15.18 APU Flashcards
what is the function of an APU?
small gas turbine (free turbine) turboshaft which can supply bleed air and electrical power for aircraft systems typically on ground
what will the APU use to give constant speed?
its own system as it runs at a constant speed
how can the APU supply pneumatic air?
it can be bled off of the compressor more commonly a separate load compressor power by a free turbine
what is a primary requirement for ETOPS?
an in flight APU?
how do in flight APU’s function?
typically started at 15,000 ft or below but can function at 40,000ft
when are in flight APU’s typically started?
in landing phase to avoid taking compressor air from main engines to be used for reserve thrust
what is the purpose of the ECB electrical control box?
it monitors start speed, turbine speed and EGT
how is the fire extinguisher activated on a APU?
automatically
what are the three operating modes of an APU?
start mode - from start to 100%
Normal speed mode - keeps the APU at 100% despite load changes
APU shut down mode - monitors APU shut down
what is the purpose of the start system?
system tests will occur to ensure systems and circuits are functioning, opens the inlet dor if the tests fail then the APU will not start
the system is monitored until 100% acceleration is reached
what is the function of start mode?
to ensure economical start and prevent APU damage
what can slow start and speeds cause in APU?
high thermal stresses
in normal mode or on speed control what is monitored?
that the APU remains constant at 100% and the EGT temperatures
what two signals are used for constant speed control?
APU speed signal
torque motor signal
what does high loading have a potential in effecting?
speed
what do high loads also cause?
increase in EGT as the engine has to work harder to maintain speed
how long can the shutdown cool down period last?
0 - 120 seconds
what is the sequence when off is selected?
remove loads
run for 0-120 seconds
fuel supply stopped
where is fuel taken from for the APU?
main engine fuel feed
what are the main components of the fuel supply for the APU?
APU fuel boost pump
APU fuel shutoff
APU fuel feed line
minimum of one fuel pressure switch to monitor fuel pressure in the feed line
why is positive fuel pressure in the APU feed line especially important?
to prevent cavitation in the high pressure fuel pump
what are the two sections of the APU air systems?
cooling
bleed air
what are the three main tasks of the APU bleed air system?
bleed supply
surge protection
if needed the load compressor supply
what does the surge protection system do?
to prevent a surge occurring from the compressors
what conditions must be met when you open the APU bleed valve?
bleed switch must be set to on
APU start sequence must be complete (operating above 95 %)
what will occur to the solenoid which actuates the bleed air supply if a shut down signal occurs?
it will be de energised
if the master switch is set to off for a normal APU shut down what will occur?
the bleed air closes and a time delay activates
when else will APU bleed air close?
if an overheat is detected near the bleed air ducts
can the load compressor be closed off?
no
what are the main components in the load compressor?
-variable inlet guid vanes
-the inlet guide vane actuator with a mechanical transmission assembly
-the APU control unit
how air be shut off from the load compressor?
actuating inlet guide vanes
what will occur when bleed air is switched on initially?
inlet guide vanes air partially opened and then adapt for supply
where are the external emergency shut down switches mounted?
-nose landing gear
-main wheel well
-sometimes on the refuelling station
when is the anti surge usually open?
during flight
what is the anti surge system?
it always bleeds off the air when surging is possible it is only installed on aircraft with no load compressor
what is the general rule for when surge control valve is open?
behind the load compressor when air slows or decreases
when will automatic APU shut down occur?
if operating limits are exceeded or important components are defective
what is the quickest way to shut down the APU?
activate the fire switch
