Oxygen masks will drop from ceiling during emergencies, how about during regular flights?
In: 9
Isn’t that what tiny holes in the windows are for?
Pressurized air taken from the high pressure stage of engines is often used to provide cabin air along with other situations where forced air is needed.
Some aircraft such as the Boeing 787 just use electrically driven compressors to pressurize air for the cabin
Airplane cabins are pressurized to the equivalent of 8,000 feet or less by diverting high-pressure air from the jet engines into the cabin, which is a **mostly* airtight pressure vessel. The masks only drop in the event that the cabin pressurization system fails, ~~most likely~~ **possibly* due to a leak in the hull.
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Commercial aircraft are pressurized meaning that they suck in outside air and force it into the cabin to maintain air pressure.
Aircraft are limited to flying at around 10,000 ft without pressurization because above that the air pressure is so low that the average human will have trouble breathing.
The problem though is that you want to fly higher because that same thin air means you can travel faster and use less fuel.
Fighter jets get around this by making crews were masks that give them air to breath.
While this is impractical for commercial planes, so instead they pressurize the entire cabin.
The masks that drop during an emergency are their to help people breath if they lose cabin pressure at altitude, at least until the plane can descend to 10,000ft where the passengers can breath outside air.
Premise: Turbine engines work by compressing air with a series of “fans” one after the other, then the air is burned with fuel, the gas expands and turns the turbine section that spins the compressor that I mentioned above. The remaining energy propels the plane.
Pressurization: given the plane has already the big compressor that is part of each jet engine, there is a system that takes part of this clean compressed air, “steal” it from the engine. This compressed air is pretty hot due to the compression work, anything between 100 and 250 Celsius. This hot compressed air is sent to an -air cycle machine- which is, simply put, a massive heat exchanger with turbines and fans in it, capable of decompressing and cool that air. By regulating that thing, you can transform the compressed air stolen from the engine into -compressed air the temperature you like-.
The -compressed air at the temperature you like- is then routed to the cabin.
There is a valve (outflow valve) that regulates how much air can be expelled by the cabin. By adjusting this valve, you adjust the pressure in the cabin.
Recap: engine compresses air, air goes to “air cycle machine”, then goes to the cabin and pressurize the cabin, the outflow valve expel the excess of pressure from the cabin.
Result: the inside of the plane is always kept at a nice temperature and at a pressure that is ok for people to live in. Also, the massive flow of air in/air out guarantees you are breathing new air and not other people’s breath and farts.
Safety: system uses air from the two engines, plus eventually the APU engine (small backup engine, too long to explain here). There are generally at least two air cycle machine, and the outflow is controlled in 4 different and independent ways. As long as you have 1 of 3 sources, 1of 2 air cycle machines, and 1 of 4 controllers for outflow, you can breathe normally in the plane. Past that, pilot will have to descend to a lower altitude, meanwhile, you have your oxy mask if you need it. Pretty safe.
Thank you all for the answers. I have always wondered how it work work for a plane that has 5 passengers vs one with 200-300+ passengers.
Just out of curiosity, does it work the same way for propeller planes?
Slight correction for comments on bleed air from the engines (most planes do this, except the 787). The bleed air is constantly pushing in the same amount of air, but there are one or more big valves at the back of the plane that let that bleed air back out, and they open and close to keep the cabin pressure constant. They can have various appearances, but [this is an example](http://www.b737.org.uk/images/pressvalves_ng.jpg).
This is also why a bullet making a hole in a plane’s skin would do nothing. Those valves would just close a little bit to compensate.
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