APU Flashcards
What’s the purpose of APU?
Auxiliary source of electrical power and bleed air during
for ground operations. In-flight, the APU is used for
backup electrical power or used for air for main engine
start. APU air cannot be used for pressurization in-flight.
Ref: AOM 2A
What indication do you have that
air inlet door has opened and
APU is ready to start?
READY light illuminates on COP.
Ref: AOM 2A
When should you check APU oil
level
15-30 minutes after shutdown to prevent over-servicing
Ref: AOM 2A
What’s the purpose of APU load
control valve?
It opens to provide bleed air from APU while aircraft is
on ground, as well as in-flight if needed, for main engine
start. It’s controlled by APU BLEED AIR switch.
Ref: AOM 2A
When is APU bleed air available?
After APU reaches operating speed. If APU was started
under cold conditions (i.e., EGT < 149°C), bleed air
loading is delayed for 60 seconds unless attempting main
engine start in-flight.
Ref: AOM 2A
When selecting APU BLEED
AIR switch to ON during ground
operations, what valve opens in
addition to load control valve?
Isolation valve
Ref: AOM 2A
True/False: Load control valve is
prevented from opening in flight
as a function of Weight-On-
Wheels (WOW).
True
Regarding the APU, two things
occur with right engine lower
cowl door open. What are they?
R COWL OPEN light illuminates on APU panel and
ECU inhibits the start while on ground in order to protect
engine cowl.
Ref: AOM 2A
What function does ECU perform
if APU limits are exceeded?
It automatically shuts down the APU on ground (non-
essential mode) and in some cases, in flight (essential
mode).
Ref: AOM 2A
Name the things that occur when
APU MASTER switch is selected
to ON.
Illuminates ON legend in APU MASTER switch,
performs initialization and APU prestart Built-in Test
(BIT), checks APU oil temperature, commands APU
inlet air door open, and commands APU fuel shutoff
valve open
Ref: AOM 2A
What happens when APU
MASTER switch is selected to
OFF?
Switch legend extinguishes, APU fuel shutoff valve
closes and APU, if running, immediately shuts down;
inlet door also begins to close.
Ref: AOM 2A
When does READY light
illuminate?
If all prestart conditions are passed, ECU commands the
signal to illuminate READY light (within 10 to 16
seconds), indicating APU is ready to start.
Ref: AOM 2A
Can the APU be started if
READY light fails to illuminate?
Possibly: give it a try
Ref: AOM 2A
What’s the significance of ON
legend in APU START switch?
When illuminated, it indicates that starter is engaged. It
extinguishes at starter cutout by 60%.
Ref: AOM 2A
What happens when APU STOP
switch is depressed?
If APU is running (on-speed), APU loads are shed and
APU enters cool-down mode. If on ground and up to
20,000 feet, ECU decreases APU RPM on linear scale
(1/2% per second) for 60 seconds (to 70% RPM), then
shuts down APU. If above 20,000 feet, ECU maintains
APU RPM for 60 seconds, then shuts down APU.
Ref: AOM 2A
