If you’ve ridden in an airplane even once, you will have noticed that the windows have a very particular shape: They’re not quite rectangular, not quite round, but more of an oval shape. And much like all parts of aircraft design, there’s nothing random about that choice.
The geometry of window design is a sophisticated response to an unforeseen issue presented by the birth of the Jet Age during the early 1950s. De Havilland Comet planes, designed and manufactured in Britain, officially entered service in 1951 as the world’s first commercial jet airliners. Unfortunately, they soon faced a series of midair disasters, which involved the planes breaking apart in flight, killing all passengers aboard.
The culprit? Rectangular windows. Mick Oakey, the managing editor of The Aviation Historian, a quarterly journal about classic airplanes and the history of flying, says that sharp internal corners on a metal structure are notorious for being “stress-raisers.” Oakey explains: “They act as a focus for the forces of bending, twisting, and stretching, and cracks or breaks are much more likely to appear in those corners than anywhere else.”
When you add in cabin pressurization, it creates a cycle of stress, with the pressure on the plane’s body increasing and decreasing during takeoff, cruising, and landing. The resulting fatigue caused the metal to weaken and eventually crack, leading otherwise brand-new planes to tear apart while flying.
“This was catastrophic for the early Comets because, as the world’s first commercial jet airliner, it flew higher and faster than its rivals and so encountered the destructive effects of fatigue before the phenomenon was widely understood,” Oakey says.
Rounded windows quickly became the standard once studies emerged about the weakness in the squared-off designs. As Oakey explains, the best design from an engineering standpoint would be to have no windows at all because glass and window frames are relatively heavy. “Rounded windows provide the best compromise between safety, minimal weight, and passenger comfort,” he says. The oval shape disperses stress more evenly rather than concentrating it in corners. And, of course, the shape allows window-seat passengers to get a view of the world outside.
Over the years, the shape has been only slightly refined, but windows today look much as they did in the near-immediate wake of the Comet disasters. “Eventually, the world airliner industry standardized on ‘upright’ ellipses, deeper than they were wide, and ‘squashed’—with slightly straighter vertical sides—in order to fit a large number of windows easily between the closely spaced circular frames, or hoops, of the fuselage structure.”
Oddly enough, there was at least one exception to the square vs. oval windows debate: Oakey points to France’s Sud Aviation Caravelle planes, which had round-cornered triangular windows in the late 1950s.
