Could planes have square windows today? Engineers say yes, but here’s why they don’t

With today’s materials and engineering, it is entirely possible that square airplane windows could withstand cabin pressure far better than they did in the 1950s. Yet in practice, there is little reason to make the switch: safety records, efficiency and decades of testing all point to circular or oval windows as the best solution for commercial aviation.

The preference traces back to a hard lesson from the early days of jet travel. In the early 1950s, the de Havilland Comet, the first commercial jet airliner, flew with large square windows and broke apart in mid-air several times. Investigations found that cabin pressurization caused fatigue cracks to form around the sharp edges of the square windows, which spread through the aircraft structure until it failed.

The underlying engineering problem is straightforward: sharp corners act as stress concentrators, where pressure is unevenly distributed, allowing cracks to form and grow. Rounded or oval window shapes eliminate this risk. Modern aircraft fly at altitudes of 30,000 to 40,000 feet, where cabins must be pressurized to simulate breathable conditions, and the resulting pressure difference causes the plane’s body to expand after takeoff and retract on landing — a cycle repeated tens of thousands of times over an aircraft’s lifespan.

Oval windows also offer aerodynamic advantages, since their curves fit more naturally into the fuselage, reducing local stress points and geometrical discontinuities. Each window is also built from three layers: an outer layer that bears the maximum pressure load, a middle backup layer, and an inner plastic layer that protects against scratches and helps with noise and temperature insulation.

A small bleed hole on the window’s middle layer balances pressure between the inner and middle panes and prevents condensation, keeping the window clear throughout the flight. Together, these design choices show why, despite modern advances, aircraft manufacturers have stuck with rounded windows for more than 70 years.