Have you noticed that your airplane cabin windows have a small hole towards the bottom of the window panes?
This tiny hole is called a “Bleed Hole”, or a “Breather Hole”.
Airplane Cabin Windows, more than a view
A pressurized airplane enables you to breathe and stay in a comfortable temperature setting when flying.
To install windows in a pressurized airplane the windows need to be strong.
Airplane cabin windows consist of three layers of window, designed to equalize the low outside pressure of the airplane with the high inside cabin pressure of the airplane.
There are 3 passenger cabin window layers
3 layers are necessary to make this system work.
It also serves as a fail-safe structural construction.
If the outer layer window pane fails, the middle window pane takes over.
1. The outer window pane (closest to the outside)
.47 inches (12mm) thick, this window pane is the thickest one.
It seals the cabin completely from the outside air, and it is also the most structurally sound window pane.
2. The middle window pane
.23 inches (6mm) thick, this is the window pane with the bleed hole.
The hole balances the pressure which exists between the cabin and the air gap between the outer and middle window panes.
3. The inner window pane (closest to you, the passenger)
.16 inches (4mm) thick, this one is an insulating barrier.
It is also called the “scratch pane”, since it protects the middle pane from scratching.
This pane acts as an insulating barrier by protecting us from the cold outside air temperatures at cruising altitude.
The outside temperature at cruise is typically around minus 76 Fahrenheit (-60 degrees Celcius).
The cabin window hole serves two different purposes.
The hole allows pressure to equalize between the cabin and the the air gap that exists between the window panes.
Therefore, and thanks to the hole, the in-flight cabin pressure is mostly “pushing” on the thickest outer pane. Remember, the inside cabin air pressure is much higher than the pressure of the outside air.
The purpose is to release moisture from the air gap between the window panes.
This prevents fog and frost from excessively forming on the window panes.
The frost and fog you sometimes see is from water condensation. This as a result of the cold outside air continuously hitting the cold surface of the window.
Why is the frost pattern round?
It’s from a mixture of the window surface temperature, cabin air humidity, and the rate of airflow through the bleed hole.
But doesn't that hole affect cabin pressurization?
No. That tiny air leak going through that window hole does not affect cabin pressurization.
Cabin air does not leave the airplane through that hole.
The air is ultimately kept inside the structure of the airplane, kept secure by the outer window pane.
What if we did not have that small hole in the cabin window?
If we did not have that tiny hole to equalize the air pressure: the difference of air pressure between the window panes would become so great and unequal that the window pane may bust!