How does aircraft air-conditioning and multiple FMS CDU work?

The air conditioner works about the same as it does in your house. If you are referring to a larger aircraft, it would have an air cycle machine.. which works on the principle of expansion through a turbine to remove heat.

As far as FMSs, it depends on why type you are referring to, whether they are synced or independent. An NZ-2000 with 3 CDUs, has three FMSs, however only two can be active at a time due to the ASC-B bus that controls them. So the third sits as a spare.. but cannot be coupled without one of the others being taken out.

Universal FMSs, very similar, where the third is up and alive, but unless you intend to select the #3 FMS as a Nav source..it sits in the back of the ped as a spare.

Sync is nice so if you put a flight plan into one it will push into the others... if independent.. then you have to load each one.

Air cycle air conditioning uses heat exchangers combined with an air cycle machine to cool the air.

First. The air is taken off the engine and put through a precooler to get the temperature down to about 350F.

Then it passed through the first heat exchanger which cools the air. The amount the air cools is based on the temperature difference between the hot and cool air... so even 110F air at Phoenix will cool the 350F air.

Before going to the second heat exchanger, the air is heated up again by the compressor of the air cycle machine. This increases the temperature differential again allowing the second heat exchanger to remove as much heat energy as the first.

The air is still too hot... so finally it is run through the turbine stage of the air cycle machine which removes all of the heat that was added at the compressor. The result is air that is well below freezing.

Then the air is passed through a water separator and mixed with warmer air for the cabin.

https://en.wikipedia.org/wiki/Environmental_contro...

https://en.wikipedia.org/wiki/Asynchronous_system

Too much crap to type.