Without it, you would still be able to fly, however, the planes wouldn’t be allowed to go any higher than 15,000 feet without everyone on board wearing an oxygen mask. The reason for this is, the air is so thin as you get higher, that there is simply not enough for your body to take in and allow you to breathe. For this reason, the aircraft is pressurized with air, so that there is enough of it for you to consume, and allow you to breathe. To understand pressurization, you first have to understand the engine(s).
A jet engine is actually a pretty simple concept. It has four stages.
i. Intake- the engine has a big fan in the front of it that will turn at very fast speeds, this fan is simply sucking air into the engine.
ii. Compression – After the air is sucked into the engine, it goes through a series of compressor blades that will essentially take all of that incoming air, and squeeze it together to make it denser.
iii. Ignite – That compressed air then has fuel squirted into it, after this, it will then enter what’s known as the “Hot Part” of the engine. This is where the magic happens, that compressed air with fuel in it, is then ignited to create thrust.
iv. Exhaust – All of that hot air that is created from this air being ignited is quickly jetted out the back of the engine, this in turn, pushes the plane forward.
Now that you understand how the engine works, I can explain pressurization a little bit further. Stage two of what was just previously discussed, the “Compression Stage” is used to make the air denser for the fuel to be injected in to. Well prior to that fuel being injected into that dense air, little valves called “Bleed Air Valves”. These valves allow some of that compressed air to leave the engine, and come into the cabin, allowing their to be more air for a person to breathe. Now despite what some may say, the air IS recirculated. There is a valve in the cabin of the plane called the “Outflow Valve” that allows some of that air to leave the cabin, for if it didn’t, the cabin of the plane would be much like a balloon, and just keep filling, and filling, and filling… We don’t want that. So the outflow valve will constantly regulate the cabin, to an altitude that would allow a person to breathe. For example, if the plane is flying at 35,000 feet, your body is at 35,000 feet as well, however, there is enough dense air inside of the cabin that the inside of the cabin is much like you’re in Colorado, or at 5,000 to 6,000ft.
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