Why Plane Windows Are Round, Not Square

The Comet and a mystery in the sky

The de Havilland Comet was the world’s first commercial jet airliner, and in 1952 it was a marvel. It flew higher, faster, and more quietly than anything else carrying passengers. It also had large, handsome, roughly square windows.

Within two years, the dream cracked open. In 1954, two Comets broke apart in midair within months of each other, scattering wreckage into the Mediterranean. There was no warning, no obvious cause, no surviving explanation from the crews. The aircraft were grounded, and one of the most thorough engineering investigations of the century began.

Pieces were recovered from the seabed and painstakingly reassembled. A test fuselage was placed in a giant water tank and pressurized over and over to simulate years of flights in a matter of weeks. Eventually the metal failed, and the failure told the whole story. The cracks began at the corner of an opening in the skin, where stress had been quietly concentrating with every cycle until the fatigue became too much. Tiny fractures grew invisibly, flight after flight, until the structure could no longer hold.

The corners were not just a weak point. They were a countdown, ticking away one pressurization at a time.

The lesson was paid for in lives, and the industry never forgot it. The Comet’s square-ish windows became a permanent warning carved into the way every airliner since has been designed.