15.18 APU Flashcards
What does the APU do?
Supplies bleed air and electrical power.
Where is the APU used?
On the ground
How are free turbines designed?
To run as constant speed without a CSDU.
What are the 2 air supply’s from the APU?
From compressor directly.
Separate Load compressor (LC) = only supplies air
What are the 3 sections of the APU?
Power.
Bleed air.
Access gearbox.
What is electrical and mechanical power required for ?
Engine starting
Pneumatics for cabin air conditioning system.
Shaft power for other pneumatic and hydraulic systems
Back up power.
Ground operation power.
Up until what heights can the APU function?
40 000ft
What type of combustion chamber is used on modern APUs?
Annular type
What monitors the start sequence and turbine speed of the APU?
Electrical Control Box (ECB)
Describe the APU fire extinguishing system?
Automatic.
What are the 3 operating modes of the APU?
Start= 0-100%
Normal= Maintains 100% speed
Shut down= Cool down
How is the APU start sequence operated?
Manually.
What is the start sequence of the APU?
- Electrical power supplied by batteries or GPU.
- System test.
- Air inlet door opens.
- APU control unit monitors acceleration until 100%
Why is APU acceleration monitored Upto 100%?
To optimise APU start sequence.
Protect APU against damage.
What automatically controls the APU acceleration process?
Speed signal.
What controls the fuel metering of the APU?
APU control unit.
Electrically controlled for all acceleration.
What are the 2 tasks of The APU control unit?
Ensures generator drive remains at 100%
No excessive EGT is reached.
What are the 2 signals for APU constant speed control?
Speed signal
Torque motor signal.
What is the purpose of the torque motor signal?
Adjusts fuel flow to maintain 100% speed.
What is the effect of high bleed loads on the APU?
Slows rotor,
Uses more fuel
Increases EGT
How is APU shutdown initiated?
Master switch is set to OFF.
How long is the cool down period of the APU?
0-120s
During APU shutdown, what occurs before fuel is removed?
Loads are removed and a cool down period.
Where does the APU get its fuel from?
Main engine fuel feed supply.
Which is the function of pressure switches?
Monitors fuel pressure.
Why do we want positive fuel pressure?
To prevent cavitation at the HP pump.
What are the 2 bleed air sub systems?
Cooling air.
Bleed air.
What is cooling air used for?
For the oil cooler and ventilation.
What are the tasks of bleed air?
Bleed supply.
Surge protection = prevents surge conditions of APU compressors.
Load compressor control.
When does the APU start sequence end?
When speed is over 95%
What are the 2 conditions to open the APU bleed valve?
Bleed switch is ON
Start sequence is complete.
When does the bleed valve open?
When the solenoid is energised
Bleed valve closes when master switch is set to OFF. When else does the APU bleed valve close?
When pneumatic controllers detect an overheating condition near the bleed air ducts.
What can be said about aircraft with Load compressors and why?
They are more fuel efficient because the LC can be adapted to the bleed air demand.
Can the Load compressor be switched off?
No because it is fixed to the rotor shaft.
Can the air inlet of the LC be closed?
Yes by IGVs
What position are the IGVs during engine starting?
Fully open as maximum airflow is needed.
When are compressors likely to surge?
When bleed loads change.
What are the 2 surge protection systems?
Anti surge system= for aircraft without a LC
Surge protection= for aircraft with a LC.
When does the anti surge valve operate?
Open during flight.
It always bleeds off air when surging is possible.
When does the surge control valve opens?
When airflow behind the LC decreases or stops.
How does an APU automatic shutdown differ from normal shutdown?
There is no cool-down period.
How else can the APU be shutdown?
By external emergency shut down switches.
What is the quickest way to shutdown the APU?
By activating the fire switch.
