Why Do Airplanes Have Those Tiny HOLES In Their Windows?

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Image: via Slate

When you take a flight, among the first questions that pop into your head is whether to go for a window or an aisle seat. Some – particularly those with bladders that apparently shrink as the cabin pressure rises – insist on the latter so that they can quickly escape when nature calls.

Conversely, the curious traveler will often opt for a window seat – partly to take in fluffy clouds and meandering rivers and roads, but also so that they can fall asleep against a solid surface rather than a stranger.

If you’re a regular window seater, then you’ve probably spotted the small hole at the base of the pane itself – and quite possibly the small specks of frost that sometimes form around it.

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This tiny hole raises some questions. For example, why is it there? Could it potentially suck us out? And the presence of frost – that can’t be a good thing, surely?

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It’s then that a passenger’s confidence in their adjacent plastic pane may start to wane. Will this window onto the world below really keep the outside out and the inside in?

A cool head prevails, though, when you remember that you’re far less likely to meet your end in a plane crash than a car accident. The respective odds, according to Discovery, are as far apart as one in 11 million and one in 5,000.

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Ice around a plane window's hole
Image: Leena J

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And rather than the tiny hole in the window – known as the “breather hole” – being a cause for concern, it’s actually, probably unsurprisingly, there to keep passengers and crew safe.

In an email to online magazine Slate, GKN Aerospace technology director Marlowe Moncur revealed that the hole’s job is to “allow pressure to equilibrate between the passenger cabin and the air gap between the panes.” This in turn enables the cabin pressure to be applied to the outside-facing pane only.

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Thanks to this “two-pane air-gap design” the window is able to withstand considerable fluctuations in atmospheric pressure, effectively preventing it from breaking and passengers being thrown out into the abyss below. Phew.

These changes in pressure shouldn’t be underestimated. At cruising altitude the pressure falls so low – 3.4 pounds per square inch, in fact – that a person wouldn’t be able to survive it.

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For this reason, cabin pressure is kept at approximately 11 pounds per square inch for the duration of the flight, and it’s because the windows are designed so cleverly that they’re able to withstand the disparity.

Crystals like cracks on a window
Image: Thu Le

There are, in fact, three panes inside plane windows. The outer and middle ones effectively contain the variance in pressure between the cabin and the sky outside, with the breather hole ensuring that only the outer pane withstands the pressure.

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And while the outer two panes are part of an airplane’s structure, the passenger-facing one – which is simply fitted to the cabin’s lining – isn’t, which is why you might sometimes hear it rattle.

Because, then, there are effectively a pair of panes on the outside – one facing outside and another in between this and the inner pane – the window is actually pretty safe.

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In the extremely unlikely event of the outer pane breaking, for example, the middle one can act as a backup – even with its breather hole.

Strange looking ice on a plane window
Image: Matthew

Yes, while there would inevitably be a small pressure leak, the hole is so small that any effect on cabin pressure would be negligible.

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What’s more, even then the inner pane would only become slightly more important, for its main function is really to shield the middle one from scratches and fissures – and, of course, from passengers’ grubby mitts and greasy foreheads. It’s basically there to take one for the team.

Sunlight catches a plane window
Image: Ag gy

Interestingly, Boeing aerospace engineer Bret Jensen told Slate that the breather hole has an additional role to play: to prevent the majority of frost and fog from collecting inside the window. How so? By releasing any unwanted moisture.

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And while the formation of frost and fog can’t be completely prevented, some quite pretty formations can occur when humid cabin air passes through the hole and meets the freezing outer pane.

Mid-flight photography, through the window
Image: duncan c

Next time you fly, then, you might just view that little hole in the window, and the flimsy looking pane that sits in front of it, a little differently. They’re far more sophisticated than they look.

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