Why are aircraft windows not square?

Your children’s question of the month for September (2017)
Laura, 11, from Giessen

Most of us don’t really care about the shape of the airplane window when looking out of it – it’s the view that counts!
But there is an important reason for the round shape, as there is for everything you see on an aircraft!

Airplanes actually used to have square windows. Then the introduction of jet engines and pressurized cabins suddenly made it possible for aircraft to fly much higher, progress that made sense for many reasons:
1. The higher an aircraft flies, the thinner the air and the lower the air pressure. This means that the aircraft experiences less resistance and requires less fuel.
2. For passengers, flying at high altitudes also makes flying more pleasant as there is less turbulence higher up.

To meet these new pressure ratios, lots of changes had to be made to aircraft. But one detail wasn’t changed right away. Only after a couple of bad accidents in the 1950s did it become clear that the shape of the windows also needed to be changed!

But why is the shape so important?

At cruising altitude (usually around 10,000 m), air pressure is far too low for us passengers and this is why the pressure in the cabin is artificially raised. Quite simply: the higher you fly, the greater the difference in pressure between the inside and outside.
Square windows, or more precisely the corners, do not allow the acting forces to “flow past” without incident, but in fact strengthen the impact of these forces on the aircraft housing.

To exclude this danger, aircraft windows have been round now for many decades.