Why no airliner has square windows

On 10 January 1954, a De Havilland Comet of the British airline BOAC — the world's first commercial jet airliner — broke apart in mid-air off the island of Elba: all 35 people aboard were killed. Three months later, another Comet went down the same way near Naples, with 21 on board. Nothing had struck them: the aircraft simply tore itself open in mid-flight. The entire fleet was grounded, and the Royal Navy fished the first aircraft's wreckage from the seabed so it could be pieced back together in a hangar. British investigators sank an entire fuselage into a water tank at Farnborough, their aeronautical laboratory, and inflated and deflated it thousands of times, mimicking the pressurisation of every flight. Around the three-thousandth cycle, the answer appeared: metal-fatigue cracks — tiny fractures that grow with every cycle — born in the corners of the fuselage openings, where a right angle concentrates all the stress. A square corner hoards the load; a curve spreads it out. Ever since, everything that opens on a pressurised aircraft — windows, doors, hatches — has had rounded corners. The Comet flew again four years later, rebuilt with oval windows, but by then Boeing had already taken the sky from it. Next time you rest your forehead against that oval window, you will be looking at the lesson of 1954.