Why do all airplane windows have a small hole?

This tiny detail may go unnoticed by most, but is crucial for a plane. Photo / Unsplash

It may have been weeks, months, or years since you graced the inside of an aeroplane but when you next do, take a closer look at the window and see if you can spot a little hole.

Before you fear the worst, this isn't an engineering oversight or mistake. In fact, without these little gaps, there could be serious issues on board.

Known as a 'breather hole' or 'bleed hole', these gaps only go through one of three acrylic layers that make up a plane window.

The middle layer contains a small hole that helps regulate the air pressure changes as an aircraft climbs to cruising altitude.

At 33,000 feet high, the pressure difference between outside the plane and inside can put a large strain on the windows.

According to Mark Vanhoenacker, a British Airways pilot writing for Slate, as an aircraft climbs higher, the air pressure drops both outside and inside the plane.

However, the pressure drops far lower outside as the aircraft's pressurisation system will keep the cabin at a comfortable and safe level for passengers.

The middle and outer panes, says Vanhoenacker, are "designed to contain this difference in pressure between the cabin and the sky."

The layer closest to the passenger is known as the 'scratch pane', and simply works to protect the more important outer panes.

This is safer for the aircraft and also makes the cabin more comfortable for passengers.

Additionally, it also allows moisture to leave the aircraft and prevent windows from fogging up. Although, from time to time you may notice a small pattern of frost form during particularly cold spells.

As for the shape of a plane window, there is a tragic reason why they are round, not square.