The engineers who design aircraft account for all those physics-related questions to make planes a very safe form of transportation. That’s why you might notice a tiny little hole in every airplane window. That hole, known as a bleed hole, is a crucial element in the structure because it helps to regulate air pressure.
Air pressure and the amount of oxygen in the air decrease the higher you go above sea level. And low air pressure and limited oxygen aren’t ideal for humans. That’s why airplanes are pressurized — to keep us alive and comfortable throughout the journey.
Airplanes have been meticulously designed and extensively tested over the decades, and rapid decompression resulting from a structural failure is a rare event thanks to safety features like airplane window design.
Perhaps unsurprisingly, windows are something of a weak spot on airplanes — they’re made of acrylic, which isn’t as strong as the various metals that typically make up the rest of the fuselage. (Though these days, some fuselages are made out of composites like carbon fiber.) Passengers probably wouldn’t enjoy flying in airplanes without windows, and having an outside view in an emergency situation is crucial for evacuations, so rather than eliminating windows entirely, windows are designed to be as strong as possible.
That starts with the shape of the windows: Airplane windows are round because pressure is more evenly dispersed over a rounded shape. Then there’s the bleed hole, which is designed to help alleviate some of the pressure exerted on the window.
“An airplane window actually has three panels: an outer pane to deal with the air pressure difference; a middle pane with the bleed hole, the tiny hole you see, which helps balance the air pressure; and a thin inner pane, also called a scratch pane, which helps protect the middle and outer pane from damage from airplane cabin activities,” says the Airbus spokesperson. The bleed hole helps to balance the air pressure between the outer and middle panes.
And if the inner pane, or scratch pane, were to break, absolutely nothing would happen at all, since it’s just a scratch protector and not a structural element. ”
There’s one more reason for the bleed hole — it releases moisture from the space between the panes, preventing fogging or frosting. So the next time you’re up in the air taking photos out the plane window, you’ve got that little bleed hole to thank for keeping the acrylic as clear as possible and for keeping you safe and sound.
