Fans of window seats on airplanes are likely familiar with the small hole located at the bottom of the window – a tiny mystery that plays an essential role in the technical design of aircraft.
Any hole on a commercial airplane could cause passengers to feel anxious about their safety. However, there is a small hole in airplane windows. What is the purpose of this hole?
The Small, Confusing Hole in Airplane Windows
A Small Hole with a Big Role
Sitting next to the airplane window to gaze at the sky is a favorite pastime for many travelers. If you’ve flown before, you may have noticed a small hole located at the bottom edge of the window. Many people worry that this hole could lead to a loss of pressure when the airplane reaches altitudes above 10,000 meters, resulting in an accident. In reality, this tiny hole plays a crucial role. This seemingly simple detail serves to balance the pressure inside the aircraft.
Passengers sitting by the airplane window often wonder about the small hole in the window. It is not merely a design feature; these holes are the result of strict physical principles and safety regulations. In aviation science, these small holes are known as breather holes or bleed holes, located in the second layer of glass.
To understand better, it is essential to know about the construction of airplane windows. The window consists of three layers of glass: the outermost layer which withstands external pressure, the middle layer with the small hole, and the innermost layer made of acrylic to protect the passengers. The purpose of the inner acrylic layer is to safeguard the two outer layers.
The small hole plays an important role in airplane windows. (Photo: BI)
The small hole, referred to as “vent hole” or “pressure equalization hole”, allows air from the cabin to pass through, balancing the pressure between the layers of glass, ensuring that only the outermost layer is directly affected.
As the airplane ascends higher, air pressure decreases. The outside air pressure is much lower than that inside the aircraft, which can cause harmful physical effects on the airplane windows. The internal pressure balancing system will regulate the pressure inside the aircraft to a safe and comfortable level for passengers.
The acrylic layers in the middle and outermost parts of the airplane window must withstand the forces caused by pressure differences. Both of these layers can handle the pressure force, but thanks to the small hole in the window, only the outermost layer bears the load.
An Important Factor
Marlowe Moncur, the CTO at GKN Aerospace, states: “The purpose of the small breather hole on the middle panel is to balance the pressure between the passenger cabin and the space between the acrylic panels, so the pressure from the passenger cabin will only affect the outermost panel.”
In the rare event that the outer acrylic panel cannot withstand the pressure (which is almost never the case) and cracks, the middle panel will act as a backup for the outer panel. Naturally, the small hole in the middle panel will allow a small stream of air to pass through, but the pressure balancing system will handle the remaining issues.
In the uncommon case of one of the glass panels breaking, the backup structure of the glass panels ensures that pressure does not compromise the window’s integrity. The remaining panels can still withstand the pressure, ensuring passenger safety.
The hole may seem simple, but it is not. (Photo: BI)
Bret Jensen, a senior aerospace engineer at Boeing, shares: “This hole helps prevent moisture and snow from accumulating on the window. This explains why your window does not fog up every time the airplane passes through clouds.”
On long flights, a thin layer of snow can accumulate around the breather hole, as many flights may reach altitudes where outside air can drop to -57 degrees Celsius.
Video explaining why there is always a small hole in airplane windows.
These small holes in airplane windows, though tiny, play an indispensable role in ensuring safety and comfort for passengers in flight.
