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15

What powers the airconditioning on most airliners is APU or main engine compressor bleed, which is being supplied at fairly high pressure, say, 80 psi, and temperature, around 3-400F, to the air conditioning packs. You normally have a choice to select APU or main engine bleed. You will obviously use APU bleed on the ground before engine start, but also may ...


13

I was a tech writer in another life. Under ATA100/2100, the Air Transport Association system of categorization of systems for maintenance manuals, the APU is Chapter 49 (Airborne Auxiliary Power). Power Plant chapters are 70 and up. (Some OEMs also use a equivalent numbering convention for engineering drawings to align them to ATA100/2100, where, say, if ...


7

It's a function of the relative demand each service puts on the APU. You have to remember that bleed is power consumed; you're just taking it in a different form. As a turboshaft jet engine, the primary limitation on total power is the amount of heat the turbine can take. When you take bleed from the compressor, the turbine is forced to generate a lot of ...


6

On the 737, the APU is typically started from battery power; this is normally done on the ground, but it could be started this way in flight as well. (The 737 doesn't have a RAT.) The APU is typically operated only on the ground for short-haul flights, although there are cases such as a generator that failed in flight when it's started & used in the ...


5

In the U.S., auxiliary power units are categorized under Powerplant for the purposes of maintenance. The FAA Aviation Maintenance Technician Handbook - Powerplant, Volume 1 lists auxiliary power units in the category of turboshaft engines on page 1-38.


3

You are getting the APU mixed up with the RAT: The Auxiliary Power Unit (APU) is an engine powered generator, either a gas turbine or a piston engine. APUs deliver enough power to start engines, power cabin lights, cockpit instruments and radios, and in some cases power hydraulics. Often the APU can only be run at lower altitudes where the air is thicker A ...


3

If it's the flap closing, you should be able to verify that from the APU ECAM page. Based on the time frame you mentioned, most likely it's the simulated overspeed, which is part of the shutdown sequence and is basically the signal that closes the fuel solenoid (which you can also verify from the APU page). APU flap closing comes later. For more on the ...


2

It's not a normal A/C unit Aircraft do not use a Freon based compressor vapor-liquid transition heat pump type arrangement the way every home, car, boat and train does. They use a simpler cycle, because most turbine aircraft have a great abundance of compressed air available, or are obliged to have compressors anyway for pressurization. The compressed-air A/...


2

The running engines power commercial airliner interior climate and environmental control through their built in electrical generators and bleed air systems. The units that supply the interior air in the correct temperature and pressure for passengers are sometimes called PACK as an acronym for Pneumatic Air Cycle Kit. When the engines are off, the APU (...


2

They can leave one engine running to take the place of the APU. If that's not possible for whatever reason, the interior just gets warm.


1

It is a "power plant", so it would be classified as such. An APU isn't much different than a small jet engine.


1

Yes battery power will supply both the APU's igniter system and its starter/generator (a single motor that switches to generator mode after start, similar to turboprops). So you need the APU to start the engines (notwithstanding other bleed sources) but you need the battery to start the APU (or an external DC power source - the APU will be fed from a DC bus;...


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