Why are airplane windows round?

Why are airplane windows round? The short answer

An airliner's cabin is pumped full of air so you can breathe comfortably high up, and that makes the whole body of the plane swell very slightly on every flight, like a balloon.⁹ The swelling stretches the metal skin. Wherever there's a hole, such as a window, the stretch has to flow around it. At a sharp corner, the stress crowds into one tiny spot; around a smooth curve, it spreads out.¹² Repeat that stretch thousands of times and a crack can slowly grow from a sharp corner. This is called metal fatigue.³⁴ Rounded windows remove that sharp weak spot.

Engineers learned this the hard way in the 1950s, from the world's first jet airliner, the de Havilland Comet.¹

A plane is a flying balloon

Airliners cruise around 11 kilometres (about 36,000 feet) up.⁷ Up there, the air pressure is only about a quarter of what it is at sea level: roughly 3.4 pounds per square inch instead of 14.6.⁷ That's far too thin to breathe comfortably, so the plane pumps extra air into the cabin.

The cabin isn't pumped all the way back up to sea-level pressure, though. US rules say the cabin must feel no higher than 8,000 feet (about 2,400 m) in normal flight,⁶ and some newer airliners keep it nearer 6,000 feet (about 1,800 m).⁹ Keeping the difference smaller keeps the stress on the plane's body smaller, because that stress depends on the difference between the pressure inside and outside.⁹

Even so, the air inside pushes outwards much harder than the thin air outside pushes in. The cabin swells a tiny amount, then relaxes when the plane lands and the pressures even out.⁹ That's one "pressure cycle" per flight, and a plane flies thousands of them.⁴

1 flight = 1 stretch-and-relax of the whole cabin

In the player: the outside pressure in the balloon toy comes from the standard-atmosphere formula; the smooth rise of the cabin's "feels like" height and the size of the swelling are simplified (the swelling is hugely exaggerated so you can see it).

What is a stress concentration?

Imagine the pull in a sheet of metal as lines of force flowing through it, like water in a river. With no hole, the lines run straight and evenly. Cut a hole and they must squeeze around it, so the metal right beside the hole carries extra load.¹¹⁰

How much extra depends on the shape. As far back as 1898, the engineer G. Kirsch worked out that the edge of a small round hole in a big plate pulled in one direction feels, at its most-pulled point, about three times the pull far away.² Later, Charles Inglis showed that for a long narrow hole the pile-up gets far worse as the ends get sharper (as the radius of curvature shrinks).² Engineers call these hot spots stress concentrations, and they're exactly where cracks like to start: at sharp corners, notches and cut-outs.²

Engineers estimate the peak with a simple formula for a pointed, oval-ended hole:¹⁰

peak stress ≈ (1 + 2√(a ÷ ρ)) × the stress far away

Here a is half the length of the hole across the pull and ρ (rho) is the radius of the curve at its end. A circle has ρ = a, which gives the 3× above. Make the corner sharper (smaller ρ) and the number climbs. The "corners" and "your turn" toys use this rule of thumb to colour the stress; it's a teaching sketch, not a full engineering simulation.

A rounded window still makes some extra stress; the point is that it spreads it out instead of focusing it on one tiny spot.¹

Why a notch makes paper tear so easily

You can feel a stress concentration with a sheet of paper. Try to pull a sheet apart by its edges and it's surprisingly strong. Make one tiny cut in the edge first, and it tears easily, starting right at the notch. The notch is a very sharp corner, so all the pull crowds into its tip.²

What is metal fatigue?

A material that can take a load once can still fail if the load is put on and taken off many times. That's fatigue.³ Bend a paperclip back and forth and it eventually snaps, even though a single bend does nothing much.¹³

In an airplane, the repeated load is the cabin pressure going up and down on every flight.⁴ Fatigue damage starts as tiny cracks, too small to see at first, and they grow a little with each cycle.⁵ Where do they start? At stress concentrations: sharp corners, notches and even rivet holes.²⁸

In the player: the "your turn" crack-growth numbers are made up to show the idea; they aren't real data for any aircraft.

The de Havilland Comet: how engineers found out

The de Havilland Comet first flew in 1949 and began carrying passengers on 2 May 1952, the world's first pressurised jet airliner.¹¹² It flew faster, higher and more smoothly than the propeller airliners before it,¹ and flying higher meant its cabin had to hold back a bigger pressure difference.⁸

In 1954, two Comets broke up in flight, and the whole fleet was grounded while investigators worked out why.¹

The water-tank test

Investigators put the body of a Comet (registration G-ALYU) into a huge tank. They filled the tank and the cabin with water, then pumped extra water into the cabin until the pressure matched a real flight, let it out, and repeated this again and again to copy flight after flight. Using water instead of air was safer: water barely squashes, so it stores far less energy than squeezed air, and a split is much less violent.¹

That fuselage had already made about 1,230 real flights. After about 1,830 more "flights" in the tank, some 3,060 in total, the metal split open, starting at the corner of a squarish escape-hatch window.¹ Wreckage recovered from the sea then showed the first crack in one of the accidents had started at squarish radio-antenna windows in the roof, with the "unmistakable fingerprint of fatigue".¹ The FAA's summary: the squarish windows created stress concentrations much higher than expected, and the production Comets were reaching only about 1,000 pressure cycles instead of the 16,000 a test fuselage had survived.¹

The Comet changed to oval windows, rules changed to require fatigue tests on complete, fresh pressure cabins, and the whole industry learned how pressurised bodies wear out.¹²

Source: FAA Lessons Learned.¹ Other histories give slightly different counts (for example 3,057).⁸

Were the Comet's passenger windows square?

Not quite. The Comet's big passenger windows were rectangular with slightly rounded corners, and propeller airliners of the time generally flew lower, so their cabins held back a smaller pressure difference. The cracks that mattered began at smaller, squarish cut-outs (the roof antenna windows and an escape hatch window), helped along by rivet holes and thin skin nearby.¹⁸ The lesson, though, was the same for every opening: round off the corners.

What are airplane windows made of? Three panes and a tiny hole

A passenger window isn't ordinary glass. It's three layers of tough, clear acrylic plastic:⁷

• Inner "scratch" pane: the one you can touch. It protects the others.
• Middle pane: has a tiny breather hole (also called a bleed hole) near the bottom. It's the spare, built to hold if the outer pane is damaged.
• Outer pane: the strong one that holds back the cabin air.

The breather hole lets air pressure even out between the cabin and the gap behind the middle pane, so during flight the cabin pressure presses only on the outer pane. It also helps stop the window fogging up.⁷

Are airplanes safe to fly?

Yes. Lessons like the Comet's are a big part of why. Over the last ten years in the US, the death rate per passenger mile in cars and other passenger vehicles was about 1,200 times the rate on scheduled airlines (National Safety Council).¹¹ Today's airliners are designed and regularly inspected to help find fatigue cracks before they threaten safety.¹⁵

Common misconceptions

Keep exploring

Curious how the plane gets up there in the first place? See how planes fly. And for the stuff in ordinary windows, see how glass is made. If the engine hum bothers you on a flight, find out how noise-cancelling headphones work.

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