Why airplane windows are oval

It all started with the de Havilland Comet. The first commercial jet passenger aircraft from the 1950s.

Remember the typical Douglas DC-4 and the old piston-powered planes in general? They had square windows, which gave a better view. Such planes usually flew at an altitude of about 3,000 meters, where pressurization was not necessary. Not exactly out of luxury, though. The higher the altitude, the lower the air resistance, the higher the speed, and the lower the fuel consumption. But piston engines could not climb higher than 5,000 meters without turbocharging, precisely because of the low air density — but that’s another story.

When designing the first jet aircraft, which thanks to its engines could climb up to 10,000 meters, it had to be pressurized. They fitted it with square windows — following industry habit.

After several years of Comet operations, the aircraft began disappearing from radar right in mid-flight. No SOS signals, no interceptions, nothing. The plane would take off and vanish en route. They searched the sea for wreckage — nothing. Fishermen occasionally pulled up small pieces of metal, about 50 cm in size, but who could say what they were.

Several more years later it was discovered that at the corners of square windows, when pressurized, the points of greatest stress appeared, metal fatigue and microcracks developed instantly. The fuselage lasted literally just a couple of years (about 750–1500 pressurization cycles) before in flight it would simply tear apart and the fuselage would explode.

That’s why modern fuselages now have a circular cross-section (not just for aerodynamics), and the windows are oval.