Why airplane windows have round corners

If you’ve ever had the window seat on an airliner, you’ve probably noticed that the corners of the window were rounded instead of squared. Ever wondered why?

The answer is simple: to keep that window from killing you.

Enter the jet age

To understand why, we need to take a trip back to the year 1954.

The early ’50s were an exciting time in the aviation industry. After being dedicated full-time to building warplanes for most of the 1940s, the end of World War II meant that manufacturers could get back to the everyday civilian business of building planes to haul people and cargo. And the pressures of the war had prompted an enormous wave of innovations in aviation technology, opening possibilities to make those planes faster, longer-ranged, and more comfortable than they had ever been before.

Geoffrey de Havilland

Geoffrey de Havilland, 1929.

One who seized on these possibilities was a British aeronautical pioneer, Geoffrey de Havilland. By 1950, de Havilland was already a legend. He had designed many of the United Kingdom’s most successful aircraft during World War I, and then went on to found his own company, de Havilland Aircraft Company Limited, which built a reputation for innovative, high-performance design with such planes as the Moth and Mosquito.

With peace restored, de Havilland had turned his attention to the question of what civilian aviation would look like in the new postwar era. And his attention gravitated towards one of the many new technologies the war had spawned: the jet engine.

Ever since the Wright Brothers had first flown at Kitty Hawk, nearly all airplanes had been driven through the sky by the same type of mechanism they had used: engines fitted with propellers. Propellers had proven to be adaptable and reliable, and so they flourished; but as engines got bigger and planes got faster, propellers couldn’t keep up. Additionally, propeller engines were raucous beasts, their loud noise and vibrations making the plane’s ride less comfortable the bigger they got. That wasn’t an issue with warplanes, but for civilian air transport, where passenger comfort was a priority, it was a big issue.

De Havilland saw that the jet engine could solve both these problems at once. With all their moving parts contained inside their enclosure, jet engines didn’t suffer the aerodynamic penalties that propellers did, so they could propel planes through the sound barrier and beyond. And unlike propellers, jets were relatively quiet and still, which meant a jet airliner could whisk its passengers through the sky in quiet comfort.

A jet airliner, in other words, was an airliner that could revolutionize aviation — and that meant the first company to bring one to market stood to make an absolutely fantastic amount of money. So de Havilland set out to make sure that company was his company.

Riding a Comet

The result of that effort was unveiled to the world in 1949. It was called the de Havilland Comet.

de Havilland Comet

BEA de Havilland DH 106 Comet, 1969. Photo credit: Ralf Manteufel. Licensed under GFDL 1.2.

The Comet was like nothing the world had ever seen. Sleek and streamlined, powered by four jet engines housed inside its wings, it looked like the future. Its cruising speed of 460 miles per hour beat its propeller-driven competitors, like the Douglas DC-6, by nearly fifty percent. And its pressurized cabin let it soar through the sky at 35,000 feet, high above storms and turbulence, giving passengers a smooth, comfortable ride.

For all its innovations, though, there was one thing about the Comet, as originally designed, that was utterly conventional.

It had square windows.

The Comet was an immediate success. The first Comets off the production line went to Britain’s national “flag carrier” airline, British Overseas Airways Corporation (“BOAC”). On May 2, 1952, a BOAC Comet made the world’s first-ever jet passenger airline flight, and smooth, quiet Comet service quickly proved popular with the world’s glitterati — including Queen Elizabeth and the royal family, who took a ride on a Comet in June 1953.

Airlines from around the world began to place orders with De Havilland for Comets of their own. Britain, struggling to find its economic footing again after the losses of two world wars, appeared to be on the verge of becoming the world’s leader in commercial aviation.

Which is why it quickly become a national crisis when, just a few months after that triumphant royal flight, Comets began to mysteriously fall out of the sky.

Falling stars

In its first year of service, two Comets were involved in crashes causing loss of life. The first, operated by Canadian Pacific Airlines, crashed outside Karachi on March 3, 1953, killing all 11 people on board. The second, operated by BOAC, crashed shortly after departing Calcutta on May 2 of the same year, this time causing 43 fatalities. While these crashes raised some initial concerns, investigations attributed them both more to pilot error than to any fundamental problem with the Comet, which meant that after some minor improvements de Havilland’s flagship could continue to operate as normal.

The next year, however, saw two more Comet disasters.

BOAC Flight 781 took off from Rome’s Ciampino Airport at 10:31 AM local time on January 10, 1954, en route to London’s Heathrow Airport. Twenty minutes later, as the Comet climbed past 27,000 feet, communications with her pilot were suddenly cut off. Shortly thereafter, off the nearby island of Elba, fishermen saw wreckage falling from the sky. 35 lives were lost.

