Rounded holes in the thin fuselage are structurally more sound, and much less prone to stress fractures. Stress fractures in a pressurized cabin can lead to explosive decompression and outright structural failure.
The seriousness of the issue was highlighted when the first commercial jet, the De Havilland Comet of Britain, was plagued with three crashes shortly after its introduction in 1952. Much to their shock, thorough investigations revealed that the main culprit in all three crashes was likely metal fatigue. And more of the deterioration started at the corners of the Comet’s large, rectangular windows. Up to 70% of the aircraft’s ultimate stress under pressure was concentrated on the corners of the aircraft’s window. The comet was then redesigned with a stronger fuselage and round windows.
If round windows are best, why do Boeing and Airbus provide us with oval ones?
Here’s the explanation: Having round windows would necessarily mean more solid material in the gap between windows. By elongating the windows vertically, aircraft designers can provide more viewing area (more surface area devoted to windows) and also better accommodate passengers of differing heights.
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