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What is an Air Cycle Machine?

Patrick Roland
Patrick Roland

An air cycle machine is the system that provides fresh, temperature-controlled oxygen to an airplane cabin. This complex machine avoids using freon-like air conditioning units and eliminates the moisture associated with those systems. It comes in different types, with each providing a different level of circulation. This machine is the industry standard for providing air to a cabin and has many other benefits, such as its reliability and environmental impact.

This machine begins its complex process at the engines. Bleed air is the excess air from these enormous turbines, and it is fed into a heat exchanger. The bleed air is too hot for comfort at this point, so the natural air temperature in the sky, usually hovering around the freezing mark, is used as a natural form of coolant. This air is sent to a centrifugal compressor in order to compress the air for efficiency purposes, but this also heats the air up again. The air once again passes through a cooling process using the outside temperature as it gets closer to being expelled inside the plane.

An air cycle machine provides temperature-controlled oxygen to the cabin of an aircraft.
An air cycle machine provides temperature-controlled oxygen to the cabin of an aircraft.

The compressed air is expanded and fed into an inlet fan of the air cycle machine. The fan draws the cooled air through the cabin to the individual air vents and then to the general air vents. The temperature of this system can be raised by a control panel that allows uncooled bleed air into the system to raise the temperature to a desired level.

Depending on the size of the cabin, the air cycle machine can be any of four sizes. The smallest and simplest is one consisting of a turbine and a fan. From this point, more compressors and turbines can be added. The most complex machine consists of two turbines, a fan, and a compressor.

This device has been preferred by airlines for many reasons in addition to its ease of use. One of the main reasons is the fact that it is environmentally friendly because it does not use harmful chemicals such as freon. The system does not create moisture and vapor, so it tends to last much longer than traditional air conditioning systems. By utilizing the bleed air properly, it also erases the need for a separate heating system because the engine air creates as much heat as necessary.

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Discussion Comments


@umbra21 - Well, there are a few things you can do to make it easier. For one thing, you should drink a lot of water. I know people don't like doing that on long haul flights because they think they'll have to get up and go to the toilet all the time, but it's really important to stay hydrated, particularly in that dry air.

The other thing you can do is seal your lips with chapstick and try not to lick at them at all, because that can dry them out further. Also, the hot towels that many airlines offer periodically throughout the flight can really help.

Finally, try getting a small bottle of water and spraying your face with it whenever you start feeling a bit blah.

I find that some airlines are better than others about making sure the air isn't too dry coming off the air cycle machine, although I'm not sure what they do. The more expensive the airline, the better the air quality, I think.


@browncoat - The only problem is that it's so well controlled that I find it very artificial. It doesn't bother me so much when it's a short flight, but I had to fly half way across the world a few months ago and after sitting in airports and airplanes for over 30 hours my lips were completely chapped and sore. They took a few days to come right.

I guess if the aim is to remove as much moisture from the air as possible, in order to keep the machinery sound, they are going to end up making it too dry for the average person, especially over long term exposure.

Which is not to say that I think they should do differently, mind you. I'm all for keeping the machinery sound. I just wish that it didn't have to be at the cost of my eyes and lips!


I never tend to notice the air in an airplane, so I guess that's a sign that the air cycle machine does its job well. I've actually always thought they made the right choice in making the air fairly cool, so that you will want to use a blanket, just because the opposite would be very oppressive.

If I feel too warm, and I'm in an airplane, there's just not going to be a way to cool down and I can imagine feeling a bit panicky. If you have a bunch of people who feel that way, then you've got trouble. After all, once you get down to the last shirt, there's nothing more you can do to cool down, while you can always put on another bit of clothing if you're cold.

I guess it's all carefully calibrated to be just the right temperature.

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    • An air cycle machine provides temperature-controlled oxygen to the cabin of an aircraft.
      By: Pavel Losevsky
      An air cycle machine provides temperature-controlled oxygen to the cabin of an aircraft.