What Are Those Small Holes On Airplane Windows?

If you’ve ever been in an airplane, you might’ve noticed that there’s a small hole in the bottom of your window. If you haven’t noticed it before, look for it next time; it’s on every plane. If you have noticed it before, you might have wondered whether it should be there and whether it was safe or not. The hole is actually an important component of the window, and contrary to what you might have thought it did, the hole actually helps the safety of the window. In this article, we will be discussing the function of the hole and what it actually does.

In order to understand why the hole is there, we must first know about the difference in air pressure between the cabin of the airplane and the outside of the plane. The higher up in the atmosphere, the thinner and less pressurized the air is. Since humans need to be in more pressurized air with tolerable levels of oxygen, the cabins of airplanes need to be pressurized. This creates a high pressure difference between the inside of a plane and the outside world. The difference in air pressure creates stress on the windows of a plane, and that’s where the holes are involved.

Jet Airways flight cabin forgets to pressurize cabin, causes nosebleeds -  Vox
Credit: Vox

An airplane window consists of 3 different layers of panes. The most inner layer is the one exposed to the passengers and prevents damage to the other panes. The middle pane is where the hole is located, and the outer pane is the layer of glass that endures most of the air pressure. The hole in the middle pane is called a “bleed hole”, and it is what allows the air to flow to the third pane. There is an air gap between the middle and outer glass panes, and the bleed hole allows the air pressure to balance out or reach an equilibrium between this gap and the cabin. The hole diverts much of the air pressure onto the outer panel of glass and allows the middle pane to function as a fail-safe pane in case the outer pane breaks. Essentially, the purpose of the hole is to ensure that there is a second layer of protection if one of the panes breaks from the air pressure stress.

Why is there a double window in an aeroplane? - Quora
Credit: Quora

Not only does the bleed hole play one of the most important roles in the safety of an airplane window, but also it contributes to the comfort of the passenger. Bleed holes are also what prevents windows from fogging up on a plane. The cold air on the outside of the plane and the heated inside could easily create fog on the windows of a plane, but the bleed hole prevents this. When pressured air escapes through the bleed hole into the gap between the two glass panes, it also allows moisture from the air gap to be released. This stops the window from building up fog or freezing up, and it is also why there is sometimes a ring of ice around the hole in cold conditions.

What's That Thing: Why are there holes in airplane windows?
Credit: slate.com

In the end, the bleed hole is the complete opposite of what people usually first think it is. The bleed hole is not a safety hazard or a dangerous mistake, but rather the complete opposite. Even though the bleed hole is smaller than your finger, it is responsible for one of the largest roles in the safety of a window. The bleed hole enables a second layer of protection just in case something happens, and although the chances of the outer pane of the window breaking from air pressure are unlikely, a backup form of protection is extremely important. Next time you see one of the tiny holes when flying, know that it plays a crucial role in keeping you safe. 






Cover Image: Nairobi Wire

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