APU Flashcards
Why do we have an APU?
To allow the aircraft to be self sufficient
What prevents fire propagation from the APU compartment?
Stainless steel fire walls
On an APU without a load compressor, what is the source for the bleed air?
Main compressor
What type of generator is used on an APU and why?
3 phase AC generator because it is the most used type on any aircraft
How many APU mounts are flexible and why?
2 to allow for thermal expansion/contraction
What is the purpose of the flap diverter?
Increases ram air
Prevents turbulence and fluids getting in
Why does the APU actuator intake door use 28V DC?
Because it’s from the battery, the only internal source in the ground
What could happen if the APU door position switch fails?
Can be manually overrided
APU might not start, could damage itself, etc.
APU start mode lasts till what speed?
Cut off speed- 100%
When the APU master switch is pressed on, what happens in the control system?
All areas with closed circuit breakers receive power
Pre start checks
Start up procedures
What signals does the APU control unit use in normal mode?
APU speed
Torque motor
What are the three APU shut down modes and what happens in each?
Normal- 2 minute wait/cool-down
Auto- immediate, based on parameters
Emergency- immediate shut down
Why does the APU need a cool down time?
To allow the starter motor to cool, to avoid thermal stressing
Why are there external APU shut down switches on the aircraft?
To initiate shut down during maintenance or operations on the ground
What is the purpose of the APU booster pump and when is it on?
It ensures a positive supply of fuel coming from the tanks.
Found in left hand main wheel well
How does the APU control unit know the position of the IGV actuator?
Feedback signals sent by the actuators sensors
What’s the problem with an ani-surge system?
Not very fuel efficient
Where does a surge protection get its flow signals from?
Flow sensor
How is the APU cooling fan driven?
Gearbox
Major differences in oil system between engine and APU
De-oiling valve
Oil cools APU generator
What happens if APU oil pressure gets too low or too high?
Auto APU shut down
The APU control unit normally situated where?
In pressurised area before the fire wall in the tail
APU starter motor type
Dc with brushes
APU ignition exited power source?
Dc supply
What are the three starter limitations for an APU starter motor?
Duty time
Cycle time
Duty cycle
