Two crashes that forever changed the shape of airplane windows

Look at any passenger aircraft window today and one feature stands out: it has no sharp corners. That design change followed two fatal crashes involving the Comet, the world's first jet airliner.

Britain introduced the Comet in May 1952. It flew twice as fast and twice as high as other aircraft of its time, turning journeys that had taken days into trips lasting hours. But its nearly square windows became part of a painful lesson in aircraft engineering.

On January 10, 1954, a Comet flying from Rome to London broke apart in midair about 8,000 meters above the sea near Elba. Thirty-five people died. On April 8, another Comet traveling from Rome to Cairo broke apart during its climb and fell into the Mediterranean, killing 21 people. After the second crash, the entire fleet was grounded and its flight certificate withdrawn.

Investigators subjected a complete aircraft, provided by the airline, to repeated pressurization cycles in a huge water tank. They filled the cabin with pressurized water and then released it, repeating the process. Each cycle represented a flight, as the aircraft expands during ascent and contracts during descent. Water offered a safer substitute for air if the structure failed.

On June 24, 1954, after more than 3,000 cycles, the fuselage split at the corner of an opening. Pressure spreads across a pressurized fuselage, but a sharp corner concentrates stress. Tiny cracks can grow with every flight until the damage becomes large enough to cause failure. Sir Arnold Hall, who led the testing, concluded that metal fatigue caused by repeated changes in cabin pressure had destroyed the aircraft.

The usual retelling is incomplete. The fatal crack near Elba did not begin at a passenger window, but at an opening on the upper fuselage. Investigators also found that the skin was too thin and that rivet holes left tiny cracks. Rounded windows formed part of a wider reinforcement program, not the only change. On the modified Comet, emergency-exit openings remained square. Since then, passenger aircraft have not been built with sharp-cornered windows.