Why Are Airplane Windows Rounded?

The rounded corners of an airplane window look like a small stylistic choice. They are actually part of a lesson learned from the mysterious destruction of some of the world’s first passenger jets.

The de Havilland Comet entered commercial service in 1952. Its pressurized cabin allowed passengers to fly smoothly above much of the weather, and its large windows were nearly rectangular. Then several Comets broke apart in flight. Investigators had to determine why aircraft that appeared sound could fail so suddenly.

A Comet fuselage was placed inside an enormous water tank and repeatedly pressurized, reproducing the stress of takeoff, high altitude flight, and landing. The tests revealed metal fatigue around openings in the cabin. Sharp corners concentrate stress rather than allowing it to spread smoothly through the surrounding material. Repeated pressurization could begin a tiny crack near a window or another cutout. With each flight, the crack grew until the fuselage could no longer contain the pressure.

The window shape was not the only engineering issue found in the Comet, and it is too simple to blame every accident on windows alone. Rivet holes, construction methods, and other stress concentrations also mattered. But the investigation made the danger of sharp cornered openings unmistakable.

Modern passenger windows use rounded shapes so that stress flows around their edges more evenly. Their layered panes, small pressure holes, and reinforced surroundings perform other safety functions as well.

Every rounded window quietly carries the memory of the moment aviation learned that even a corner can become dangerous when an aircraft breathes in and out thousands of times.