Turbine Powered APUs Flashcards
Name the main components of a typical APU
- A small power turbine for power and bleed air
- electrical generator
When is the greatest demand placed on an APU?
When supplying bleed air
How is an APU started?
With its own electrical starter and battery power
Where does the APU get its fuel supply?
From the aircraft main fuel tanks
At what speed does a gas turbine APU operate and how is the speed maintained?
At or near it’s rated speed regardless of electrical or pneumatic loads imposed. The APU fuel control automatically adjusts the fuel flow to maintain the rated speed.
What is used to prevent a heavily loaded APU from exceeding its maximum EGT?
A load control valve modulates the pneumatic load to maintain the EGT’s within limit
How is an APU shut down and why is this procedure used?
After the APU is unloaded by closing the bleed air valve, it is run for a specified amount of time to allow the EGT to cool and stabilize. This takes around three minutes. If a heavily loaded APU is abruptly shut down without any cooling period, Damage maybe occur as a result of thermal shock.
What powers the variable inlet guide vanes used on some APUs to regulate compressor intake airflow?
Fuel pressure
What is the function of an APU on modern transport category aircraft?
To supply ground, electrical and pneumatic power to aircraft systems when the engines are not operating, and some aircraft is a back up source of in-flight electrical power
Name at least two uses for APU produced pneumatic power
Engine starting, ground air conditioning, anti-icing
How is the cool down for many APUs accomplished prior to shut down?
By running the APU for at least three minutes after closing the bleed air valve
What is the operating effect of following a required cool down prior:
The APU’s EGT is lowered and stabilized
How is an APU’s operating speed maintained despite variations in the loads imposed?
By the APU’s fuel control automatically adjusting the fuel flow
