Why are airplane windows round?
Sharp corners concentrate stress into a single point, and at cruising altitude a pressurised cabin pushes hard enough on that point to tear the fuselage open.
Simple intuition
The plain reason, in everyday words
Think about opening a bag of crisps. It is surprisingly hard to tear the plastic in the middle, but once there is a tiny notch in the edge, it rips open easily. The notch gives the tear somewhere to start. A square window is a permanent notch built into the side of an aeroplane. Up at cruising height the air inside the cabin is pushed to a much higher pressure than the thin air outside, so the whole body of the plane is squeezed outwards like a balloon. All that force has to travel around the window openings. At a sharp corner it piles up in one tiny spot instead of spreading out. Round off the corner and the force flows around it smoothly, with nowhere to pile up.
Round windows are about aerodynamics or reducing drag.
The windows sit flush inside the fuselage skin and have essentially no effect on drag. The shape is a structural decision about where stress accumulates.
The Comet crashed because jets were a new and inherently risky technology.
The jet engines were not the problem. The failure was a metal fatigue problem in the pressurised fuselage, and it would have happened to a propeller aircraft flying the same pressure cycles.
The window glass is what has to be strong.
The passenger-facing pane is not the pressure-bearing part at all. Cabin windows are usually multiple panes, with a middle pane carrying the pressure load and a small breather hole in the inner pane equalising the gap. The structural question is about the hole cut in the aluminium, not the transparency filling it.
Once you can see stress concentration, you start noticing it everywhere: the rounded corners of a phone screen, the curve where a spanner's handle meets its head, the reason a scratch on a windscreen turns into a crack. It is also a clean example of how safety engineering actually advances — not by predicting every failure in advance, but by investigating the first one thoroughly enough that it never repeats.
Who worked it out
The mathematics was in place long before the accidents. Charles Inglis published his analysis of stress around elliptical holes in plates in 1913, and Alan Arnold Griffith's 1921 work on brittle fracture explained why real materials break far below their theoretical strength.
What problem forced it
The theory became urgent in 1954, when two Comets broke apart within four months and the Royal Aircraft Establishment ran a full-scale water-tank fatigue test to find out why.
How it changed since
The findings reshaped airworthiness regulation. Fail-safe and damage-tolerant design requirements entered certification rules, full-scale fatigue testing became standard, and the aviation industry adopted the practice of publishing accident investigations openly so that every manufacturer learns from each failure.
Metal fatigue and crack growth
Explains why the damage accumulates over thousands of flights rather than appearing all at once.
Why aircraft cabins are pressurised at all
The load that causes the problem only exists because of a different design decision about human physiology.
Written for Curio rather than collected from a forum — it is part of the curated corpus that ships with the platform. The references it draws on are listed under Sources.
Why are arches so strong?
An arch turns a downward load into a sideways squeeze, and stone is enormously strong in squeeze and almost useless in pull.
A natural next question