Small hole in the windows of commercial aircraft?

Ever seen a hole in the window of a commercial aircraft similar to the one on the right? Ever wanted to know the reason why but forgot to search for it during the day afterwards?

There are actually a few layers in the window if you look closely. The 1st layer (outer most layer) is to keep the pressure inside the plane, 2nd and 3rd layers are protective layers called the fail safe pane vent. The protective layers are present to prevent the passengers from scratching the primary window. Even a small scratch could cause the window to crack due to the high pressure of the cabin.

The small hole has several functions. The first is to let moisture to escape from the layers in-between in case it gets trapped in there. If there is moisture present, it may cause fogging (like in your car) and due to the low temperatures while in the air, it may freeze! The 2nd reason is to avoid corrosion of the window in case moisture gets in. This is one of the major cause of plane accidents. The 3rd reason is used for pressure differential and the final reason is the function of the fail safe pane vent.

We shall discuss a little about pressure in the aircraft so you will have a better understanding on reason 3 and 4.

Pressure Changes as Altitude Increases/Decreses

As the aircraft increases or decreases in altitude, the atmospheric pressure outside changes (refer to picture above).

When the plane is at a high altitude, the pressure inside the cabin is much larger then the pressure outside. This is due to cabin pressurization. Cabin pressurization is the pumping of compressed air into the cabin to maintain a safe environment. We cannot use the thin air outside the aircraft to breath because we may develop hypoxia (deprivation of oxygen supply), altitude sickness, decompression sickness and numerous other physical problems.

The small hole (drilled in the plexiglass window) is there to equalize pressure between the inside of the cabin and the actual window which is the outer pane. It allows air in and out to minimize the pressure difference between the panes of glass. It is also called the fail safe pane vent because just incase the outer pane fails, the air between blows out and the inside pane keeps the cabin relatively sealed.