and requiring that a crack be detectable and arrestable before it becomes critical.
- Sharing the findings. Britain published the results in full rather than protecting them commercially. Boeing and Douglas used them directly. The 707 and DC-8 were safer aircraft because the Comet failed first and Britain said why.
The Comet itself was redesigned and returned as the Comet 4, but the two-year grounding cost it the market. The 707 arrived and took it. It is a hard irony: the aircraft that lost the jet age is the reason the jet age was survivable.