Why are airplane windows always round? — the "square window" crash story mixed up two different windows

Have you ever looked out of an airplane window? They are always round or oval — never square. But the windows in your house and on a bus are often square. Why?

Here is a story you may hear: in 1954, the world's first jet airliner, the de Havilland Comet, broke apart in mid-air — twice in one year. Investigators blamed metal fatigue, and people said the square corners of the windows "trapped" stress until cracks grew. So everyone switched to round windows.

The story is only half true. Wikipedia has a special section called "Square window myths" that debunks it. The report was actually pointing at the square cut-out for a radio antenna (the ADF) above the cockpit — the fatigue crack started at a rivet hole near that corner. Surviving Comets show the passenger windows already had rounded corners. The real reason for the oval windows was glue: de Havilland switched to Redux adhesive bonding to remove rivets, which saved weight and removed crack-starting holes.

But the science in the story is real. Sharp corners concentrate stress, and cracks love to start at sharp corners. Scientists tested a whole Comet fuselage in a giant water tank, pressurising it again and again. After 3,057 cycles, it burst — and the break started at the escape hatch cut-out, which was still rectangular.

Try this (with an adult): cut a small square hole and a small round hole in a thin plastic bag. Pull both sides gently. The square hole tears from its corners and keeps growing; the round hole just stretches a little. That is why engineers don't like sharp corners.