If you’re a fan of the window seat, you’ve probably noticed a tiny hole at the bottom of the plane’s windows – and wondered why on earth that tiny thing is there.
It turns out that the hole is called a breather hole or a bleed hole and it is stuck between two other acrylic panes, which means that when you look through an airplane window, you actually look through three different panes.
The first of these panes – the one you can touch and dirty up with fingerprints – is called a scratch pane. The middle pane is the one with the bleed hole, and the whole system is completed by the outer pane – the most important one, since it protects you from the pressure difference outside.
While both the outer and middle panes have the power to resist the pressure from outside the plane, the brunt of the work falls on the outer window because it’s the final barrier between you and the clouds.
Before exploring how these windows work, it’s important to understand a little bit about how atmospheric pressure changes when you travel.
If everything is working properly, you should not even notice the pressure dropping outside your window.
As the plane reaches higher elevations, the air outside the plane becomes thinner, containing less oxygen and pressure.
According to the American Vacuum Society, you experience roughly 1.0 kilogram per square centimetre (14.6 pounds per square inch) of pressure at sea level. This is what we’re used to feeling and breathing. It’s pretty comfortable.
When you get in an airplane and climb all the way up to a cruising altitude of 10,679 metres (35,000 feet), this pressure drops down to a mere 0.2 kilogram per square centimetre (3.4 pounds per square inch).
While you’re airborne there’s a giant different between the pressure inside the cabin of a plane and that outside of the plane.
So, what does all this have to do with the scary-looking hole in the window? Well, this hole actually reduces the pressure on the middle pane, so that only the outer pane experiences the force of cabin pressure – and it undergoes this pressure more gradually during the flight.
“[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,” Marlowe Moncur, director of technology at GKN Aerospace – a passenger window manufacturing company – explained to Slate.