Operation Manual Part 2

Aerosoft – Digital Aviation
CRJ-70 & CRJ-900
AOM PART 2
Systems Description
VOL
5
5-1-5
26-Jul-2017
AUXILLIARY POWER UNIT
The CRJ700 and CRJ900 are both equipped with an Allied Signal RE 220 (RJ) auxiliary power unit (APU). The
APU is technically a gas-turbine and it is used to provide electrical power and even more important
compressed air.
The APU is basically comprised of three stages: a compressor followed by a combustion chamber and a
turbine. The APU is started by an electrical motor which rotates the APUs main shaft until it gained enough
speed so that fuel can be injected into the combustion chamber. The fuel burns and hence tries to expand
which accelerates the air even further. The turbine on the other hand drives a generator – so the turbine
converts the accelerated airflow into electrical power. As you can see the basic principle of an APU and an
aircraft engine are technical the same. The major difference is that the APU is not used to produce thrust
therefore APUs are also much smaller than aircraft engines.
Apart from electrical power, bleed air is also drawn from the APU to supply the packs or also the engine
starters (the aircrafts engines are usually started with pneumatic air).
Thus, it is mainly use during aircraft preparation and engine start. Nevertheless, it may also be used as a
backup once an engine failed / was switched off.
The air-inlet-flap of the APU is used to control airflow into the APU. The flaps position is controlled
automatically by the built-in electronics.