Plane porthole: what is the little hole for?

If you have already occupied a seat on the porthole on a plane, you have probably noticed the small hole at the bottom of each porthole. It is not a manufacturing defect. It has a determining function relating to the pressure in the cabin.

To start, you should know that most portholes consist of three thicknesses. The inner layer is a simple plexiglass covering preventing passengers from contacting the external windows. “Since the outside air can go down to -60 ° C to 11 or 12 km above sea level, there is a risk of catching frostbite by touching the windows,” explains Heinrich Grossbongardt, aviation expert, at the German travel site “Travelbook”.

The external glass is thicker and is the only one attached to the device fuselage. This means that it must withstand the cabin pressure during the flight. Depending on the flight altitude, the pressure inside the cabin is indeed significantly higher than outside the fuselage.

Between the outer window and the central glass is a air chamber. “Since during a flight, the air pressure in the cabin is not constant, it must be compensated in the air chamber,” explains Heinrich Grossbongardt.

And this is where the little hole in the porthole comes into play. “It allows the air to go out as a climb and go downhill”. It is therefore a kind of ventilation valve which maintains the balance of the air pressure between the windows and ensures the relatively constant maintenance of the pressure in the cabin. It avoids, moreover, the windows to get buried and allows passengers to have a clear view of the outside.