APU Flashcards
Why is positive fuel pressure extremely important?
Prevent cavitation at high pressure fuel pump.
What is an advantage for maintenance of a vent push button?
Bypasses APU master switch.
Purge the APU fuel feed line without someone in cockpit.
(APU fuel boost pump started and APU fire shut off valve opens)
What is the APU air system divided into?
Cooling air system
Bleed air system.
What 3 tasks does the bleed air system have?
- Bleed supply
- Surge protection
- Load compressor control
Why do APU’s have load compressors?
Reduce fuel consumption by APU.
Can APU’s supply electrical power and bleed air at the same time?
Yes, but normally only one used.
When bleed air switched off, EGT decreases.
How is the air inlet to the load compressor closed off?
Inlet guide vanes. (Mechanically actuated)
When are APU’s most likely to surge?
When bleed load changes.
How is compressor surging prevented?
Bleeding off air that is blocking compressor discharge.
Where are anti-surge systems installed?
APU’s with no load compressor.
Normally opened during flight, not very fuel efficient
What is the best method of preventing surging?
Preventing surging on APU’s with load compressors, where airflow is monitored and only bled if compressor surging occurs.
What is the airflow in the compressor discharge duct equal to?
Difference between static and total pressure.
Used to open and close surge control valve
Why does the oil system have a de-oiling valve?
Improve APU start sequence.
What does the a APU system cool?
APU generator.
Where is the APU oil reservoir?
Lower part of the APU gearbox.
What are the 3 types of supply pump for the APU oil system? (Always positive displacement pump)
Gerotor type
Gear type
Cane type
How is air removed from the APU oil system?
Carried by scavenged oil.
How does the air/oil separator work?
Has vanes Mounted on a drive gear which rotates and separates the air/oil mist.
After an air/oil separator where does the oil go?
Returns to the sump (gearbox)
by gravity
What would an additional vent valve installed in the APU exhaust duct help?
Pressurise the gearbox. (Approx 4-5 psi)
Important at high altitudes.
What are the advantages of a pressurised gearbox?
Supports the oil supply pump and minimises oil foaming.
What happens if the oil temperature becomes too high?
APU automatically shuts down.
What does the master switch for the APU do?
Supplies electrical power to the APU control unit which starts its power up test.
After successful test, fuel shut off valve opens.
Where is the APU start relay usually located?
Electronic equipment compartment.
How are APU’s usually started?
DC motors on APU gearbox.
What type of DC motor is the starter motor?
Brush type.
Brush wear indicators show if the brushes on the starter motor are worn
How do you check the brush wear indicators?
Small window, with an indicator pin which shows the length of the brushes.
Or electrical sensors which send wear levels to cockpit.
What is the voltage feedback signal on the APU starter for?
Feedback signal for troubleshooting to show what component has caused APU start problem.
What happens when the starter relay unit is opened at starter cut off speed?
Battery is disconnected and APU continues to accelerate without starter support.
Why does the battery and starter motor become very hot?
Very high currents are needed to accelerate the APU rotor.
After each APU start must be cooled before using again
How is the APU ignition system usually powered?
Direct current. (From APU control unit)
What does the ignition exciter do?
Converts low voltage DC power into high voltage for igniter plugs.
Is the ignition system activated in parallel with the APU start system?
Yes.
When is the APU ignition switched off?
Happens either at cut out speed or when the APU reaches operational speed.
What does the APU generator push button do?
Connects the APU to the aircrafts electrical circuits.
During a fire warning test for the APU, what also occurs when external power is available?
Aural warning
As well as red light illuminating and squib and discharge lights illuminating
What would a decrease in battery voltage show on start indications for the APU?
APU starter is accelerating the APU.
After starter cut out, battery voltage increases again as it draws energy from APU
When are the ECAM indications only available?
If AC power is switched on.
What is the quickest way of shutting down the APU?
Activate the fire switch.
What is the purpose of the APU?
Supply pneumatic bleed air and electrical power for different systems.
In flight, in the event of bleed air and Engine AC generator failure, what can done?
Run the APU to supply pneumatics and electrical power
What section of the APU drives the compressor and gearbox?
The power section.
What accessories will the power section of the APU drive?
- Fuel pump
- oil pump
- cooling fan
- ac generator
What is the load compressor?
Ensures sufficient pneumatics are supplied to the aircraft pneumatics system
What comes from the APU compressor for the aircraft?