The loss of Flight 781 made headlines worldwide. and an investigation began immediately. This was before the age of black boxes and all the other modern infrastructure that has been developed to account for air disasters, however, so it proceeded slowly.BOAC voluntarily grounded its entire Comet fleet on January 12 to perform its own internal inquiry; after another round of minor improvements, Comet service resumed again on March 23rd.

It was just days later that another Comet was lost.

This time it was South African Airways Flight 201, another Comet departing from Rome. The flight took off at 7:32 PM local time on April 8, 1954, headed for Cairo. Her captain checked in with ground control at 8:05 PM, confirming the flight’s estimated time of arrival. And then nothing was heard from Flight 201 ever again. No wreckage was ever found; 21 lives were lost.

The investigation

Finally, after four crashes and dozens of fatalities, the British government stepped in. “The cost of solving the Comet mystery must be reckoned in neither money nor manpower,” Prime Minister Winston Churchill declared. The Comet’s certificate of airworthiness was revoked until the cause of the crashes could be conclusively identified. de Havilland’s production line fell idle and Comets worldwide began to gather dust as the question of the Comet’s safety — a question upon which hung the future of Britain’s aerospace industry — awaited resolution.

Piecing together wreckage from crashed Comets, the investigators suspected that, whatever the root cause had been, what had ultimately brought the Comets down was a phenomenon called explosive decompression.

As noted above, one of the Comet’s exciting new features had been a “pressurized” cabin. At high altitudes, there isn’t enough oxygen in the air for people to be able to breathe; this had always limited the maximum altitude at which planes could fly. Pressurizing the aircraft — sealing it air-tight, and then pumping it full of breathable oxygen — allowed passengers inside to breathe easily, even as the air outside became too oxygen-thin. It was this innovation that allows pressurized aircraft like the Comet (and all modern airliners) to soar high above the weather and turbulence that buffet planes at low altitudes.

Pressurizing an aircraft isn’t risk-free, though. With the air inside the cabin now being held at a higher pressure than the air outside, it becomes critical that the sealed cabin remain sealed until such time as the plane reaches ground level and the pressure inside and out lines up again. If that seal is broken, air from inside will rush out — sometimes so violently that the force of it rips the entire plane apart.

But just observing that the crashed Comets had suffered decompression didn’t answer the underlying question. It was one thing to be able to say that something had ruptured the integrity of the Comet’s sealed cabin; it was quite another to be able to say exactly what that thing was. So the investigation proceeded.

Eventually, a series of extraordinary tests — including submerging the entire cabin of a Comet in a giant water tank, and then repeatedly pressurizing and de-pressurizing it in three-minute intervals — found the answer. And it all went back to the one thing about the Comet that wasn’t revolutionary.

It all went back to those square windows.

Stress kills

It turns out, the investigators found, that when an aircraft’s interior is pressurized and de-pressurized repeatedly, over and over again for many months, the strength of that aircraft’s metal body slowly weakens — a phenomenon that became known as metal fatigue. And when the holes you cut into that body to hold windows have sharp corners like squares do, thanks to a process called stress concentration the weakness builds up much faster in those sharp corners than it does elsewhere. Eventually cracks start to form in those corners, one of those cracks gets wide enough to let the pressurized air in the cabin rush out, and… boom.

Explosive decompression.

Two of the Comet’s innovations, each safe enough on their own, had when combined spelled disaster. Jet engines allowed quick, quiet air travel, but were most efficient at higher altitudes. And there had been airliners with pressurized cabins before, but since they were propeller-driven they flew at lower altitudes where the stresses on the airframe were less severe. Not until the jet-powered Comet would an airliner with a pressurized cabin fly high enough to stress its windows beyond the breaking point.

None of the plane’s designers had realized it, but the Comet had been doomed from the day the first one rolled off the assembly line.

The aftermath

Once the cause of the Comet crashes had been discovered, de Havilland was quick to act. All existing Comets were refitted with new windows with rounded corners, and all Comets produced afterwards came with the safer windows built in. And round-window Comets would go on to prove themselves no more dangerous to fly than any other aircraft.

But the solution came too late.

Too late for the Comet; its reputation now compromised beyond repair, that crowd of airlines wanting to buy it for themselves rapidly dispersed.

Too late for de Havilland; the Comet’s faded sales prospects, combined with some other high-profile disasters, drove the cash-strapped company to sell itself to competitor Hawker Siddeley in 1960.

Too late for the British aerospace industry, which saw competitors like Boeing and Lockheed, slower to market with their own jetliners but perceived to be more safe, seize and hold the crown of king of the Jet Age for America.

And, of course, too late for the 24 crew members and 86 passengers who died when the Comets they had boarded — those beautiful, doomed Comets, with their world-beating technology and luxurious accommodations and fatal square windows — had in an instant transformed around them from symbols of the future into tumbling heaps of wreckage.

Which is why, when you sit in the window seat of an airliner today, your window has round corners.