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Why there’s a tiny hole in airplane windows?

You don’t have to be an aeronautical engineer to know that an airplane cabin is pressurised to keep us from passing out as we soar through the skies, 36,000 feet above sea level. It also stands to reason that – to keep that pressure contained – the
cabin can’t have any holes in it. So why, you might be wondering, is
there a scary-looking small hole in every airplane window? Well, in
short, despite what it seems, it’s there for your safety.
A British Airways pilot who reports about aviation for Slate, these
tiny window holes are actually called ‘bleed holes’, and they’re wedged
between two other panes of acrylic material, meaning that when you look
through an airplane window, you’re actually peering through three
different panes.
The first of these panes – the one you can touch and dirty up with
fingerprints – is called a scratch pane. The middle pane is the one with
the bleed hole, and the whole system is completed by the outer pane –
the most important one, since it protects you from the pressure
difference outside.
While both the outer and middle panes have the power to resist the pressure from outside the plane, the brunt of the work falls on the outer window because it’s the final barrier between you and the clouds.
Before we go into how these windows work, though, it’s important to
understand a bit about how air pressure changes when you’re flying. If
everything is working properly, you shouldn’t even notice the pressure
dropping outside your window as you sit there watching a movie or
catching up on your social media feed (if you’re lucky enough to have
The cabin of an airplane
creates pressure by pumping in conditioned air that mimics the air
pressure you’re used to feeling on the ground. By doing so, you can
survive the trip without passing out or suffering from hypoxia – a condition with fatal complications that arises from a lack of oxygen. But, as you reach higher and higher elevations in your comfortably
pressurised cabin, the air outside the plane becomes thinner, containing
less oxygen and pressure.
It’s kind of hard to imagine this shift in pressure, but using numbers seems to make it a bit easier. According to the American Vacuum Society,
you experience roughly 1.0 kilogram per square centimetre of pressure at sea level. This is what we’re used to
feeling and breathing. It’s pretty comfortable.
When you get in an airplane and climb all the way up to a cruising
altitude of 10,679 metres (35,000 feet), this pressure drops down to a
mere 0.2 kilogram per square centimetre. This means that while you’re airborne there’s a giant different
between the pressure inside the cabin of a plane and that outside of the
plane – and all the air inside the plane desperately wants to get
outside to fix the imbalance.
So what does all this have to do with the small scary-looking hole in
the window? Well, that hole actually reduces the pressure on the middle
pane, so only the outer pane takes the force of the cabin pressure –
and it experiences that pressure more gradually during flight.
So that means that if the outer pane somehow was broken by debris, we’d
still have the middle pane to protect us from the lack of air pressure
Source: Slate