Three de Havilland Comets Taught Us to Round the Windows

On January 10, 1954, BOAC Flight 781, a de Havilland Comet, broke up at altitude over the Mediterranean and fell into the sea near Elba. All 35 people aboard died. Three months later, on April 8, a second Comet on South African Airways Flight 201 broke up off Naples. The Comet was the world's first commercial jetliner, and within three years of entering service it had killed 99 people across three in-flight breakups.

The Royal Aircraft Establishment at Farnborough was given the wreckage and a third aircraft to test. They built a tank big enough to submerge the fuselage in water and pumped pressure into the cabin in cycles, simulating climb and descent thousands of times in days. The test fuselage failed near a corner of one of the rectangular automatic-direction-finder windows on the cabin roof. The cracked metal was sectioned and examined. The same crack pattern was found at the same corner of the salvaged Mediterranean wreckage.

The culprit was metal fatigue, then poorly understood in pressurized airframes. Square corners concentrate stress at the corner itself, where the load lines bunch up. Over thousands of pressurization cycles, the aluminum skin around those corners developed microscopic cracks that grew slowly until one cycle popped them open and the cabin came apart in seconds. The Court of Inquiry's 1955 report laid this out in detail and reshaped how every jet built afterward would be designed.

That is why your Boeing or Airbus window is an oval. The shape spreads the same load smoothly around its perimeter, with no corner for cracks to start at. The lesson cost three airframes, the de Havilland company's lead in jet aviation, and 99 lives.