Aircraft Pressurization System: A Complete Guide

The only way that you’re able to breathe at the extreme altitudes that aircraft fly at is because of the pressurization system. Here’s how it all works.

Aircraft pressurization systems are designed to keep the air inside the plane’s cabin fresh, clean, and breathable. At higher altitudes, the air is too thin to breathe and supply enough oxygen to the brain. Aircraft pressurization systems ensure there is enough oxygen in the air to breathe properly.

If you’re reading this article, chances are high that you’ve flown on a plane before. You might even be a pilot (or thinking about becoming one). In either case, you might be wondering how it’s possible to breathe while flying since the air is so thin at high altitudes. Well, the answer to that question is aircraft pressurization. In this article, you’ll learn how aircraft pressurization works, why it’s necessary, how planes can handle it, and any possible side effects.

As you’re browsing the web trying to learn everything you can about aviation and flight, you’ve come to the right place. At SkyTough, our number one priority is providing our readers with the best content that they can find on the web. To make that a reality, we’ve combined our own knowledge and experience with that of other experts in the field to really narrow down all the intricacies of aircraft pressurization systems.

‍What Is Cabin Pressure In An Aircraft?

Air pressure is something that the vast majority of us likely take for granted on a daily basis. Here on the planet’s surface, air pressure is a comfortable 14.7 psi at sea level. At this pressure, we don’t really feel any actual pressure on us, it’s easy to breathe, all seems well. But as we take aircraft tens of thousands of feet into the air, the air pressure changes drastically.

The higher you go into the air, the lower the pressure is. And with lower air pressure, things start to get a little difficult for us humans. Things that we don’t really think about — i.e., breathing and staying conscious. We’ll get more into the reasons why planes need to be pressurized shortly, but here’s the general gist.

The goal of aircraft pressurization systems is twofold. First, as mentioned above, humans need to have higher air pressure than what’s outside at high altitudes in order to breathe. Secondly, if there was no air circulation, the air inside an aircraft would get stale and dirty very quickly.

With aircraft pressurization systems, we’re able to provide enough air pressure to keep us humans breathing properly while simultaneously exchanging the air in the cabin every two to three minutes. This makes aircraft air incredibly clean and fresh, far more so than what you breathe in on a daily basis at home or in the office.

So how do these aircraft pressurization systems work?

How Does Aircraft Pressurization Work?

Although aircraft pressurization system is a mouthful on its own and it sounds difficult, it’s actually one of the simplest systems on an aircraft. Most of it is done automatically, and the components keep the aircraft pressurized pretty much on their own, without any input from the pilot or anything like that.

There are two major things needed for an aircraft pressurization system to work— fresh (pressurized) air coming in and old stale air going out. In most modern aircraft, the pressurized air comes from bleed air from the airplane’s engines. This is excess air from the engines that is pressurized, cooled, and redirected into the cabin to keep pressure.

While most aircraft use engine bleed air to keep the cabin pressurized, some new airplanes use electric air compressors. These compressors are powered by the plane’s electric system, and pump fresh outside air into the cabin of the plane. But no matter how the plane gets fresh air in, it needs to be able to get rid of the old air in the cabin.

To do this, planes have something called an outflow valve. This valve is a motorized door that’s typically mounted near the rear of the aircraft. Some planes will have two outflow valves depending on the size of the plane, and it is the key component to keeping the cabin pressurized during flight.

The outflow valve is used to modulate pressure in the cabin to keep it at the necessary levels for human comfort while also exchanging the air in the cabin regularly. If pressure is too low, the outflow valve will close to allow pressure to build. When the pressure gets too high, the outflow valve opens, and the excess pressure vents outside of the plane.

It’s a really simple system that keeps the cabin pressurized with fresh, clean air at all times!

Why Do Planes Need To Be Pressurized?

The cabins inside of an airplane need to be pressurized to enable passengers and crew to breathe. This is because at higher altitudes, air molecules are further apart from each other than they are at lower altitudes. If you’ve ever heard of the air getting thin as you go higher, this is what that means. The air literally does get thinner as the molecules spread across.

For humans, this means that we take in less air when we breathe, meaning we get less oxygen than normal. And if there isn’t enough oxygen being supplied to the brain, the brain will begin to shut down and eventually fail. With how high planes fly, the air is so thin that you would likely remain conscious for less than a minute.

This is why planes need to be pressurized, to ensure that the air is dense enough (i.e. there’s enough air pressure) to provide adequate oxygen to the brain. That’s also why if the plane depressurizes during flight, oxygen masks will drop down. It’s all about breathing, oxygen, and ensuring your brain gets what it needs to function!

