# How do big planes like 737 manage fresh air for passengers during flight?

How do the big planes (e.g. A380 or 737) flying on high altitudes manage to get fresh air for passengers and crew during the flight?

I know the inner space of the plane used for passengers and crew is sealed and the pressure is maintained within some range.

However in standard flight altitude of about 10-14 km there is not enough oxygen to breathe. I am not sure whether the standard oxygen ratio is still about 21% and you only have less air (lower pressure).

We talked two possible solutions and maybe I am still wrong:

1. using some pumps and small pressure tanks to suck in air from outside till we get higher pressure and then use some valves to "replace" the breathed up air with the fresh one

2. use some pressure tanks loaded with oxygen + some cleaning devices to clean the air and add oxygen to keep the same oxygen level - but this would need to get the oxygen before take-off and such tanks would be really dangerous

So how is it in reality?

• The ratio of oxygen is constant. It's only because the partial pressure is reduced that you find it difficult to breath. Airliner hulls are far from sealed. There are many inlets and outlets to condition air and to manage the pressure altitude. Those movies you see where someone shoots a bullet through the fuselage leading to explosive decompression are nonsense. You could put a fist sized hole in the fuselage and apart from noise and being able to see it, you would hardly notice. Oct 1, 2014 at 14:56
• Think of the cabin as a balloon with air coming in from some source (ex. bleed air) and with a controlled leak (outflow valve). By adding and venting air You control the pressurize of the cabin to maintain the air pressure found at a certain altitude. This gives you same amount of oxygen (partial pressure O2) as found as if you were walking at that altitude. Oct 1, 2014 at 16:22
• Details on how the A/C works: Why is air mixed with bypass air on the A/C of an aircraft?
– mins
Mar 7, 2016 at 7:44

They bleed hot compressed air from inside the jet engines, cool it down and pass it into the cabin. There are outlet valves in the fuselage that allow stale air out and which control cabin pressure and air-refresh rates.

A few aircraft types, such as the 787, now use a compressor to take in air from under the aircraft. They do this because it can be more fuel-efficient (not for any other reason). It is arguable whether it creates an additional fire hazard.

Airplanes basically draw in outside air, compress it, and send it to the cabin. RedGrittyBrick lists some related questions about this process. Most airplanes use bleed air from the engines, using the compressors in the engines to do the compressing. Although in normal operation this will only bring in outside air, engine issues can cause fumes to enter the cabin. The 787 uses an electric compressor that draws from separate intakes.

The problem with breathing at high altitudes is not necessarily the lack of oxygen, but the low pressure. Humans need a certain partial pressure of oxygen for the lungs to work properly, and as pressures drop, the oxygen is less effective at getting into the bloodstream, which results in hypoxia. So keeping the cabin pressurized is what allows the occupants to breathe normally.

For more information about the cabin environment, see this paper. Ventilation can be expressed in "outside air changes/hour", or how often the total volume of air is replaced by new air. Airplanes have 10-15 changes/hour (every 4-6 minutes), which is 2-3 times more changes/hour than hospital delivery and operating rooms, and 4-15 times more than a typical building. New air is constantly being circulated into the cabin, while old air escapes through relief valves.

Bleed air from the engine is used to power a Pneumatic Air Cycle Kit (PACK) which takes in outside air heats it and compresses it.

The mechanism is entirely mechanical using turbines and heat exchangers and engine bleed air is isolated from the air destined to the cabin to avoid oils and fumes entering the cabin.

The answer to How hot would pressurized air get if the air conditioning failed? ex^plained in detail how a PACK works.

• Do they heat or cool the air? We had this before ... see RedGrittyBrick's answer for comparison. Oct 1, 2014 at 9:42
• Reading the linked article I don't get impression bleed air is isolated from the air destined to the cabin. In fact it says the exact opposite. The bleed air is destined to cabin (a mix of cooled and non-cooled streams to achieve desired temperature) and outside air is only used as coolant. Oct 1, 2014 at 9:47