The Comet and its windows
The de Havilland Comet entered service in May 1952 and was the first jet airliner in the world. It flew at about 800 kilometres an hour at heights above 10,000 metres, far faster than the propeller aircraft of the day, and the cabin was quiet and smooth. Passengers and airlines lined up for it, and British industry had a lead over its American rivals.
In 1954 two Comets broke up in flight within three months of each other, one near Elba in January and one near Naples in April. The aircraft were grounded. The Royal Aircraft Establishment at Farnborough took a Comet fuselage, submerged it in a large water tank and repeatedly pressurised and depressurised it to simulate flights.
A crack from a corner
After the equivalent of several thousand flights the test fuselage failed. A crack had started at the corner of one of the square windows, and spread along the skin. Stresses concentrate at sharp corners, and the repeated cycles of pressure had worn down the metal until it gave way.
The lesson changed the way aircraft are designed. Windows have rounded corners, structures are built so that a crack cannot spread unchecked, and every airframe is tested for fatigue. The accident also made the investigation of failures a matter of routine for the industry. We note the origin of pressurisation in our article on the Stratoliner.