If you have a window seat on a plane, you may notice a tiny hole down towards the bottom. You can’t feel it if you try to touch it, and yet it’s on every passenger window of just about every commercial aircraft.
PC: flickr, Chris Waits
Ever wonder why it was there? Welp. I found out…
The little holes are called “bleed holes” or “breather holes” and they play a huge role in saving our lives when we fly.
As we know, plane cabins are pressurized so we don’t pass out from the lack of oxygen and pressure at higher elevations. This causes a huge difference in the pressure we feel inside the plane versus what’s outside it when we’re at 35,000 feet.
The acrylic plane windows have 3 layers. The innermost layer, called the “scratch pane,” is the one you can touch. The middle pane has a bleed hole. And then there’s the outermost layer, which is stronger and subject to the most pressure.
PC: via Boeing 737 maintenance manual. Not shown: scratch pane
The outer and middle panes are more structural than the inner layer and they, along with their rubber perimeter seal, create the barrier that protects us from the outside pressure. The holes in the middle layer reduce the pressure that the middle pane experiences so that only the outer pane experiences the full force of the pressure difference. Marlowe Moncure, director of technology at GKN Aerospace explained it this way:
“”[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.”
If a pane were to “go,” it would be the outer layer since it’s under the most pressure. If that were to happen, the middle layer would still hold and the small hole would still allow for pressure to change more slowly.
And as a bonus, the breather hole also helps keep the window free of fog by wicking any moisture that could get stuck between the panes. This flyer who loves to look out the window is especially appreciative of that. 🙂
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