Module 15.18 Auxiliary Power Units Flashcards
What is an Auxiliary Power Unit (APU)?
A small gas turbine (free turbine) turboshaft engine
What does an APU supply?
Pneumatic bleed air and electrical power
The APU is the primary source of bleed air for what?
Engine starting
The free-turbine of an APU is designed to run at what?
Constant speed
Why is the APU free-turbine run at a constant speed?
To ensure it maintains a constant frequency without the need for an additional constant-speed drive unit.
Other than a normal compressor, what can the APU also have?
A load compressor
Where can air be bled from?
The compressor or separate load compressor if it has one.
Where is the APU located?
An unpressurised, fireproof compartment within the rear fuselage in the tail cone.
What aircraft certification does an APU allow?
ETOPS (Extended Twin Operational Performance) certification
APU’s are capable of operation up to what altitude?
40 000 ft
In flight, when is the APU often started?
Landing phase
Why may the APU be started during landing phase?
Unloads the main engine’s pneumatic burden and allows the engine to keep compressor air to use for reserve thrust.
What are the three main sections of the APU?
Power section
Bleed section
Accessory gearbox
What does the power section drive?
The compressor and gearbox.
What accessories does the gearbox drive?
Fuel pump
Oil pumps
Cooling fan
AC-generator
What is a load compressor?
A separate compressor used to provide bleed air.
What components make up the power section of an APU?
Compressor
Combustion chamber
Turbine
How many stages can the compressor have?
Single or multiple stage
What type of combustion chamber is used?
Annular type
In case of a failure, what controls an automatic shutdown?
Electrical Control Box (ECB)
How does the APU supply the AC generator?
At a constant speed
The control unit controls the APU in what three operating modes?
Start mode
Normal Speed mode
Shut-down mode
What does normal speed mode do?
Keeps the APU at 100% speed regardless of load changes.
What two signals does the APU control unit use for constant speed contro?
APU speed signal
Torque motor signal
How does the control unit maintain a constant speed?
It compares the actual APU speed with the necessary speed. It then changes the torque motor signal to change the fuel metering.
What does high bleed loads do to the rotor?
Slows it down because the load compressor draws more energy from the APU power section.
With high bleed loads, how is rotor speed kept at 100%?
Adding more fuel
What does the APU do when the master switch is set to ‘OFF’?
It prepares for a safe shutdown.
When does the APU allowed itself to shutdown?
After it has operated for a sufficient amount of time without a pneumatic or electric load.
How long is the cool-down period?
0 and 120 seconds
Where is fuel for the APU taken from?
The main engine fuel feed manifold.
What does the APU fuel system consist of?
Fuel booster pump
Fuel shut-off valve
Fuel feed line
Pressure switches
The APU fuel shut off valve is also known as what?
APU low-pressure fuel shut-off valve.
What does the APU fuel feed line do?
Supplies fuel from the tanks.
How many pressure switches are in the APU fuel system?
A minimum of 1
Why is positive pressure in the fuel feed line important?
To prevent cavitation at the high-pressure fuel pump.
What two subsystems is APU bleed air divided into?
Cooling air systems
Bleed air systems
What does the cooling air system do?
Supplies air for the oil cooler and the APU compartment ventilation.
What does the bleed supply system control?
The supply of the APUs bleed air to the aircraft’s pneumatic system.
What does the surge protection system do?
Prevents any surge condition of the APU compressors.
What does the load compressor surge system do?
Controls the amount of air going over the load compressor to meet the bleed demand.
Can the load compressor be switch off?
No, because it is fixed to the rotor shaft.
What is the air inlet into the load compressor closed off by?
Inlet guide vanes
What happens to the inlet guide vanes for the load compressor when the APU is switched on?
They move to a partially open position.
When can APU compressors surge?
When the bleed load changes.
How can compressor surge be prevented?
By bleeding off the air that is blocking the compressor discharge.
What are the two surge protection systems?
Anti-surge system
Load compressor surge control valve
What APUs use the anti-surge system?
Those with no load compressor.
When does the surge control valve open?
If the airflow behind the load compressor decreases or stops.
What is automatic shutdown activated by?
APU control unit
What does automatic shutdown do?
Protects the APU from damage.
What must be done in the case of an emergency shutdown?
APU must be switched off immediately without any cooling-down period.
When is APU emergency shutdown activated?
During a fire condition when the aircraft is on the ground.
What happens after the APU shuts down due to a fire emergency?
The fire is extinguished automatically by its fire extinguishing system.
What is the quickest way to shut down the APU during an emergency?
Activate the fire switch.
When is the anti-surge valve operated?
During flight to prevent surging.
Where is excesses airflow to prevent a surge discharged?
Through the APU’s exhaust duct to the atmosphere.
Where are the external emergency shutdown switches located?
Man / nose landing gear
Refuelling station
