We are all quite used to the standard curved shape of a commercial aircraft window, but believe it or not, they were once square. In 1952, three tragic commercial airplane crashes left engineers puzzled. The ill-fated airframes had two things in common—a fuselage that tore apart at cruising altitude, and square windows.
Because commercial flight demand had increased at the time, airlines needed airplanes with the capacity to travel at a higher cruising altitude of 30,000 ft.—formerly, commercial aircraft were propeller driven, and traveled at 10,000 to 12,000 ft. With the invention of commercial jet aircraft, engineers designed aircraft with a cylindrical fuselage that was pressurized. These changes meant the aircraft could operate as commuter transport at higher altitudes, with less drag. Unfortunately, the square window design remained.
In inspecting the debris, the design flaw of the windows was quickly determined. As a plane increases in altitude, external atmospheric pressure lowers. The difference between external and internal pressurization causes the fuselage to contract and expand slightly. In the case of the 1952 crashes, the stress created within a fuselage material was aggravated by the rigid edges of the square windows. Because the rigid shape interrupts the stress distribution across the surface of the fuselage, stress concentration builds at the edges of the window. As a result, the elevated stress created fractures in the material of the fuselage.
Aircraft windows are now rounded in order to ensure even distribution of stress across the airframe. Modern window engineering has multiple failsafes and the capacity to preserve pressure conditions of the cabin. Passenger windows are two panels with airspace in between. One panel operates as a fail safe to protect the interior cabin pressure even if the other panel fails.
New window technology continues to develop, and can be seen in modern aircraft cabins like that of the Boeing Dreamliner. It’s windows have multi-ply transparency material mounted on a composite fuselage. This material is designed with crack propagation resistant properties, and is able to manage more impact than its predecessors. As a result, windows made with transparencies are able to manage greater overall loads.
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