Why Are There Holes in Airplane Windows?

(Photo: Mark Vanhoenacker)

Why is there a hole in the airplane? This would normally be an alarming question. But we’re referring to the tiny hole in each window of a jetliner. It may be more noticeable when ice crystals form around them, which commonly happens. Mark Vanhoekacker of Slate explains that these holes are necessary to maintain proper cabin air pressure:

If you look closely at a typical passenger cabin window, you’ll see three panes, typically made of acrylic materials. The purpose of the innermost pane—sometimes called the scratch pane, but I like to call it the smudge pane—is merely to protect the next one.

The middle pane (with the breather hole in it) and the outer pane are more important. Generally speaking, as an aircraft climbs, the air pressure drops in both the cabin and the outside air—but it drops much more outside, as the aircraft’s pressurization system keeps the cabin pressure at a comfortable and safe level. This means that the pressure inside the aircraft during flight is typically much greater than the pressure outside.

The outer two cabin windows are designed to contain this difference in pressure between the cabin and the sky. Both the middle and the outer panes are strong enough to withstand the difference on their own, but under normal circumstances it’s the outer pane that bears this pressure—thanks to the breather hole. As Marlowe Moncur, director of technology 
for GKN Aerospace, a leading passenger cabin window manufacturer, put it to me via email: “[T]he purpose of the small bleed hole in the [middle] pane is to allow pressure to equilibrate between the passenger cabin and the air gap between the panes, so that the cabin pressure during flight is applied to only the outer pane.”

(Image: CBS)

So there’s nothing to be anxious about, Mr. Shatner. Please return to your seat.

-via Marilyn Terrell

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