Whats that tiny hole in an airplane window?

You strap to your window seat, relax and look out the airplane window. You then notice a tiny hole almost at the bottom. When you start flying higher and higher, you notice that no air enters that tiny hole. That tiny hole you see does not go all the way out the aircraft, and it is there for a reason. That tiny hole on airplane windows are actually called bleed holes, and they are there for safety reasons.

Before we go into why those holes are there, allow us to talk about what happens at 31,000ft. As you climb higher and higher, the oxygen level goes down. Notice that when you go higher in mountains like, you tend to feel a bit nausea? That's because the oxygen level at higher levels are less as compared to when you are at sea level. This lack of oxygen may lead to hypoxia which can be fatal.

How come when you fly in an airplane, you don't feel anything even if you climb to up to 30,000ft and higher? The reason for that is an airplane cabin is pressurized as it climbs higher so passengers can breath normally. Now this is good for the passengers as engine air is pumped into the cabin to mimic the oxygen level we breathe when outside an aircraft at normal altitudes. An aircraft's pressurization system is indeed good for the passengers, but not so good for the cabin and fuselage of an aircraft if it cannot bleed.

An aircraft's windows are made of three panes. The first pane or inner pane is the one you can touch or knock on. The second pane is the one with the tiny hole. The third pane is the outer one, it protects us from the pressure difference outside. There is indeed a huge difference in pressure inside a cabin and outside at 35,000ft.

The tiny hole reduces the pressure on the middle pane, allowing only the outer pane to take all the cabin pressure. Hence, it allows pressure to bleed out, in order to balance the air pressure between the middle and outer pane. Aside from allowing air pressure to bleed, the bleed hole releases moisture between the second and third pane, which prevents the window from fogging up.

So next time you ride an airplane, know that when you look out the window, you are actually looking through three window panes. At the same time, that hole is not inside the cabin which you can insert the ballpen tip in. It is located at the second pane.

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