APU Flashcards
How many apu mounts
3
What loads do the 3 mounts transmit
Vertical loads and forces from the apu to the aircraft structure
What loads do 2 mounts carry
Transmit Axial loads and forces from apu to aircraft structure
flexible to allow limited movement for thermal expansion
What loads do 1 mount carry
Carries lateral loads and is always a fixed mount for strength and stability
What’s the purpose of the apu
Supplies pneumatic bleed air and electrical power
How is the apu removed
Using a fish pole hoist
How do u hold the apu once removed
Support stand
Where do we mainly use the apu
On ground
What are the 3 parts of the apu
Power
Bleed
Accessory
What does the bleed system do
Make sure sufficient bleed air is supplied to the aircraft pneumatic system
Where’s the apu intake on airbus
Lower intake door
Where is intake in Boeing
Right hand upper side of fuselage
What does apu intake door do
Supplies air to the apu and it closes off the apu air inlet if apu isn’t running
How is inlet door operated
Electrical actuator supplied from 28v DC
Air pressure for bleed air
30-45psi
What’s another name for the apu air inlet duct and why
Diffuser due to its shape , it guides air to the apu
What do the air inlet guide vanes do
Improve the airflow
Where are the inlet guide vanes
In the air intake elbow
What does the apu drain system do
Catch and drain fluids and leakages
Why is apu drain system necessary
Prevents fire operating malfunctions and corrosion
Where do u find condensed water
Apu exhaust section after apu shut down is cooled
Where does Oil or fuel leakage come from
come from the drive seals at the apu gear box
What are the two types of drains
Direct and indirect
What’s a direct drain
Catch fluids and drain them via drain lines directly overboard
What’s an indirect drain
Catch fluids the put into a tank which is connected to a drain mast
How does the drain mast work
Airflow at the drain mast causes suction to the suction line
What does the apu supply the ac generator with
Constant speed
What does normal speed mode do
Keeps apu at constant 100%
What does start mode do
Control apu from start activation until its at 100%
How is start mode activated
Manually by switches on the overhead panel in the cockpit
How many apu switch in Boeing
One switch
What are the two individual switches used on airbus
Master switch and start switch
What’s necessary to start the apu
Start initiation and start activation
What does start initiation mean
Electrical power is supplied to the APU control unit
What happens when master switch is on
Apu control unit is supplied with electrical power and first performs a pre start test
when does apu inlet door open
When pre start test is successfully performed
What does the speed indicator and EGT indicator show when air inlet door is open
Zero
What two signals are used for constant speed control
Apu speed signal and torque motor signal
How does normal operation keep a constant speed
By comparing the actual apu speed with thr necessary speed
How does the toque motor signal work
If speed is not 100% it will change the fuel metering
How do you shut down apu
Normal
Automatic
Emergency
How is apu normal shut down
Via master switch
How does apu automatic shut down
Triggered by apu if it detects any malfunctions in important comments
How does apu shut down in emergency
By fire switch or alternate emergency shut down switch
What is the apu cool down period on normal shut down
Varies between 0 and 120s
How does apu shut down after cool down
Control unit closes off fuel supply to the combustion chamber so apu comes to a stop
How quick is automatic shut down command activated
Imminently without any cool down period
Does emergency apu shut down have a cool down period
No
How is apu fire extinguished
By a special fire extinguishing system
Where does fuel come from for the apu
Aircraft main engine engine fuel feed system
What does apu fuel shut off valve do
Serves as an isolation valve for the apu fuel feed line
What does the pressure switch in primary apu system do
Monitors fuel pressure in the apu fuel feed like directly infront of apu and triggers warming if there’s a fire
What does the second pressure switch do
Controls apu fuel boost pump
What is the apu fuel boost pump used for
Give positive fuel supply if tank boost pumps not available
What’s does pushing the vent push button do
Apu fuel boost pump is started and the apu fuel shut off valve opens
What’s the two sub system of apu air system
Cooling and bleed
What can the bleed air system be split into
Bleed supply
Surge protection
Load compressor control if apu has one
What does load compressor system do
Reduces fuel consumption of the apu
What happens when only
One power source is needed in load compressor control
The other one is shut off , when apu
Bleed if off EGT decreases
Can the load compressor be switched off
No it’s fixed to rotor shaft
What do the variable inlet guide vanes do in the load compressor
Control quantity of airflow into the load compressor & can move to any position between open and closed
How are inlet guide vanes moved
By inlet guide vane actuator
How can compressor surge be prevented
Bleeding off the air blocking the compressor discharge
Where are anti-surge systems installed
Apus without load compressors
Why is anti sure system not efficient
Air is bled off when there is no surging
Where is surge protection used
Apus with load compressors
How does surge protection work
Monitors the airflow in the load compressor discharge duct and opens a surge control valve is surge occurs
Where is airflow sensor installed
Discharge duct of the load compressor
What are the discharge duct pressure pick ups
Total pressure pick up and static pressure pick up
What’s the relationship between airflow in the compressor discharge duct and pressure
Airflow is equal to the difference in total pressure and static pressure
Where does the apu cooling system supply air to
Apu cases
Apu compartment
Apu oil cooler
What’s does the de-oiling valve do
Improve apu start sequence and on many Apus the oil system is used cool apu generator
Where is the oil resovoir
Lowest part of the apu gearbox
What type of pump is an apu oil supply pump
Positive displacement pump
How are Apus with load compressors fuel efficient
They can be switched off if bleed load is not required by the aircraft
What an advantage to apu with a load compressor
EGT limits aren’t reached which Improves lifetime of the apu
How is airflow into load compressor controlled
Variable inlet guide vanes
What are variable inlet guide vanes
Work as a modulating and shut off valve
Where does electrical power supply come from
Generators driven via apu gearbox
Why does apu not need a constant speed drive
Controlled to a constant speed by apu fuel control unit
What supplies electrical power to to the apu to do the power up test
Master switch
What type is the starter motor
Brush type DC motor
What do brush wear indicators show
Show if brushes on the starter motor are worn via small inspection windows
What do electrical brush wear sensors do
Electrically signal the control unit when worn on newer Apus
When is starter relay energised
When apu start switch is activated
What happens at starter cut off speed
Apu control unit opens starter relay
What happens when starter relay opens
Disconnects battery from the apu starter motor
What must you do after an apu start
Cool down sufficiently before you can use them again
What are the 3 starter limitations
Starter duty time
Starter cool down time
Starter duty cycle
Battery voltage check
Must be more then 23v
What shows apu starter is accelerating the apu
Decrease in voltage during apu start
What’s the quickest time to shut down the apu
Activate the fire switch