Pneumatic bleed air
What are centrifugal flow compressors resistant from?
FOD
What type of combustion chamber are found on modern APU’s?
Annular
Can-type on older
What type of turbines will you find on small and larger APU’s?
Centrifugal on small
Axial on larger with two or more stages
What is the air pressure usually delivered by the APU?
30 to 45 psi
What 2 ways can methods of bleed air are there on APU’s?
Compressor
Individual load compressor
Why is fuel efficiency and life time of an APU with an load compressors increased?
Compressors can be switched off I’d bleed air is not required by the aircraft.
(EGT limits not reached as easy as air is not taken from combustor)
How can the airflow be changed on demand from the pneumatic system?
Inlet guide vanes which can modulate in any position between open and closed.
Why do APU generators not need a constant speed drive?
Is always controlled at a constant speed by the APU fuel control unit
What compartments is the APU and it’s components enclosed by?
APU compartment,
Equipment compartment
Muffler compartment
What equipment does the APU equipment compartment house?
Air-intake actuator
APU extinguisher bottle
Fuel supply line
Bleed air duct
How many mounts support the APU?
3
Right/left Aft and forward mounts
What connect the APU mounts on large aircraft?
APU mount links
Links and rods which transmit forces to structural points
What is each APU mount designed to take?
2 for flexibility allowing for thermal expansion/limited movement
1 for lateral loads from APU to the aircraft
What loads are all 3 APU mounts designed to take?
Lateral loads
What is the name for the hoist equipment used to remove an APU for overhaul?
Fish pole hoist and a support stand
What aircraft is an intake flap usually found on?
Airbus
Where is the APU intake door usually found for Boeing aircraft?
Right hand upper side of the fuselage
What does the APU intake door prevent?
FOD, birds and aerodynamic drag in flight
How is the APU inlet door operated?
With an electrical Actuator
Operated by 28 volts dc power
If closed, what will stop the APU for operating?
The intake door, must be open to operate.
What is the APU intake nose?
Forms optimum aerodynamic surface for lower APU air intake
What does a diffuser do?
Increases static pressure of airflow passing through it
APU inlet duct
What does the APU intake elbow form?
Air duct between the air inlet duct and the APU air inlet
What are the two types of APU drain?
Direct (catch fluids and drain overboard)
Indirect (catch fluids and drain to special drain tank connected to drain mast)
During flight, what does airflow at the drain mast cause?
Suction in the suction line.
What modes does the APU control unit operate?
Start (until reaches 100% speed)
Normal speed mode (remains at 100% speed)
APU shut down mode
For APU start, what is the start initiation?
Electrical power supplied to APU control unit and carries out pre-start checks
What does the APU control unit carry pre-start checks on?
Electrical circuits and condition of the sensors
Once pre-start checks have been completed, what should happen next?
APU air inlet door is opened
What 2 signals will the APU control unit use to ensure it remains at 100% speed?
APU speed signal
Torque motor signal
(Maintain speed by comparing actual speed with necessary speed)
What are the 3 methods of APU shut down?
- manual by the master switch
- automatically when APU malfunctions
- emergency (fire switch or emergency shut down switch)
When will an APU shut down?
Once it has operated for a sufficient time without electrical or pneumatic load.
(Between 0-120s can be adjusted)
In automatic/emergency shutdown mode, how is the APU stopped?
Imminently without cool down
Where are the APU emergency shut down buttons on the aircraft?
Nose wheel
Main wheels
Refuelling station depending on type or size of Ac.
How is the APU extinguished of a fire on modern aircraft?
Automatically by a special extinguisher
What are the primary and secondary fuel systems on the APU?
Primary - LP fuel system (tanks to fuel control unit)
Secondary - HP fuel system (fuel control unit to fuel nozzles)
What system will allow fuel for the APU to be taken from any tank?
Cross-feed manifold
What is the purpose of the APU boost pump?
Gives positive fuel supply to the APU during the start sequence when the fuel pressure is too low.
What is the APU fuel shut off valve for?
Serves as an isolation valve. Will be shut when the APU is not running.
What’s the purpose of fuel feed line?
Supply’s fuel from the tanks to the APU via the side walls in the bulk cargo compartment.
What is the minimum number of pressure switches In the APU fuel system?
1
Second switch operates boost pump if pressure in cross feed is too low
