15.18 APU Flashcards
What is the purpose of the APU
Supplies pneumatic bleed air and electrical power for the different aircraft systems
When are APUs mainly used
When the aircraft is on the ground and engines are not running
On modern twin engine aircraft what must the APU provide in the event of engine or AC generator failure
Bleed air and electrical power in flight as a back up source
What 3 main sections is the APU split in to
- The Power section
- The Bleed section
- The auxiliary gearbox section
What does the power section on the APU drive
The compressor and the gearbox, the gearbox drives accessories such as
- The fuel pump
- The oil pumps
- The cooling fan
- The AC generator
The APU must ensure that sufficient bleed air is supplied to the pneumatic system. How is this done
With a compressor called a ‘Load Compressor’
What is the APU compressor used for
Used to provide the air for combustion
On some APUs the compressor is used to supply the pneumatic system
On most APUs what type of compressor would be found
A centrifugal compressor
What are the advantages of a centrifugal compressor
- Good performance
- A long lifetime
- Resistant to Foreign Object Damage (FOD)
What type of combustion chamber would be found on modern APUs
The annular type
On older APUs you may also find the can-type combustion chamber
What type of turbine would be found on;
- small APUs
- large APUs
Small APU - Centrifugal
Large APU - Axial with 2 or more stages
At what pressure does the APU supply bleed air
30-45psi
What are the two methods of APU bleed air supply
- One uses the compressor of the power section
- The other uses an individual load compressor
What component improves the life of an APU
A load compressor.
It can be turned off if bleed air is not required reducing the EGT and the load from the compressor
How is the airflow into the load compressor controlled?
With Variable Inlet Guide Vanes (VIGV)
They work like a modulating and shut of valve, they move into any position between open and closed
Why does the APU electrical generator not need a (CSD) constant speed drive when engines do
The APU is controlled to a constant speed by the APU fuel control unit
The APU and its components are enclosed by what compartments
- The equipment compartment
- The APU compartment
- Some aircraft also have a muffler compartment
In the APU equipment compartment what type of APU equipment does it contain
- The air intake actuator
- The APU fire extinguisher bottle
- The fuel supply line
- The bleed air duct
What is special about the APU compartment
It is entirely fireproof and serves as a fire compartment. The APU fire walls prevent any possible fire propagation to the fuselage
How many mounts support the APU
3
What are the functions of the 3 engine mounts
One is always fixed and is for strength and stability
The other two are flexible and because the must permit limited movement for thermal expansion
On large aircraft, how is the APU supported?
APU mount links
They are rods or tubes that transmit the forces from the mounts to the aircraft structural points
What type of loads must the engine mounts transmit
All 3 must transmit vertical loads from APU to aircraft structure
Two mounts transmit axial loads from APU to aircraft structure
One mount transmits lateral loads and forces from APU to aircraft structure
Is removal of the APU required for complete overhaul?
Yes
How is the APU removed from the APU compartment
With hoist equipment also called a fish pole hoist.
It is used as a mini crane to manually crank the APU up and down
What are the major components of the APU air intake system
- The air intake door
- The air intake door actuator
- The air intake nose
- The air inlet duct
What is the air inlet door also known as
The air intake flap
It supplies air to run the APU and closes when the APU is not running
It prevents birds and debris entering the APU and reduces aerodynamic drag in flight
How is the APU inlet door operated
An intake door actuator powered by 28v DC power
It has a manual override if the door cannot be electrically operated
What are the 3 main components of the APU inlet duct system
- The air intake nose
- The air inlet duct
- The air inlet elbow
What is the purpose of the air intake nose
Forms an optimum aerodynamic surface on the lower APU air intake
What is the APU inlet duct often named
A diffuser because of its shape.
