The de Havilland Comet's First Flight on July Twenty-Seventh, Nineteen Forty-Nine, and the Square Windows That Taught the World How Metal Really Breaks

The de Havilland DH.106 Comet made its first flight on July 27, 1949, at Hatfield aerodrome north of London, becoming the world’s first jet airliner and pulling commercial aviation out of the propeller age. Within a few years, a series of fatal in-flight breakups traced the cause to metal fatigue cracking at the corners of the aircraft’s square windows - a discovery that permanently changed how aircraft are designed, inspected, and retired. It is the reason every airliner window you see today has rounded corners.

What Was the de Havilland Comet?

The Comet was the first jet-powered airliner ever to carry fare-paying passengers. Built by de Havilland - the same firm behind the wooden wartime Mosquito - it broke completely with the piston airliners of its day.

Where a Constellation or a DC-6 bristled with propellers, rows of cylinders, and a roar that rattled your teeth, the Comet had a smooth silver fuselage, gently swept wings, and four de Havilland Ghost turbojet engines buried inside the wing roots to keep the wing clean.

She seated roughly 36 passengers in what was, for 1949, genuine luxury: large seats, tables, and generous square picture windows so travelers could look down on the clouds from an altitude no airliner passenger had ever reached.

Why Was the Comet Such a Breakthrough?

After the Second World War, crossing an ocean meant piston airliners flying low, down in the weather, on trips that could take the better part of a day with a fuel stop in the middle. It was loud, rough, and slow.

A British committee, meeting during the war while the bombs were still falling, asked what would come after the piston engine. De Havilland answered with a jet built for passengers - an idea most of the industry thought was madness.

A turbine engine wants to fly high, in thin, cold air where drag is low, the engine runs efficiently, and the ride is smooth. The challenge was building a pressurized cabin that could carry ordinary people up to 40,000 feet, day after day. Nobody had done it. De Havilland did it anyway.

The First Flight: July 27, 1949

On that summer evening, de Havilland’s chief test pilot, John Cunningham, climbed aboard. A celebrated wartime night-fighter ace nicknamed “Cat’s Eyes Cunningham” - a cover story that concealed the secret airborne radar he was actually using - Cunningham was known as a cool, nerveless hand.

He ran up the four turbines, and instead of the bark and roar of pistons the aerodrome heard a smooth, rising whistle it had never heard before. He rolled down the grass and lifted off into the evening light. The jet age for passengers was born.

How the Comet Changed Air Travel

The Comet entered service with BOAC (British Overseas Airways Corporation) in 1952, and it was a sensation. It cruised at around 480 mph while piston airliners plodded along some 200 mph slower, and it flew above the turbulence - passengers marveled that a coin set on the table wouldn’t move and their coffee never spilled.

Routes like London to Johannesburg and London to Tokyo saw travel times cut roughly in half. Britain - still on rationing and digging out from the war - had leapt ahead of everyone, while Boeing and Douglas in America were caught flat-footed, still bolting propellers onto airframes.

For about two years, the Comet was the future.

Why Did the Comet Start Crashing?

The trouble began quietly. In the spring of 1953, a Comet broke up in a storm shortly after taking off from Calcutta, and investigators reasonably blamed the weather.

Then, in January 1954, a Comet named Yoke Peter took off from Rome and climbed out over the Mediterranean near the island of Elba. The day was clear. With no warning, the aircraft simply broke apart at altitude and fell into the sea. Everyone aboard was lost.

BOAC grounded the fleet, engineers made dozens of modifications chasing every theory - fire, engine failure, sabotage - and, finding nothing conclusive, returned the Comet to service under enormous pressure.

Two weeks later, a second Comet, again out of Rome, broke apart in clear conditions while climbing over Naples and fell into the sea. The fleet was grounded for good.

The Water Tank Test That Solved the Mystery

Britain launched one of the most important accident investigations in aviation history. Divers recovered wreckage of Yoke Peter from the floor of the Mediterranean - one of the first great deep-water recoveries ever attempted - and investigators decided to test an entire Comet fuselage to destruction.

They built a giant water tank around a complete fuselage. Water was the key: if you pressure-test with air and the structure lets go, the explosion destroys the evidence, but water simply floods out. They filled the cabin, pressurized it, released the pressure, and repeated the cycle over and over - cramming years of flights into weeks of testing.

After the equivalent of thousands of flights, the fuselage cracked open. And the crack started at the corner of a window.

What Is Metal Fatigue - and What Did the Corners Teach Us?

Every time a cabin is pressurized, the fuselage swells slightly like a balloon; when it descends, it shrinks. That happens on every single flight. Flex metal back and forth enough times and it grows tired, developing tiny cracks - and those cracks begin wherever stress concentrates.

Stress concentrates at sharp corners. The Comet’s beautiful square windows focused stress at their corners, where invisible cracks grew a hair with each pressurization cycle until one reached its limit and the pressurized fuselage unzipped in an instant. That is what destroyed Yoke Peter and the aircraft lost over Naples.

It was not a storm, and not sabotage - it was a sharp corner and the brutal arithmetic of metal fatigue, which airframe engineers had barely begun to understand.

To their lasting credit, the Comet’s designers and British investigators did the humble thing: they admitted they had reached the edge of human knowledge and published all of it. That is why every airliner window today is a rounded oval with no sharp corners - a direct physical monument to the lives lost on the Comet, and a lesson Boeing, Douglas, and the rest of the world learned essentially for free.

The modern discipline of tracking pressurization cycles rather than just flight hours, the idea that a structure has a defined fatigue life, and the practice of inspecting for cracks before they find you all trace straight back to that water tank in 1954.

What Happened to the Comet Afterward?

The story doesn’t end in tragedy. De Havilland went back to the drawing board and rebuilt the aircraft with thicker skin and rounded windows, bringing it back as the Comet 4. In 1958, a Comet 4 flew the first transatlantic jet passenger service, London to New York.

By then, though, America had caught up and surged past. The Boeing 707 and Douglas DC-8 were bigger and longer-legged, built on everything the Comet had bled to teach. Britain lit the torch; America carried it the rest of the way.

There’s a final chapter, too: the Comet airframe was adapted into the Nimrod maritime patrol aircraft, which served the Royal Air Force hunting submarines well into the 21st century. A design first flown on the grass at Hatfield in 1949 was still on duty more than 50 years later.

Key Takeaways

• The de Havilland Comet first flew on July 27, 1949, and became the world’s first jet airliner, entering service with BOAC in 1952.
• Two fatal 1954 breakups near Elba and Naples were traced to metal fatigue cracks starting at the corners of the Comet’s square windows.
• A landmark water-tank fatigue test reproduced the failure after the equivalent of thousands of pressurization cycles, proving the cause.
• The investigation is why all modern airliner windows have rounded corners and why aircraft life is tracked in pressurization cycles, not just flight hours.
• The redesigned Comet 4 flew the first transatlantic jet passenger service in 1958, and the airframe later served as the RAF’s Nimrod into the 21st century.