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
