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 the 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 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
Try it at home: notch or no notch?
- Take two strips of paper the same size.
- In the middle of one long edge of the first strip, snip a tiny cut, just 2–3 mm deep, with scissors.
- Hold each strip by its short ends and pull slowly apart. The notched one tears much more easily, and the tear starts at the notch.
- Now cut a small square hole in the middle of one strip and a round hole of the same width in another, and pull again. Watch where the tear starts: usually at a corner of the square. (A plastic bag stretched over a bowl works too: poke a small slit and a small round hole and see which one runs.)
Ask an adult before using scissors, and point the blades away from you.
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 when something gave way it wouldn't burst apart.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 plenty of propeller airliners of the time had similar windows without trouble, because they flew lower with 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 mile travelled in cars was about 1,200 times higher than on scheduled airlines.11 Today's airliners are designed so that damage is found long before it can matter, and their structures are regularly inspected for fatigue cracks.15
Common misconceptions
"Round windows are just for looks." No. The shape decides where stress piles up, and sharp corners focus it into one spot.
12
"A round hole has no extra stress at all." A round hole still makes the stress at its edge about three times bigger. It just spreads that along a smooth curve instead of focusing it.
2
"One flight could crack a square window." Fatigue cracks grow a little at a time, over thousands of pressure cycles.
15
"The Comet had perfectly square passenger windows." They had slightly rounded corners. The critical cracks started at smaller squarish cut-outs.
8
"The hole in the window is a leak." It's only in the middle pane. The outer pane still seals the cabin.
7
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.
FAQ
Why are airplane windows round, in simple words?
Every flight, the cabin is pumped full of air and swells very slightly, which stretches the metal skin. At a sharp corner that stretch crowds into one tiny spot, and over thousands of flights a crack can start there. A rounded corner spreads the stretch out along a smooth curve, so no single spot takes all the strain.
Did square windows cause plane crashes?
In 1954 two de Havilland Comet jet airliners broke apart in flight. Investigators found metal fatigue cracks that started at the corners of squarish openings in the fuselage: small radio-antenna windows in the roof and, in a water-tank test, an escape-hatch window. After that, designers gave windows and other openings well-rounded corners.
What is the tiny hole in an airplane window for?
A plane window has three panes. The small breather (or bleed) hole is in the middle pane. It lets air pressure even out so that only the strong outer pane holds back the cabin air, keeping the middle pane as a spare. It also helps stop the window fogging up.
What are airplane windows made of?
Passenger windows are made of layers of tough, clear acrylic plastic, not ordinary glass: an inner scratch pane you can touch, a middle pane with the breather hole, and a thick outer pane that holds back the cabin pressure.
What is metal fatigue?
Metal fatigue is when a part that is strong enough to take a load once fails after the load is put on and taken off many times. Tiny cracks form, often at a sharp corner or hole, and grow a little with every cycle. Bending a paperclip back and forth until it snaps is a simple example.
What would happen if a plane window cracked?
The outer pane does the work of holding in the cabin air, but the middle pane is built to take over if the outer one is damaged. Airliners are designed so that one damaged part doesn't lead to disaster, and they're inspected regularly.
Why are ship portholes round too?
Mostly for the same reason. Any hole in a loaded sheet of metal makes the stress pile up around its edge, and a round hole spreads that extra stress along a smooth curve instead of squeezing it into sharp corners.
Sources
- FAA Lessons Learned, de Havilland DH-106 Comet 1
- Encyclopaedia Britannica, Mechanics of solids: Stress concentrations and fracture
- Encyclopaedia Britannica, Materials testing: Fatigue
- FAA, Aging Airplane Program: Widespread Fatigue Damage, Final Rule (2010)
- FAA Advisory Circular 120-104, Preventing Widespread Fatigue Damage
- 14 CFR § 25.841, Pressurized cabins (Cornell LII)
- ScienceAlert, Here's why there's a tiny hole in airplane windows (quoting GKN Aerospace)
- Wikipedia, de Havilland Comet (accident investigation and "square window myths", with cited sources)
- Wikipedia, Cabin pressurization (with cited sources)
- Wikipedia, Stress concentration (with cited sources)
- National Safety Council, Injury Facts: Deaths by Transportation Mode
- Encyclopaedia Britannica, Comet (airplane)
- Britannica Kids, Fatigue