Why are airplane windows round?

The first jet airliner

On 2 May 1952 the de Havilland Comet became the world's first jet airliner in passenger service, flying to Johannesburg^{2, 6}. Because it flew far higher than propeller airliners, its cabin was pressurised to about 8.25 psi above outside air, about double that of earlier airliners^{2, 5}. Every flight inflated the fuselage slightly; every landing let it relax².

Two crashes and a giant water tank

On 10 January 1954, Comet G-ALYP broke up near Elba after leaving Rome, killing all 35 on board⁵. After modifications, flights resumed on 23 March⁵. On 8 April, G-ALYY broke up near Naples, killing 21, and the Comet was grounded^{2, 5}.

The Royal Navy recovered about 70% of the Elba wreck^{2, 9}. Sir Arnold Hall, director of the Royal Aircraft Establishment at Farnborough, led the technical investigation^{14, 15}. A complete Comet, G-ALYU, was sunk in a water tank and its cabin pressurised again and again; water hardly compresses, so a rupture would not destroy the evidence^{2, 5}. After about 3,060 cycles, earlier airline flights included, it split near a corner of the forward escape hatch^{2, 5}.

The real culprit

G-ALYP's break began in the cabin roof, around two small windows for a direction-finding antenna^{5, 6, 7}. A fatigue crack had grown from a bolt hole near the rear window over about 1,290 pressurised flights^{2, 5}. Later research found a small manufacturing defect at the countersunk edge of that bolt hole, likely hidden under the bolt head and hard to spot on inspection^{2, 4, 14}. Any hole in a stretched sheet piles up stress at its edge, about three times the average in the textbook case, and more at tight corners^{2, 5, 13}.

De Havilland's test section had survived 16,000 cycles⁵, but it had first been pressurised to twice the working load, which probably strengthened its corners and hid the danger^{2, 5}. The court blamed fatigue for Elba and called the same cause for Naples 'at least possible'⁹. A later analysis argues the rivets and bolts around the windows mattered more than their squarish shape².

Lessons built into every airliner

Later Comets, including the Comet 4, got oval windows and thicker skin around openings; around curved edges, stress flows past with little build-up^{2, 5, 6}. The Comet 4 flew the first transatlantic jet service on 4 October 1958, and Comets carried passengers until 1980^{2, 6}. Rival jets such as the Boeing 707 used 'fail-safe' structure with backup load paths and ways to stop cracks⁵. Today, US rules demand full-scale fatigue test evidence and inspections based on how fast cracks can grow⁸.

The window you look through today

A typical cabin window has three panes, usually acrylic^{11, 12}. The outer pane carries the cabin pressure, flight after flight^{11, 12}. Behind it sits a thinner pane (often called the middle pane; technical documents say 'inner pane') that is a fail-safe: if the outer pane breaks, it must hold the pressure alone^{10, 12}. In 2022 the FAA ordered some windows on Embraer jets replaced because that backup pane was too thin¹⁰.

Its tiny bleed hole lets cabin air into the gap between the panes, so the pressure on both sides of the backup pane evens out and the load rests on the outer pane^{11, 12}; a Boeing engineer says it also lets moisture out, reducing fog and frost¹¹. The pane you can touch is a thin, non-structural scratch pane that protects the others^{11, 12}.