Why All Airplane Windows Are Round (And the Disaster That Happened When They Were Square)
When you glance out your airplane window at 30,000 feet, that curved little portal seems like a simple design choice. But the reason it is round has nothing to do with style. It is the result of one of the biggest lessons in aviation safety, a deadly one that reshaped how airplanes are built.
In the early years of flight, windows were not part of aircraft. The Wright Brothers focused primarily on powered flight, rather than passenger comfort. As airplanes improved and started flying higher, cabins had to be sealed and pressurized to keep passengers safe.
By the 1940s, aircraft could finally carry people long distances in comfort, and engineers began refining many features of passenger cabins, including windows, to make high-altitude travel both practical and safe.
The Jet Age Disaster
Image via Wikimedia Commons/RuthAS
The de Havilland Comet entered service in 1952 as the first commercial jetliner. It was faster and more advanced than any aircraft built before it, flying at altitudes near 30,000 feet and ushering in the Jet Age of passenger travel. Only a few years later, the program faced disaster. Between 1953 and 1954, several Comet planes disintegrated midair after metal fatigue caused structural cracks around the windows. The accidents killed dozens of passengers and forced authorities to ground the fleet for a full investigation.
Investigators found that the constant pressure changes during flight caused fine cracks to form in stressed sections of the fuselage, especially around the corners of windows and hatches. The square shape of the windows focused strain at those points, allowing the cracks to spread until the structure failed. Tests later showed that sharp corners carried far more stress than any other part of the aircraft, and engineers realized the window design itself had to change.
Round Window Revolution
Engineers solved the problem by changing the shape. They replaced square windows with rounded ones to remove the corners where stress built up. The new design distributed pressure evenly and greatly reduced the risk of cracking. Over time, it proved durable through thousands of pressurization cycles, and the round window became a permanent feature on every pressurized airliner that followed.
Rounded windows also better endure repeated changes in cabin pressure. Every time a plane climbs or descends, the fuselage expands and contracts slightly. Over many flights, these fluctuations could weaken sharp-cornered openings, while a curved frame flexes gently with the pressure instead of resisting it, keeping the airframe intact. The same principle is applied in spacecraft, submarines, and deep-sea vessels, where round portals handle extreme external or internal pressure more safely than angular ones.
Beyond the Shape
Image via Getty Images/frankpeters
Modern airplane windows are made from acrylic, not glass, because the flexible acrylic is lighter and stronger. Each window typically consists of three layers that work together for strength and safety. The thick outer pane takes the brunt of external pressure at altitude.
The middle pane features a small hole, known as a breather or bleed hole, which allows pressure to equalize between the inner and outer sections. The thin inner pane serves as a protective barrier for passengers, helping to prevent scratches or damage to the structural layers.
Today, rounded windows are more than just a safety feature; they represent the evolution of engineering through research, testing, and innovation. Every pressurized passenger jet in the sky follows the lessons learned from the Comet disasters of the 1950s. The curved window design, now so ordinary, is a quiet yet vital example of design informed by experience.