The duct guides the air to the APU
What is the purpose of the APU air intake elbow
It forms the air duct between the air inlet duct and the APU air inlet
There are inlet guide vanes in the elbow to help improve airflow
What is the function of the APU drain system
Catch and drain fluids and leakage from critical APU areas
This is important to prevent fire, operation functions and corrosion
Where would you normally find fuel leaks
From the inlet guide vane actuator, the fuel manifold or the lowest point of the combustion chamber after a wet APU start
Where would you find water leakage
Condensed water comes from the APU exhaust section after the APU is shut down and cooled
Where would you normally find oil leakage
The drive seals at the APU gearbox. The gear box drive seal drains are the same as on the aircraft engines
What are the two main type of drains
- Direct drains
- Indirect drains
What do direct drains do
Catch the fluids and drain them via drain lines directly overboard
What do indirect drains do
Catch the fluids and drain them to a special drain tank, the drain tank is connected to a drain mast
What are the 3 separate operating modes that the electronic APU control unit does to run the APU
- The start mode controls the APU from the start activation until it has reached its 100% speed
- The normal speed mode keeps the APU at 100% speed regardless of load changes
- The APU shut down mode monitors and controls APU shit down
How is APU start mode activated
Activated manually by switches on the overhead panel on the cockpit
What 2 actions are necessary to start the APU
- Start initiation (Electrical power is supplied to the APU control unit)
- Start activation
What will the APU control unit do when the APU master switch has been turned on
It is supplied with electrical power and performs a pre start test
The pre start test ensures that the APU control unit operates properly me that the important sensor signals are available
What happens when the APU control unit has carried out its pre start up test
The APU air inlet door is commanded to open, the fuel supply is prepared, the fuel shut off valve is opened and the APU fuel boost pump is switched on
When the master switch is on the APU Indication is available in the cockpit, what parameters will it show
The speed indicator and the EGT indicator show zero and the air inlet door is open
What is the APU control units task after the APU has got up to operational speed
Make sure that the generator drive remains constant at 100% and that no excessive EGT is reached
What two signals does the APU control unit use for constant speed control
- APU speed signal
- The torque motor signal
The APU maintains constant speed by comparing the actual APU peed with the necessary speed. It then changes the torque motor signal to change the fuel metering
What are the 3 ways to shut down an APU
- The normal way is to manually turn off the APU master switch
- Automatic shut down is activated by the APU control unit because of malfunctions in important APU components
- An emergency shut down is activated by the fire switch or alternate emergency shut down switch
After the APU has been running and the master switch is selected to OFF what happens next
The APU is only allowed to shut down after it has operated for a sufficient time without pneumatic or electric load.
The cool down period varies between 0-120 seconds and can be adjusted during maintenance on some APUs if required
Why is APU automatic shut down used
Protects the APU from damage if operating limits are exceeded or important APU components are defective
In automatic shut down the stop demand is activated immediately with out any cool down period
How long is the cool down period on an APU when shut down in the normal way?
0-120 seconds and can be adjusted during maintenance
Where are the external emergency shit down switches for the APU
- At the nose landing gear
- In the main wheel well
- On the refuelling station depending on aircraft type or size
How is an APU fire extinguished in an emergency
Automatically by a special fire extinguishing system
What is the primary fuel system of the APU also named
Low pressure fuel system
The system starts at the fuel tanks and ends at the fuel control unit of the APU
What is the secondary fuel system also called
The high pressure fuel system
Starts at the fuel control unit and ends at the fuel nozzles on the combustion chamber
What are the main components of the primary fuel system
- The APU fuel boost pump
- The APU shut off valve, also known as APU low pressure fuel shut off valve
- the APU fuel feed line
- Pressure switches
What serves as isolation valve for the APU fuel feed line
The APU fuel shut off valve
What is the APU fuel line vent switch
Located on the inner firewall of the APU compartment.
If the button is pushed the APU fuel boost pump is started and the APU fuel shut off valve opens
The vent push button bypasses the APU master switch. Advantage for maintenance because they can purge the APU fuel feed line with out a person in the cockpit
What two subsystems is the APU air system divided in to
- The cooling system
- The Bleed system
What 3 main tasks does the bleed system have
- Bleed supply
- Surge protection
- If the APU has a load compressor, load compressor control
What is the purpose of the APU air cooling system
Uses air from the APU intake to cool the APU, the APU compartment and air cooled components like the oil cooler or the AC generator
APUs can supply electrical power and bleed air at the same time. Can one function be turned off
Yes
Can the load compressor be switched off
No as it is fixed to the rotor shaft
However the air inlet to the load compressor is closed off by inlet guide vanes. This means the air cannot enter the load compressor
What are the main components of the APU load compressor system
- The variable inlet guide vanes
- The inlet guide vanes actuator with a mechanical transmission assembly
- The APU control unit
What are the main components in the inlet guide vane system
- IGV actuator assembly
- The transmission assembly
- The load compressor inlet guide vanes
What are the IGV actuator assembly sub components
- The hydraulic actuator
- The torque motor
- The linear voltage displacement transducer LVDT
How can compression surging be prevented
By bleeding off the air that is blocking the compressor
What are the two surge protection methods found on the APU
- The anti surge system
- The surge protection system (With load compressor only)
What does the anti surge system do
Installed on APUs without a load compressor
The system uses an anti surge valve which bleeds off air when surging is possible
This is opened during flight. It is not fuel efficient as it also bleeds off air when there is no surging
What does the surge protection method do (With load compressor)
Surge protection system monitors airflow in the load compressor inlet duct and opens the surge control valve if the compressor surges.
