Why do airplane windows always have a small hole in them?

Your kids’ question of the month in March (2017)
Johanna, 9, from Bremen

Have you ever sat next to the window on an airplane? If so, you might have noticed that at the bottom of the pane there is a small hole – but why?
Airplanes are fascinating ... but incredibly complicated! To ensure that they can carry passengers across the world safely, every single tiny detail like this is hugely important.

This is why airplanes also have special windowpanes.

Normal windowpanes, like those you have at home, would not be able to withstand the air pressure above the clouds – if an airplane was equipped with normal panes, the glass would break as the airplane flies upward because the air pressure drops at height.

This is why windows in most passenger aircraft have have three panespanes – an inside one, a middle one, and, of course, an outside pane. The inner pane that you can touch is there solely to prevent contact with the outer pane. This is because above the clouds, the air (and thus also the pane) is very cold: up to minus 60 degrees!

But why the small hole?

There are two reasons for the hole: one is that flying wouldn’t be quite so great if you didn’t have such an amazing view, agreed? The small hole allows the moisture that forms between the two panes to escape and therefore the windows don’t steam up.

The second reason is the much more important one: the hole is there to equalize the air pressure. Once an aircraft climbs high, the air pressure around it decreases. In the cabin, however, the air pressure stays almost the same. The outer pane is the thickest and therefore the strongest of the three panes and must maintain the pressure in the cabin. The small hole ensures that the air between the two outer panes can circulate and the cabin pressure is only borne by the strongest pane.