Air pressuration/depressuration and how is it done?

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Watching this YouTube video [https://youtu.be/-L7o6HtX8Vg?t=157](https://youtu.be/-L7o6HtX8Vg?t=157) where a navy seal breaks down movie scenes, and he mentioned that before the guys perform a HALO jump cabin has to be depressurized so that they don’t breathe in thin air. But doesn’t the cabin already have air?

And then why are cabins pressurized in the first place, and how is that done?

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Think of it as a certain mass of air packed into a certain volume (in this case, the volume being an aircraft fuselage), the more air per unit of volume, the higher the pressure. On an aircraft this is done through the AC system, a vent usually near one of the engines draws in air, runs it through a compressor and over a cooling unit and condenser of sorts to get rid of the moisture, and then into the cabin (or in the case of most turbofans the air is taken directly from one of the compressor stages, I think 2nd or 3rd, but I’m not a fan tech). The air can be heated by running it through a radiator warmed by the exhaust, but that’s not entirely necessary all the time. The more air you compress and put into the aircraft, the higher the pressure.

Now the higher up in the atmosphere you go, the less air there is, and the lower the pressure. But at those altitudes a human would very quickly suffer and die from hypoxia among other things, so the trick is to take more air in, and cram it into the fuselage so you can bring the interior of the aircraft to an altitude equivalent of something survivable, or hopefully comfortable (anyone that’s flown on something like a beech 1900 knows that’s not always the case lol). Now, with all the extra air in the cabin, if they simply opened the back gate it would all rush out at once and cause a **lot** of problems for the aircraft, all that stress on the structure suddenly leaving would damage some major components, so they bleed it back out usually into the engine or through a bleed valve at the back of the aircraft over a relatively short period of time (usually around a 3000f/m equivalent maximum).

If you have a pressurized aircraft the pressure in it is higher than on the outside. If you just open it there would be a sudden high-speed stream of air out, you depressurize the aircraft in a controlled day. A sudden decompression is called explosive decompression.

The reason you pressurize aircraft is that air pressure drops with altitude and if you go high enough the pressure is so low that you do not breathe in enough oxygen. The simple way for people in the aircraft to solve it is to keep the pressure in the aircraft higher than the outside.

There is another way if the air pressure is not to low. That is to breathe pure oxygen or just a gas mixture with higher oxygen content than regual air. So when you do a HALO jump you have tubes with oxygen so you can breathe. The crew of the aircraft needs to have a mask on too that provides enough oxygen.

This is only possible to a point because when the pressure get to low breathing pure oxygen do not provide enough oxygen for you to survive, At that point, you need pressure suits or pressurised aircraft to stay alive,