Engineers discovered the hard way why plane windows had to be round.

The de Havilland Comet jet — which had been the world’s first commercial jetliner — began climbing to cruising altitude to carry its 35 passengers and crew to London.

About 15 minutes into the flight, which was operated by the British Overseas Airways Corporation (BOAC), the captain spoke to another pilot when the transmission ended mid-sentence. The Comet jet disintegrated in the sky and crashed into the Mediterranean Sea, killing everyone on board.

Just two years earlier the de Havilland Comet had been the toast of skies when it took flight on the world’s first scheduled commercial flight.

But with the tragedy outside Rome, and a second fatal crash just months later, the Comet would leave a unique impact on the future of aviation — it would be the reason why you never, ever see square windows on a plane.

WHY THE COMET WAS SUCH A BIG DEAL

The de Havilland Comet became the world’s first commercial jetliner in 1952, and in terms of design, it was a total game-changer.

In an age of propellers, this British-made jet had four turbojet engines. It had that sleek, bullet-shaped fuselage that planes still have today. It could fly higher than its competitors, it had a pressurised cabin, and its wings swept backwards, which was unheard of at the time.

It also had large, square windows — a desirable feature for the ever-expanding jetset.

The de Havilland Comet was the world’s first passenger jet, and one of biggest pioneers in aviation.Source:News Corp Australia

And as the first jet in history to make a scheduled, commercial flight, the Comet helped make air travel possible for millions of people. In its first year, it flew 30,000 passengers, including Queen Elizabeth. It took passengers from London to places such as Tokyo, Singapore, Johannesburg, and Colombo.

But the Comet’s most important contribution to aviation wouldn’t be measured by its successes but the most horrific of failures, as a new explainer in the UK’s The Telegraph reveals.

WHAT WENT WRONG?

In 1953, just a year after the Comet entered commercial service, the world’s first fatal jetliner crash happened on a Canadian Pacific Comet flying from London to Sydney.

The jet crashed on takeoff at Karachi in Pakistan, killing all 11 passengers and crew. The tragedy was blamed on a wing configuration flaw.

That was one of a few, and occasionally fatal, crashes Comet jets experienced in those early years. But it was two crashes in 1954 that would change things forever.

The first of those incidents involved the ill-fated flight 781 from Rome, which exploded 20 minutes after takeoff. Investigators found a confusing pattern of injuries among the recovered bodies: fractured skulls and ruptured lungs.

Just months later, all 21 passengers and crew were killed when South African Airways flight 201, from London to Johannesburg, crashed into the Mediterranean Sea off Naples.

The crash of that Comet jet prompted the British minister of transport to ground Comet’s fleet. Meanwhile, recovered bodies showed similar head and lung injuries to those on flight 781.

Investigations into the crashes eventually found both were caused by in-flight metal fatigue failure, which led to explosive decompression and midair breakup.

Here’s where the part about window shape comes in. Picture: Justin Benson-CooperSource:News Corp Australia

SO WHAT’S THE LINK TO WINDOWS?

As The Telegraph this week explained, the square shape of the Comet’s windows played a big role in the metal fatigue that caused those crashes.

The sharp corners of the windows put the surrounding metal under extra stress in high altitudes — as much as two or three times more than other places on the plane. The stress was concentrated in the four corners of every window, causing the metal fatigue.

Following the investigations, de Havilland made a number of changes to its aircraft design, including rounder windows. The lack of sharp corners allowed the stress to flow more evenly around the edges of the window.

“We often learn the most from our failures, this is particularly true for advancements in the field of engineering. Unfortunately for the engineers in the aviation industry, the prices to pay for failure are high,” said Brian McManus, a former research and design engineer Brian McManus in a recent video on his YouTube channel Real Engineering.

“The flip-side of this unforgiving industry, is that it consistently provides learning opportunities for engineers, because failure is not an option when peoples lives are at risk.”