How Does The Pilot Pressurize An Airplane?

This is where the magic of machinery comes into play, making pressurization of the aircraft incredibly simple for the pilot. In most aircraft, all the pilot needs to do is set the altitude of the airport that they’ll be landing at and the airplane’s electromechanical systems will take care of the rest.

The outflow valve will work in conjunction with the bleed air (or electric compressors) to keep the aircraft pressurized automatically. Along with the valves and door you’ll read about shortly, the entire process is automatic so the pilot doesn’t have to constantly worry about cabin pressure!

How Does A Plane Stand Up To Pressurization?

When you think about what’s really happening when an aircraft is pressurized, it might make you wonder about how the fuselage can stand up to this extreme difference in pressure. With the cabin being pressurized and the air outside being so thin, the fuselage wants to pretty much blow apart due to the significantly higher pressure inside. Or crumble in on itself if the pressurization system fails and the pressure outside is too great.

How do planes handle this to be able to stay together and keep everything onboard safe?

There are three main features that aircraft designers have included on planes to ensure that they can stand up to the pressurization of the cabin. Let’s take a brief look at each one.

Positive Pressure Relief Valves

The fuselage of an aircraft is only designed to handle so much pressure in either direction. As the aircraft is being pressurized, it’s important that the pressure inside does not build up too high. If there is too much positive pressure inside the cabin, there’s the risk of the structure failing, including windows and doors being blown out.

To account for this, planes have positive pressure relief valves installed on the fuselage. These valves are spring-loaded and will open automatically if the pressure inside the cabin gets too high. Through these butterfly valves, the excess air pressure is vented to the outside where pressure is much lower. Once the pressure inside gets back down to where it should be, there won’t be enough pressure to force the valves open and they’ll automatically close and seal.

Negative Pressure Relief Doors

If there is negative pressure within the cabin, that means that the pressure inside is lower than the pressure outside, which is not ideal. This will typically only occur during rapid descent or if the pressurization system fails. If the pressure outside the plane is greater than inside, this can be bad because windows, doors, and the fuselage itself are not really designed to handle this direction of pressure.

To help with this, airplanes have negative pressure relief doors installed (typically on the side of the plane’s fuselage). These simple, spring-loaded doors will open inward if the pressure outside is greater than inside to allow higher pressure air to flow into the cabin and equalize with the outside air. Once the pressure is even (or close to even) the springs will force the doors closed again.

Rounded Edges And Corners

This is a big one that you may not think of right away. Think about the fuselage of an aircraft. Everything is… round, isn’t it? The cabin itself is more or less tubular. The doors that you enter and exit through all have curved edges and corners. Even the windows, they’re not square like the windows in your house, right?

That’s done for a reason. Corners and edges create very small areas of high stress density. And with the extreme pressure differentials and high speeds that airplanes fly at, these areas of stress concentration could lead to cracks and premature failure. Simply rounding off all edges and corners greatly reduces the chance of something like this happening, down to nearly zero.

Are There Any Effects Of Flying In A Pressurized Cabin?

As a passenger on an airplane, aircraft pressurization seems vital, and that’s of course because it is. But it’s also not a natural phenomenon, having pressurized air that we can breathe at those heights. Some people fear that there might be some adverse effects of flying in airplanes due to this cabin pressure. So are there any effects on the human body of flying with aircraft pressurization?

Actually, there are a couple of things you might notice when flying at altitude in a pressurized cabin. But thankfully, the potential effects aren’t anything that’s exactly bad for the human body, more of just a bit of an irritant than anything. The two main side effects on the human body both stem from the same thing — the air inside a pressurized cabin has very low humidity.

So what does this low humidity have to do with the human body?

Since the pressurized air has such low humidity, you’re more likely to be less hydrated than you would normally be. This is one of the reasons that flight crews and airlines offer beverage service, and also why you should accept that bottle of water to make sure you stay hydrated! If you decide to drink alcohol while flying, the effects of dehydration will be further compounded. So be sure to drink plenty of water with any alcohol while flying!

Secondly, you might notice that the food you’re eating tastes a little more bland than normal. While you may attribute this to the food that they serve on airlines, you’ll also notice it even if you bring your own food onboard. This is because the low humidity can reduce your taste and smell significantly, causing the food to taste bland.

As you can see, these side effects are nothing to really worry about. Your taste and smell will be back to normal as soon as you get back to normal air. And the sense of dehydration will wear off quickly as well, and not really put you in any danger of total dehydration. So don’t worry about any potential adverse effects of flying in a pressurized cabin — there’s nothing to worry about!