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, 11,000 metres (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.
According to Mark Vanhoenacker, 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 Wi-Fi).
To make you a happy little traveller, 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 (unless you’re a mountain climber), 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 (14.6 pounds per square inch) 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 (3.4 pounds per square inch).
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.
"[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," Marlowe Moncur, director of technology at GKN Aerospace – a passenger window manufacturing company – explained to Slate.
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 outside. Sure, it'd have a small hole in it, but that's nothing the plane's pressurisation system couldn't compensate for.
While this tiny hole plays an important role in keeping us safe, it also helps keep the window panes from fogging up – a result of the temperature difference between the inside and outside of the cabin – allowing us to stare out into the clouds.
Thank you, little window hole. Who knew something so simply could be so important?