Are you ever on the runway preparing to takeoff on a flight, and notice a tiny little hole near the bottom of the window? Your immediate reaction is, “We are about to be 39,000 feet in the air and there’s a hole in my window. What the frig?” Well that small hole is done on purpose and has an important safety feature so don’t freak the fuck out.
The airplane window actually consists of three different panes, typically made of acrylic materials. There is the innermost pane, often called the scratch pane, which stops passengers from busting the window out. There is also the middle pane and the outer pane.
The middle pane has a tiny hole in it called a breather hole or a bleed hole. These tiny holes are so important because as the plane ascends the air pressure drops and pushes on the plane. The cabin has a safe and comfortable air pressure thanks to the aircraft’s pressurization system, but that pressure from the outside is looking for a place to enter the airplane.
The breather hole allows pressure to enter the small air gap that sits between the middle and outer panes of glass. The outer pane takes most of the pressure, and the middle pane acts as a fail-safe in the unlikely event that the outer pane is destroyed. If the outer pane failed the bleed hole would allow a small leak of air through, but it is nothing the airplane’s pressurization system couldn’t easily deal with.
The breather hole also has another useful function; releasing moisture from the air gap to prevent windows from fogging up or frosting over. You may notice a small amount of frost or moisture near the hole, but you can still easily see out the window. That is because at the height of 40,00-feet the temperature drops to -70 degrees.
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