July 6, 2025, 6:10 am | Read time: 2 minutes
If you’ve ever had a window seat on a plane, you might have noticed a tiny hole at the bottom of each windowpane. But what’s the purpose of it? TRAVELBOOK explains why these holes are essential in two ways.
Airplanes are among the most complex inventions of humanity. Every tiny detail in the construction of a machine must be correct to ensure passengers travel safely from point A to point B. This includes seemingly trivial things like the windows–TRAVELBOOK has already explored the question of why their shape must never be square.
Just as important as the shape is the quality and composition of the windows. After all, even a layperson can guess that you can’t simply insert a plain glass pane into the openings, as it would shatter under pressure. When the plane ascends, the air pressure around it decreases.
Airplane Windows Consist of 3 Panes
The windows in most passenger airplanes are composed of three panes. The inner pane, which passengers see and can touch, is actually just a Plexiglas cover designed to prevent direct contact with the outer panes, as aviation expert Heinrich Grossbongardt explains to TRAVELBOOK. “This is necessary because the outside air at an altitude of eleven or twelve kilometers can be as cold as minus 60 degrees, and touching the actual panes could cause frostbite.”
Also interesting: Why the Lights Are Turned Off During Takeoff
But what is the function of the small hole that connects the panes of the airplane window? For one, it prevents the panes from fogging up, ensuring passengers have a clear view outside. More importantly, the tiny openings serve a second purpose related to pressure.
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Like a Vent Valve
Since only the outermost, thickest pane is connected to the aircraft’s fuselage, it must withstand the entire cabin pressure. Between the outer and middle panes is an air-filled cavity. “Because the air pressure in the cabin is not constant during flight, there must be a pressure equalization for the cavity,” explains aviation expert Grossbongardt. And that’s exactly what the small hole in the airplane window is for: “It ensures that air can flow out during ascent and back in during descent.”
The hole in the airplane window thus serves as a kind of vent valve. It keeps the air pressure between the panes balanced and ensures that the pressure inside the cabin remains relatively stable.