What are the main components of a typical surge protection system
- The airflow sensor
- The surge control valve
- A control unit
- The surge air exhaust duct
The surge protection airflow sensor is installed in the discharge duct of the load compressor. What are the two pressure pickups it uses
- Total pressure pickup
- Static pressure pickup
The airflow signal is used to open and close the surge control valve
What components does the APU cooling system supply
- APU cases
- The APU compartment
- The APU oil cooler
- Sometimes APU generator
What are the two differences found on an APU oil system apposed to a main engine oil system
APU system have a de oiling valve which helps to improve the APU start sequence and on many APUs the oil system is also used to cool the APU generator
What components does an APU oil system always have
- A reservoir
- An oil supply pump
- An oil cooler
- An oil filter
- Oil supply lines
- Oil scavenge lines
The lower part of the APU gearbox forms the oil reservoir. What components would we find on here for oil
- A filling tube
- A sight glass
- A pressure fill connection
- An overflow connection
What are the 3 types of supply pump which can be used in the APU oil system
- The gerotor type pump
- The gear type pump
- The vane type pump
What is done with unwanted air/oil mixture in an APU once it has done its job
It is carried back to the gearbox by the scavenge oil and by external vent lines
The air is then removed from the scavenge oil by an air/oil separator and then released into the APU exhaust duct
On some APUs there is an additional vent valve which is installed in the vent line of the APU exhaust duct, what is this for
This valve helps to pressurise the gearbox
In region of 4-5psi, it is very important if the APU is operated at high altitude
It supports the oil supply pump and it minimises oil foaming
What components would a generator oil scavenge system have
- A scavenge pump which is usually installed in the APU gearbox
- A separate oil scavenge filter
- A scavenge oil return line to the APU gearbox
What would happen if generator oil temperature becomes too high
The APU control unit initiates an automatic APU shut down
What is the function of the APU master switch
The master switch supplies electrical power to the APU control unit which starts its power up test
What type of motor is attached to the APU gearbox which is used for start up
DC
What type of motor is the dc starter motor
Brush type
What are the main components of a typical APU starter
DC motor and clutch
What is the purpose of brushwear indicators
They show if the brushes on the starter motor are worn out
The indicators are usually small inspection windows. Behind the window are the indicator pins which show the length of the brushes
What is the electrical equivalent of brush wear indicators
Electronic brush wear indicators
The sensors send signals to the APU control unit when the brushes reach their wear limits
Other than electrical brush wear indicators what other electrical signal would the APU send to the cockpit
A signal is sent from the starter motor to the APU control unit. The APU control unit knows how much voltage is available at the starter motor terminals.
The signals and information would be used for trouble shooting purposes
Why must you have a cool down period before starting an APU after it has been running
Due to the starter motor needing a very high current to accelerate the APU rotor the starter motor and the battery become very hot
What are the 3 starter limitations that must always be observed during an APU start
- The starter duty time
- The starter cool down time
- The starter duty cycle
They make sure the starter equipment is not over stressed or over heated
What are the main components of a APU ignition system
- The APU control unit
- The ignition exciter
- The ignition leads
- The igniter plugs
What is the purpose of the ignition exciter
Converts low DC voltage in to the high voltage for the igniter plugs
How long after the fuel/air mixture starting to combust in the combustion chamber does the APU ignition switched off
This happens at either starter cut out speed or when the APU reaches operational speed
Why must the APU generator start button be in the OFF position when APU is started up
To make sure that the APU generator is not connected to the aircraft electrical circuits by mistake
What are the most important indications that you need to pay attention to during APU start up
- The battery voltage
- The speed Indication
- The EGT Indication
How long after the APU master switch has been selected to off does the APU fully close down
After the cool down cycle has completed
What is the hoist equipment called that is required to remove the APU?
The fish pole hoist
What happens to the EGT when the bleed air is switched off?
It decreases
On Boeing Aircraft, how many switches are there to start the APU?
1 switch; a master switch with an additional start position
