If you’ve sat by the window on a plane, you must have noticed that little hole in the lower portion of window. There is a hole in the window? Yes, there actually is. And sometimes you might even notice the snowflake of frost forming near it. This tiny hole is called a breather hole or a bleed hole, and it serves an important safety function. If you look closely, you’ll see that the passenger window has three panes, typically made of acrylic materials. The purpose of the innermost pane, which you can see forehead smudges on, is called the scratch pane and is merely to protect the next pane. But the middle pane with the breather hole in it and the outer pane are more important.
As an aircraft climbs, the air pressure drops in both the cabin and the outside air where the pressure inside the aircraft during flight is typically much greater than the pressure outside. The outer two cabin windows are designed to contain this difference in pressure between the cabin and the sky. Both the middle and the outer panes are strong enough to withstand the difference on their own, but under normal circumstances it’s the outer pane that bears this pressure—thanks to the breather hole.
According to Marlowe Moncur, director of technology for GKN Aerospace, “[T]he purpose of the small bleed hole in the [middle] pane is to allow pressure to equilibrate between the passenger cabin and the air gap between the panes, so that the cabin pressure during flight is applied to only the outer pane.”
In the extraordinarily unlikely event that the outer pane fails, the middle pane takes over. Although there would be a small leak of air through the breather hole—it’s nothing the aircraft’s pressurisation system couldn’t easily cope with.
Another separate but related function of the hole is to release moisture from the air gap and stop (most) fog or frost from forming on the window. So when you’re looking out at the clouds and admiring the land below you, take a moment to give thanks for the breather hole.