Turbine Auxillary Power Unit Flashcards
Name the main components of a typical APU:
A small power turbine for power and bleed air, and an electrical generator.
When is the greatest demand placed on an APU?
When supplying bleed air.
How is an APU generally started?
With its own electrical starter and battery power.
Where does the APU get its fuel supply?
From the aircraft’s main fuel tanks.
At what speed does a gas turbine APU operate and how is this speed maintained?
->At or near its rated speed (100%)
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 EGT’s within limits.
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. It takes around 3 minutes. If a heavily loaded APU is abruptly shut down without any cooling period, damage may occur as result of thermal shock.
What powers the variable inlet guide vanes (IGV) used on some APU’s 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 in some aircraft as a backup source of inflight electrical power.
Name at least two uses for APU produced pneumatic power.
(1)Engine Starting (2) Ground air conditioning (3) Anti-icing.
How is the cool-down for many APU’s accomplished prior to shutdown?
By running the APU for at least 3 minutes , period of time after closing the bleed air valve.
What is the operating effect of following a required cool-down period prior:
The APU’s EGT is lowered and stabilized.
How is an APU’s operating speed maintened despite variations in the loads imposed?
By the APU’s fuel control automatically adjusting the fuel flow.
