Why airplane windows have a so-called “drain hole” – and why you shouldn’t fall asleep on them

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Vacationers getting back on their feet after two-and-a-half years inside the COVID pandemic must reacquaint themselves with the vagaries of travel.

Some things can take some getting used to, and suddenly travelers notice small details that they may not have paid attention to before.

For example – what is that tiny little hole on commercial airliner windows, and why the hell is it called a “bleed hole”?

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The very name conjures up something rather creepy and, it turns out, it’s for a good reason (sorry, nervous flyers).

This small hole in the glass is called a “bleed hole”. Credit: Facebook/HYGO Hilarious

The hole helps regulate the pressure exerted from inside the cabin on the aircraft windows.

It ensures that if an airplane window is going to break (God help!), the outer pane goes first.

The bleed hole ensures that if an aircraft window is going to break, the outer pane goes first. Credit: Facebook/HYGO Hilarious

Most commercial aircraft windows have outer, center and inner panes, all of which are usually acrylic.

“The outer glass takes all the stress of cabin pressurization,” tech writer Robbie Gonzalez explained in an article for Gizmodo.

The inner glass is designed to maintain cabin pressure in the “extremely rare” event that the outer glass is fractured.

That’s why it’s recommended that passengers don’t rest their heads on that little bubble-shaped hole, because you’re effectively preventing it from doing its job.

It is not recommended to sleep with your head resting against the “drain hole”. File picture. Credit: Leren Lu/Getty Images

“The purpose of the breather hole, which is located near the bottom of the center window, becomes clear: it serves as a bleed valve, allowing the pressure between the air in the passenger cabin and the air between the outer and central to balancing each other,” explained Gonzalez.

“This tiny little hole ensures that cabin pressure during flight is applied only to the outer pane…thus preserving the center pane for emergency situations.”

worst case scenario

The outer, middle and inner pane system, in combination with the bleed hole, works as a sort of worst-case failsafe, and damage to the middle pane can literally bring a flight to a standstill.

If all three panes were sealed without the hole, cabin pressure would act on the inner pane.

If cabin pressure caused the outer pane to burst, the inner pane is still strong enough to hold the pressure, giving pilots time to descend to lower altitudes and decrease cabin pressure.

‘I never noticed’

When all of this was explained recently on a Facebook thread, users were amazed that such a small detail is so important to the structural integrity of the aircraft.

“The way I never noticed but I’ll watch when we fly,” one person wrote.

“Oooh, that makes sense now,” wrote another.

You could pay more attention to the bleed hole from now on. Credit: Facebook/HYGO Hilarious

So, travelers, if you’re packing your support pillow for that long-haul flight, the advice is pretty clear.

Don’t use the window as a sleeping support – technically it’s safer for you to sleep on the shoulder of the passenger sitting next to you.

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