15.18 Auxiliary Power Units Flashcards
What type of engine is the APU?
Small gas turbine turboshaft used for bleed air and electrical power
What is the APU used for?
On the ground power, also provides bleed air and electrical pose in flights as a backup
Where is the APU usually located in an aircraft?
in an unpressurised, fireproof compartment within the rear fuselage in the tail cone of an aircraft
What does the APU do
Starting power for the main engines.
Pneumatic power for cabin air conditioning systems.
Shaft power for other pneumatic and hydraulic systems.
Backup electrical and pneumatic power for in-flight operations and emergencies (see ETOPS information below).
Electric and pneumatic power for ground operations with the engines shut down.
What can the APU do in flight if an engine shuts down?
Should an in-flight main engine shutdown occur, the APU can then provide the electrical and pneumatic requirements from the lost engine
When are in flight APUs usually started?
Usually started during an aircraft’s landing phase. This unloads the main engine’s pneumatic burden and allows the engine to keep compressor air to use for reserve thrust should an aborted landing or a go-around emergency event occurs
Does the APU need a constant speed drive?
No because it runs at a constant speed
How can you get air off an APU?
A compressor or a separate load compressor
What are the 3 main sections of an APU?
The power section
The bleed section
The accessory gearbox section
What does the power section of the APU drive?
The compressor and the gearbox
This APU gearbox drives all APU accessories what are they?
The fuel pump
The oil pumps
The cooling fan
The AC-generator
What are the power section in all gas turbine engines?
A compressor
A combustion chamber
A turbine
The compressor of the power section is mainly used to provide the air for combustion, what can it be?
It can be either a single-stage or a multiple-stage compressor
What is the in flight APU starting altitude?
In-flight APUs usually require a lower starting altitude (around 15 000 to 17 000 ft) to ensure enough primary/combustion air and secondary/cooling air for start but are then capable of operation up to 40 000 ft. altitude
What is the combustion chamber on an APU?
usually an annular type, but on older APUs, you can also find can-type combustion chambers
The APU has its Electrical Control Box (ECB), what does it monitor?
It monitors the start sequence, turbine speed, Exhaust Gas Temperature (EGT), the oil and fuel supply and the bleed air outlet
What does the electrical control box control in case of failure?
Am automatic shutdown to prevent further damage
What fire protection does the APU have?
Automatic fire extinguisher
How does the APU supply the AC generator?
At a constant speed
An electronic APU control unit controls the APU in three separate operating modes what are they?
The start mode controls the APU from start activation until it has reached 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 shut-down
What is the APU start sequence?
Electrical power must be supplied to the APU, APU carries out a system test to ensure circuits and sensors are functioning correctly, the APU fuel supply is prepared, the fuel shutoff valve is opened, the APU fuel boost pump is switched on and the APU control unit activates the APU starter motor
What happens if the pre start test fails?
The APU will not start
What does the APU do until 100% speed is reached?
APU control unit monitors and controls the correct acceleration
Why does the APU control unit monitor the acceleration?
To optimize the APU start sequence and to protect the APU against damage
What automatically controls the acceleration process?
The speed signal
What does the APU control unit do?
The APU Control Unit monitors the APU acceleration rate during the start sequence. If the acceleration rate is too low, it takes a long time for the APU to reach 100% speed
What must the APU control unit do when the APU is at full speed?
Must make sure the generator drive remains constant at 100% and no excessive EGT is reached
What are the two signals the APU uses for constant speed control?
The APU speed signal
The torque motor signal
How does the APU maintain a constant speed?
The APU control unit maintains a constant speed by comparing the actual APU speed with the necessary speed
During normal operation why can the speed change?
The APU bleed load
The electrical load
The air inlet temperature
The air inlet pressure
How is the APU kept at 100% at high bleed loads?
By adding more fuel
How long can the cool down be for the APU?
It can vary between 0 and 120 seconds and can be adjusted during maintenance on some apus if required
What happens when the cool down period has passed?
the control unit closes off the fuel supply to the combustion chamber so that the APU comes to a stop.
Where is fuel taken from for the APU?
The fuel for the APU is usually taken from the aircraft’s main-engine fuel feed system
What are the main components of the APU fuel systems?
The APU fuel boost pump
The APU fuel shut-off valve, also known as APU low-pressure fuel shut-off valve
The APU fuel feed line
Pressure switches
There is a minimum of one pressure switch in the APU fuel lines what does this do?
This switch monitors the fuel pressure in the APU fuel feed line directly in front of the APU and it triggers a warning if the fuel pressure is too low
Why is it important to have positive fuel pressure in the APU fuel line?
to prevent cavitation at the high-pressure fuel pump of the AP
What are the two APU bleed air systems?
The cooling air system
The bleed air system
What does the cooling air system supply supply air for?
Supplies air for the oil cooler and the APU compartment ventilation
What are the three main tasks of the bleed air subsystem?
Bleed supply
Surge protection
If the APU has a load compressor, the load compressor control
What does the load compressor system do?
Controls the amount of air going over the load compressor to meet the bleed demand of the aircraft pneumatic system
What are the two conditions that must be met to open the APU bleed valve?
The APU bleed switch must be selected to ‘ON’.
The APU start sequence must be complete; (the APU start sequence ends when the APU speed becomes greater than 95%)
When does the start sequence end?
When it’s greater that 95%
The APU bleed valve stays open until what?
As long as the solenoid remains energised there is no shutdown signal to the APU
What is the reason for this time delay?
makes sure that the APU runs with no load during the cool-down time
When else do the APU bleed valves close?
The APU bleed valve also closes when the pneumatic controllers detect an overheating near the bleed air ducts.
Do all APUs have s load compressor?
No
What is an advantage of a load compressor?
They are more fuel efficient
Why can’t the load compressor be turned off?
Because it is fixed to a rotor shaft
How can you shut off bleed air to the load compressor?
By closing the inlet guide vanes
What are the main components of the load compressor system?
The variable inlet guide vanes
The inlet guide vane actuator with a mechanical transmission assembly
The APU control unit
What does the main engine start require?
maximum air bleed for the engine starter
When is the APU most susceptible to surges?
When the bleed load changes
How can surging be prevented?
It can be prevented by bleeding off the air that is blocking the compressor discharge
What are the two surge protection methods used on APUs?
Anti surge system and surge protection
How does the anti surge system work?
Has an anti surge valve which always bleed off the air when surging is possible
Where is surge protection used (the second method)?
On APUs with load compressors
How does the surge protection work?
Here a surge protection system monitors the airflow in the load compressor discharge duct and opens a surge control valve if the compressor surges. The excessive airflow is discharged to atmosphere through the APU’s exhaust duct
What is APU automatic shutdown always activated by?
Always activated by the APU control unit, it can be activated in any operational load
How does automatic APU shutdown protect the APU?
This protects the APU from damage if operating limits are exceeded or important APU components are defective
What are some of the reasons for automatic shutdown?
Excessive EGT
Over-speed
Low oil pressure
High oil temperature and compressor surging
Fire
Where are the external emergency shut down switches on an aircraft?
At the nose landing gear
In the main wheel well
On the refuelling station depending on aircraft type or size
