APU Flashcards
What’s the purpose of APU?
Supply flight crew with auxiliary source
of bleed air and electrical power during
ground operations.
In-flight, APU is used for backup electrical power
or used for air for main engine start.
APU air cannot be used for pressurization.
What are APU’s two on-speed
modes?
Non-essential if on ground and essential
in-flight
What’s the indication that air inlet
door has opened and APU is ready to
start?
Cyan “APU Ready” annunciation
illuminates on IRS / APU / Batt page of
OHPTSs
When should you check APU oil
level?
10-30 minutes after shutdown to prevent
over-servicing
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 inflight if needed, for main engine start. It’s
controlled by APU BLEED AIR switch.
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.
When selecting APU BLEED AIR
switch to ON during ground
operations, what valve opens in
addition to the load control valve?
Isolation valve
Regarding APU, two things occur
with right engine lower cowl door
open. What are they?
Amber Cowl Door Open annunciation
appears on APU panel and ECU inhibits
start while on ground in order to protect
engine cowl
What function does ECU perform if
APU limits are exceeded?
It automatically shuts down APU on
ground (non-essential mode) and in some
cases, in flight (essential mode).
Name things that occur when APU
MASTER switch is selected to ON.
ECU performs Power-up Built-In Test(PBIT)
Commands APU inlet door open,
Commands APU fuel shutoff valve open.
Nav lights and left main fuel pump also turn ON.
TROV opens.
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.
What’s the recommended action if
amber FAULT light illuminates?
Cycle MASTER and give it another try
What’s the significance of ON legend
in APU START / STOP switch?
It indicates that starter is engaged or APU
is running.
What happens when APU START /
STOP switch is selected when APU is
running?
APU load sheds and APU enters cooldown mode.
If on ground and up to 20,000feet, 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.
What happens if APU START switch
is depressed at any time during 60
second cool down period?
APU is commanded to 100% RPM
What’s the quickest way to shutdown
APU?
Any time APU MASTER switch is
selected off, APU immediately shuts down
(APU fuel shutoff valve commanded
closed).
When does APU Generator come on
line?
When APU reaches operating speed
Why does APU GEN switch indicate
amber ON?
It represents abnormal in-flight condition
What’s the difference between
essential and non-essential?
In non-essential mode, there are many
conditions which would cause ECU to
shutdown the APU. If this were to occur,
cool down mode is bypassed, APU’s fuel
shutoff valve is closed, and APU shuts
down immediately. When in essential
mode (in-flight), some protective
shutdowns are disabled to allow continued
operation.
What happens if APU is operating in
essential mode after landing?
APU continues to assume aircraft is in
essential mode until APU MASTER
switch is selected off (or 15 min has
elapsed)
What’s an amber APU Essential CAS
message indicate?
It means it hasn’t shutdown for one of the
malfunctions that would have caused
shutdown on ground. Don’t shut it down if
you really need it because a subsequent
start may be inhibited. If you don’t need it,
shut it down promptly.
