turbine powered auxiliary power units Flashcards
name the main components of a typical APU
a small power turbine for power and bleed air, and an electrical generator
what is the function of an APU on modern transport category aircraft?
to supply ground electrical and pneumatic power to aircraft systems when the main engines are not operating, and in some aircrafts as a backup source of inflight electrical power
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 bettery power
where does the APU get its fuel supply
from one of the aircraft’s main fuel tanks
at what speed does a gas turbine APU operate and how is the speed maintained
at or near its rated power 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 shutdown 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 cool down period is typically three minutes. if a heavily loaded APU is abruptly shut down without any cooling down period, damage could 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
how long is a typical APU cool-down period
three minutes
how is an APUs operating speed maintained despite variations in the loads imposed
by the APUs fuel control automatically adjusting the fuel flow
what type of operating practice could cause thermal shock and possible damage to an APU
abrupt shutdown of the APU when it is heavily loaded and not observing prescribed cool-down procedures
name the uses for APU produced pneumatic power
- engine starting
- ground air conditioning
- anti-icing
what is the operating effect of following a required cool-down period prior to APU shutdown
the APUs EGT is lowered and stabilized
