The de Havilland Comet was beautiful. Sleek, four-engined, impossibly quiet compared to the propliners it replaced. On 2 May 1952, BOAC Comet G-ALYP departed London for Johannesburg — the world's first scheduled jet airline service. Passengers sipped champagne at 40,000 feet while propeller aircraft laboured at half that altitude below them. The future had arrived. Two years later, two Comets broke apart in mid-air within three months of each other. The investigation that followed discovered metal fatigue — a phenomenon no aircraft designer had properly accounted for — and it changed every airliner built since.

Two Crashes, One Mystery

On 10 January 1954, BOAC Flight 781 — Comet G-ALYP, named "Yoke Peter" — broke apart at 27,000 feet near the island of Elba in the Mediterranean. All 35 aboard died. Three months later, on 8 April, South African Airways Flight 201 — a BOAC Comet on charter, G-ALYY — disintegrated over the sea near Stromboli, south of Naples. All 21 aboard died. Both aircraft had simply come apart in the sky. The Comet fleet was grounded. The Royal Aircraft Establishment at Farnborough launched the most exhaustive aircraft investigation in history.

The Water Tank

Sir Arnold Hall's team at Farnborough devised a test that had never been attempted: they submerged a complete Comet fuselage — G-ALYU, "Yoke Uncle" — in a water tank and pressurised it repeatedly to simulate the stresses of climbing to cruising altitude and descending again. Each pressurisation cycle simulated one flight. After 3,057 total cycles (1,221 actual flights plus 1,836 simulated), the fuselage burst open — the failure starting at the corner of the forward escape hatch, a cutout with the same fatal geometry as the windows. In the recovered wreckage of G-ALYP itself, investigators traced the break-up to a crack that began at a rivet hole near the rear ADF aerial window. The tight corner radii of the Comet's nearly square cutouts created stress concentrations far higher than de Havilland's engineers had predicted. Every pressurisation cycle widened microscopic cracks at these corners until the fuselage could no longer hold together. Presenting the findings, Sir Arnold Hall, director of the Royal Aircraft Establishment, concluded that the failures were the result of metal fatigue caused by the repeated pressurisation and de-pressurisation of the cabin.

Round Windows Forever

The fix was straightforward: round the windows. Oval or circular window cutouts distribute stress evenly instead of concentrating it at corners. Every commercial airliner built since the Comet — every Boeing, every Airbus, every Embraer, every Bombardier — has round or oval windows. Look out the window on your next flight: the shape exists because of two Comets that fell into the Mediterranean in 1954. The Comet programme recovered. The redesigned Comet 4 entered service in 1958 with oval windows, thicker skin, and proper fatigue testing. But the delay had been fatal to de Havilland's commercial ambitions: the Boeing 707, which benefited enormously from the Comet's lessons, entered service that same year and swept the market. The Comet was beautiful. It was first. And it taught the world a lesson written in aluminium and blood: metal gets tired, and corners kill. Sources: Royal Aircraft Establishment Court of Inquiry, FAA Lessons Learned, de Havilland Aircraft Museum, Air & Space Quarterly