Fatigue and the way the prototype was tested complete the explanation.
The early de Havilland Comet suffered catastrophic in-flight breakups. FAA’s lessons-learned history says investigative testing found especially high stress concentrations around its squarish windows and window frames. Repeated pressurization then fatigued the material around those corners.
Why corners matter
A stress concentration is a small region carrying much higher stress than the material around it. A smooth curve lets stress flow around an opening more gradually. An abrupt corner concentrates it. In a pressurized fuselage that repeatedly expands and relaxes, a local concentration can become a fatigue-crack starting point.
“Square windows caused the crashes” preserves the memorable shape and discards the repeated loading, crack growth, and testing history.
The part the short version misses
De Havilland had tested a prototype fuselage extensively. But the FAA account explains that the same specimen had first been pressure-tested at unusually high load. That process cold-worked material around the corners and improved its fatigue properties. The later fatigue test therefore used a specimen that no longer represented an ordinary production aircraft.
The test article lasted around 16,000 pressure cycles. Production aircraft were showing catastrophic fatigue around roughly 1,000 cycles. The reassuring test result had hidden the vulnerability it was supposed to reveal.
A necessary boundary
The FAA distinguishes the accidents. Its page says the official cause of the 1953 Calcutta crash remained storm-related overstress. The fatigue finding applies directly to the Elba crash, and investigators considered it applicable to the Naples crash despite limited wreckage. Treating every Comet loss as one identical “square window crash” is too broad.
Authoritative reference
Federal Aviation Administration: De Havilland DH-106 Comet 1
LossyInfo review completed 20 August 2026. The FAA page links the underlying accident reports and separates the findings for Calcutta, Elba, and Naples.