What is the function of that small hole?

Those who love window seats on airplanes are surely no strangers to the small hole at the bottom of the window – a tiny mystery that plays an indispensable role in the engineering design of aircraft.

Any hole on a commercial aircraft would normally cause passengers great concern and fear for their safety. However, there is indeed a small hole in the airplane window. So, what is the purpose of this hole?

A small hole with a big role

Sitting by the airplane window to admire the sky is a favorite of many passengers. If you’ve ever flown, you might have noticed a tiny hole located near the bottom edge of the window. Some people worry that this hole could lead to pressure loss when the aircraft reaches an altitude above 10,000 meters, potentially causing accidents. In fact, this small hole serves an important function. Though seemingly simple, it helps balance the pressure inside the aircraft.

Passengers sitting next to airplane windows are often curious about this little hole. It’s not a random design element—these tiny holes are the result of strict physical and safety principles. In the field of aeronautical science, these holes are known as bleed holes or breather holes, and they are located in the middle pane of the window.

To understand this better, it helps to know the structure of an airplane window. It consists of three layers: the outermost pane withstands external pressure, the middle pane has the small hole, and the innermost pane—made of acrylic—protects passengers. The purpose of the inner acrylic pane is to protect the outer two layers.