It is known as the “breathing hole” or “vent hole” and is among the important features of airplanes.
Some people worry about whether this hole is risky, but on the contrary, it is an engineering idea that avoids risks.
Aircraft windows consist of three layers: inner, middle and outer.
These layers are often made of acrylic material. The innermost layer protects the next layer and the passengers; It is a plastic cover. The “breathing hole” is located in the middle layer. The outer layer is the last layer to cope with the harsh weather conditions outside the aircraft.
The pressure in the cabin of a standard passenger aircraft is the same as at 2,500 meters above sea level (This altitude is up to 1,800 meters on advanced Boeing 787 and Airbus A350 aircraft).
As the plane gains altitude, the pressure outside and in the cabin begins to decrease. This is the reason for that strange feeling in our ears, we feel the effect of our inner pressure. The pressure outside is of course much lower than the pressure in the cabin. Here, the 2nd and 3rd layers in the airplane windows are in the interface between this pressure. It is designed durable enough to resist this strong difference.
Let’s come to the important function of the small hole:
This hole balances the pressure in the cabin with the pressure between the 2nd and 3rd layers. The main pressure is supported by the outer layer, so the middle layer is not loaded too much. Thanks to this tiny hole, this is possible.
When the plane rises to normal cruising altitude, ice crystals begin to form around this hole.
This is because the warm air in the cabin comes into contact with the cold window surface. The moisture accumulated between these two layers is discharged from the hole and it is ensured that the windows do not fog up and do not freeze. Otherwise, it would not be possible to watch those magnificent cloudscapes.
But what happens if it cracks and air gets inside?
In such a scenario, the pressure balancing system on the aircraft will overcome this problem as the amount of air leaking from this hole will be very low.
In summary; This small hole, which seems to be of little importance, is actually one of the indispensable features of airplanes.
Sources: 1, 2
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