M15.18 APU Flashcards
What is the certification achieved by an aircraft possessing and in flight APU?
ETOPS (Extended twin operational performance) certification
In flight APU require a lower starting altitude of what?
Around 15,000-17,000 feet
Capable of operation up to 40,000 feet
Why are APU started during aircraft’s landing phase?
Unloads main engines pneumatic burden and allows the engine to keep compressor air for reverse thrust
Three sections of an APU?
The power section
The bleed section
The accessory gearbox section
The power section of the APU drives what?
The compressor and gearbox
The gearbox drives what APU accessories?
Fuel pump
Oil pump
Cooling fan
A-C generator
On some APU a separate compressor is installed to support the pneumatic system, what is this called?
The load compressor
As with all gas turbine engines, what makes up the power section?
A compressor
A combustion chamber
A turbine
On modern APU what type of combustion chamber is used? What about old ones?
Modern is annular type
Can find can type on older
The APU has what to monitor start sequence, turbine speed, EGT and oil and fuel supply?
Electrical control box
An electronic APU control unit controls the APU in what three different modes?
The start module controls the APU start activation until it’s reached 100% speed
The normal speed mode keeps APU at 100% speed
The APU shut down mode monitors and controls shut down
APU start sequence
Manually operated switches in flight deck to supply electrical power to APU from GPU
A test is then conducted to ensure circuits and sensors function correctly
APU air inlet door opened, APU fuel supply prepared, fuel shut off valve opened, fuel boost pump switched on.
If APU start sequence fails what happens?
APU doesn’t start
Throughout acceleration the fuel metering is controlled by what?
The APU control unit
What two signals does the APU control unit use for constant speed control?
The APU speed signal
The torque motor signal
What factors affect APU operation?
APU bleed load
Electrical load
Air inlet temperature
Air inlet pressure
If APU bleed load increases if what?
The APU bleed valve is open
When the bleed load increases what else increases?
APU EGT (Exhaust gas temperature)
The APU can only shut down after it has operated for a sufficient time without what?
A pneumatic or electric load
Cool down period for APU shut down?
Varies between 0-120 seconds. Adjusted during maintenance
Where is the APU fuel normally taken from?
The aircraft’s main engine fuel feed system
Main components of APU fuel feed system?
APU fuel boost pump
APU fuel shut off valve (known as low pressure fuel shut off valve)
APU fuel feed line
Pressure switches
What does the pressure switch do in the APU fuel system?
Monitors fuel pressure in fuel feed line directly in front of APU and triggers warning if fuel pressure is too low
Having positive fuel pressure in the APU fuel feed line is important why?
prevent cavitation at the high pressure fuel pump of the APU
APU air system is divided into what two sub systems?
Cooling air system
Bleed air system
The cooling air system in the APU supplies air to where?
Oil cooler and APU compartment ventilation
Three main tasks of bleed air system in APU?
Bleed supply
Surge protection
The load compressor control (if it has one)
The load compressor does what in the APU?
Controls the amount of air going over the load compressor to meet the bleed demand of aircraft pneumatic system
What two conditions are needed to open APU bleed valve?
Bleed stitch in on position
APU start sequence must have been completed
Why are APU’s with load compressors more fuel efficient?
Airflow across load compressor can be adapted to the aircraft’s bleed air demands
As it cannot be turned off, how is the air inlet closed off for the load compressor?
Inlet guide vanes
Main components of the load compressor system?
Variable inlet guide vanes
Inlet guide vane actuator with mechanical transmission assembly
APU control unit
When can APU compressors surge?
When bleed load changes
How can APU compressor surge be prevented?
By bleeding off the air that is blocking the compressor discharge
What surge protection is used on APU without load compressors?
Anti surge system
For APU with load compressors what is the method of surge protection?
Surge protection system monitors the airflow in load compressor discharge duct and opens a surge control valve if the compressor surges.
As a general rule,what is said about the surge control valve?
If the airflow behind load compressor decreases or stops the valve opens
Automatic APU shutdown is always activated by what?
APU control unit
Where are the external emergency APU shut down switches?
At the nose landing gear
In the main wheel well
On the refuelling station depending on aircraft size and type
What is the quickest way to shut down the APU?
Activate the fire switch
What will cause an automatic APU emergency shut down?
A fire condition when aircraft is on the ground
Is the APU compartment pressurised?
No
