15.8 APU👌🏼 Flashcards
Explain APU
- small gas turbine
- supplies pneumatic + electrical power
What is the primary requirement to achieve ETOPS certification
- aircraft possession of an in-flight APU
APU require low starting altitude which is around
- 15000ft-17000ft
APUs are capable to operate up to what altitude
40000ft
APU is split into 3 main sections
- power section
- bleed section
- accessory gearbox section
What accessories are driven by the gearbox
- fuel pump
- oil pump
- cooling fan
- ac generator
2 ways APU air can be bled
- APU compressor
- load compressor
What does the power section on gas turbines contain
- compressor
- combustion chamber
- turbine
Why are In flight APU often started during landing phase
To ensure in case of go-around there is maximum engine power available
What are the combustion chambers type for modern APU
Annular type
What fire protection system is on the APU
- fire extinguisher that activates automatically
The AC generator is at a constant speed supplied by what
APU
Electronic APU control unit has 3 separate operating modules which are
- start mode
- normal speed
- APU shut down
Explain start mode control
APU from start to 100% speed
Explain normal speed mode
Remain at 100% speed regardless of load changes
Explain APU shut down mode
- monitors and controls APU shut down
2 reasons APU control unit is monitored
- optimise APU start sequence
- protect APU from damage
What are the 2 signals for constant speed control
- APU speed signal
- torque motor signal
APU fuel is taken from where
- aircraft’s main engine fuel feed system
APU cool down period varies between
0-120 seconds (can be adjusted)
When is the APU allowed to shutdown
After operating for a sufficient time without pneumatic or electric load
How can the APU be kept at 100% speed with high bleed loads?
Add more fuel (results in higher EGT)
If APU has load compressor what system does it belong to
Air bleed system
2 conditions must be met to open the APU bleed valve
- APU bleed switch ON
- APU start sequence must be complete
When APU is shut down why is there a time delay
- So APU can run with no loads to cool down
Advantage of load compressor control
- more fuel efficient
Can the load compressor be switched off
No as it is fixed to the rotor shaft
What does the surge protection system prevent
- any surge condition of the APU compressors
When can APU compressors surge
When bleed load changes
Automatic APU shutdown is activated by what
APU control unit
APU start sequence ends when APU speed becomes greater than what percentage
95%
Where are the APU external emergency shut-down switches located
- NL gear
- main wheel well
- on refuelling station
Is the automatic APU emergency shutdown activated in a fire condition when aircraft is on ground
Yes
Explain anti-surge valve
Operated during flight to prevent surging
Air inlet into the load compressor is closed off by ?
Inlet guide vanes
Surge protection system is used on what
APU’s with Load compressor
Explain how the surge protection method works
-monitors airflow
- opens a surge control valve if compressor surges
Does automatic shutdown have a cooling period
No it is immediately shut
When pressing FIRE button what occurs to APU
Immediately shut down
When a FIRE occurs at APU what occurs
Immediately shut down
