How the de Havilland Comet’s mid-air breakups gave airplanes their round windows

In the early 1950s, the de Havilland Comet became the world’s first commercial jet airliner, prized for its speed and comfort. It also had large square windows, and it broke apart in mid-air several times, a mystery that took extensive investigation to solve.

Engineers eventually found that cabin pressurization caused fatigue cracks to form around the sharp edges of the Comet’s square windows. Those cracks spread through the aircraft’s structure over time until it failed. Tests, including pressurizing an entire fuselage submerged in a water tank, confirmed that the square windows were a primary cause of the accidents.

The disasters triggered a revolution in aircraft construction. All modern commercial aircraft now use round or oval windows, along with greater overall structural strength. The shape change addresses a basic engineering problem: sharp corners act as stress concentrators, where pressure is unevenly distributed, and cracks can form and grow at these points during the repeated pressurization cycles every flight puts an aircraft through.

Modern airliners fly at altitudes of 30,000 to 40,000 feet, where cabins must be pressurized to simulate breathable conditions, creating a significant pressure difference between the inside and outside of the plane. The aircraft’s body expands during pressurization after takeoff and retracts on landing, a cycle repeated tens of thousands of times over its lifespan, making the elimination of stress-concentrating sharp corners essential.

Oval windows also carry aerodynamic advantages, since their curves fit more naturally into the fuselage, reducing local stress points. Decades after the Comet disasters, despite advances that could theoretically make square windows viable again, the proven safety record of round and oval windows keeps them the standard across commercial aviation.