auxiliary power unit Flashcards
what kind of APU does the 175 have?
Sundstrand APS 2300
Describe the APU
constant speed
single stage impeller with 2 stage axial flow turbine
independent oil system
provides pneumatic and electrical AC power
pneumatic power is used for engine starting and to supply bleed air to the AC packs of the ECS
electrical generator supplies 115 V kVA to the electrical system
what controls the APU?
a dedicated FADEC
what battery is used for the APU start?
Batt 2 powers the starter
Batt 1 powers the FADEC and EICAS for monitoring
what is the fuel source for the APU?
with only DC power, the DC fuel pump in the right fuel tank.
if AC power is avaliable and engine is not running then provided by AC fuel pump.
engine running - ejector fuel pump provides fuel to the APU from right wing tank
also possible:
left fuel tank thru the crossfeed valve
what fuel pump is used for APU start?
batteries ‘ON’ only DC fuel pump in right tank
AC powered airplane ACMP 2
right engine running ejector fuel pump
will the APU shut down automatically if speed and acceleration rate isn’t reached in starting cycle time?
YES
what does moving the APU start switch to OFF do?
initiates normal APU shutdown
1 minute cool down
no pneumatics (electrics only for 1 minute)
Can the APU shut down be cancelled?
yes, by moving the start switch back to ON before the 1 minute cool down is over
how is the APU shut down immediately?
APU emergency stop button
no 1 minute cool down period
what will happens if batteries are turned off during the 1 minute cool down period?
APU will shut down immediately
when does APU shut down automatically occur?
on ground:
overspeed
underspeed
fadec critical fault
sensor failure
high oil temp
EGT over temp
low oil pressure
fire
in flight:
overspeed
underspeed
fadec critical fault
can the APU be started after critical fault?
no
