15.18 Flashcards
What type of engine is an APU?
A small gas turbine turboshaft engine
What 2 things does the APU supply?
Pneumatic bleed air
Electrical power
Why would the APU be used in flight?
To provide bleed air and electrical power incase of a failure
What makes the APU engine very flexible and why?
The free turbine arrangement because the compressor is not affected by changes in load on the free turbine
What does the free turbine ensure?
That the APU generator runs at a constant frequency meaning there is no need for an additional constant-speed drive unit
What Is common for the free turbine to do?
Drive a separate Load Compressor (LC) to supply the aircraft pneumatic systems
Where is the APU located on large commercial aircraft?
In an unpressurised, fireproof compartment within the rear fuselage in the Tain cone of an aircraft
What 5 things does the APU provide power for?
Starting the main engines
Pneumatic power for the cabin air conditioning systems
Shaft power for other pneumatic and hydraulic systems
Backup electrical and pneumatic power in flight and in emergencies
Electric and pneumatic power for ground operations with the engine shut down
What is one of the primary requirements in ETOPS?
Possession of an inflight APU
What altitude are APUs capable of operating at?
40,000 feet, but need a starting altitude around 15000 - 17000 feet
Why are APUs turned on during an aircrafts landing phase?
To unload the main engines pneumatic burden, to allow compressor air to be used for reserve thrust loads incase of a go-around or aborted landing
What are the 3 main sections of the APU?
The power section
The bleed section
The accessory gearbox section
What drives the compressor and the gearbox?
The power section
What 4 things does the gearbox drive?
The fuel pump
The oil pumps
The cooling fan
The AC-generator
What must the bleed section of the APU ensure?
That sufficient bleed air is supplied to the aircrafts pneumatic system
On the APUs a separate compressor is used for bleed air, what is it called?
The load compressor
What does the APU have on all gas turbine engines?
A compressor
A combustion chamber
A turbine
What are the 2 types of a compressor?
Single-stage
multiple-stage
What type of combustion chamber is used on an APU?
Annular type, on older APUs you can also find can-type
What does the Electrical Control Box (ECB) monitor?
The start sequence, turbine speed, EGT, oil, fuel supply, and bleed air outlet
What does an ECB do incase of a failure?
Controls an automatic shutdown to prevent further damage
What does the APU have to protect it?
A fire protection system which automatically activates a fire extinguisher
What speed does the APU supply the AC generator with?
A constant speed
What are the 3 operating modes than an electronic APU control unit used=s to control the APU?
Start mode - From APU until it reaches 100% speed
Normal speed mode - Keeps APU at 100% speed
APU shut-down mode - Monitors and controls APU shut-down
What is the start sequence operated?
Manually by switches in the flight deck
How is electrical power supplied to the APU?
By selecting the master switch, using batteries or ground power
What does the APU do once start has been selected and why?
It carries out a system test to ensure circuits and sensors are functioning correctly and the APU air inlet door is commanded to open
Will the APU star if the pre-start test fails?
No
What are the 2 reasons the APU controls unit monitors the acceleration of the APU?
To optimise the APU start sequence
To protect the APU against damage
What is the speed signal needed for in the APU?
To automatically control the acceleration process
What does the APU control unit monitor?
APU acceleration
Why does the APU control unit stop the star sequence if the acceleration rate is too low?
Because long operation at low speeds causes high thermal stress to the APU
What 2 signals does the APU control unit use for constant speed control?
The APU speed signal
The torque motor signal
How does the APU control unit maintain a constant speed?
By comparing actual speed with necessary speed, changing the torque motor signal to change the fuel metering
What are 4 reason an APU speed can change?
The APU bleed load
The electrical load
The air inlet temperature
The air inlet pressure
When is an APU prepared for a safe shutdown?
When the master switch is set to off
When is the APU only allowed to shut down?
After it has operated for a sufficient time without a pneumatic or electric load
How long is an APU cool-down period?
0-120 seconds
What are the 4 main components for the APU fuel system?
The APU fuel boost pump
The APU fuel shut-off valve, also known as the APU low-pressure fuel shut-off valve
The APU fuel feed line
Pressure switches
Where do the APU fuel feed lines run?
Along the inner wall of the fuselage of the aircraft, behind the sidewall linings of the aft cargo compartments
What is the minimum amount of pressure switches in the primary APU fuel system?
1
What does the pressure switch monitor in the APU fuel system?
Monitors the fuel pressure in the APU fuel feed line, triggers a waning if the fuel pressure is too low
Why is positive fuel pressure in the feed line important?
To prevent cavitation at the high-pressure fuel pump of the APU
What are the 2 subsystems the APU air system is divided into?
The cooling air system
The bleed air system
What does the cooling air system supply air for?
The oil cooler and the APU compartment ventilation
What are the 3 main tasks of the bleed air system?
Bleed supply
Surge protection
Load compressor control (if there is one)
What does the bleed supply system control?
The supply of APU bleed air to the aircrafts pneumatic system
What prevents any surge condition of the APU compressors?
The surge protection system
If a load compressor is present, what system does it belong to?
The air bleed system
What does the load compressor control system control?
The amount of air going over the load compressor to meet the bleed demand of the aircraft’s pneumatic system
What 2 conditions must be met to open the APU bleed valve?
The APU bleed switch must be selected ON
APU start system completes
When is the APU start sequence complete?
When the APU speed become greater than 95%
What opens the APU bleed valve once the conditions have been met?
A solenoid energising
What does a time delay do when shutting down an APU?
Makes sure that the APU runs with no load during the cool-down time
When do the APU bleed valve also close?
When the pneumatic controllers detect an overheating near the bleed air ducts
What does the load compressor make the engine?
More efficient
Can the load compressor be switched off?
NO, because it’s fixed to the rotor shaft
What is air inlet to the load compressor closed of by?
The inlet guide vanes
What are the 3 main components of the load compressor system?
The variable inlet guide vanes
The inlet guide vane actuator
The APU control unit
What controls the position of the inlet guide vane actuator?
The APU control unit
How can compressor surging be prevented?
By bleeding off the air that is blocking the compressor discharge
What type of APU is the anti-surge system installed on?
One without a load compressor
How does the anti-surge system work?
It bleeds off air when surging is possible
What is automatic APU shutdown activated by?
The APU control unit, in any operational mode
What protects the APU from damage?
The automatic shutdown system
Is there a cooldown period in the when automatic shutdown is initiated?
No
What are 5 reasons why automatic shutdown can be initiated?
Excessive EGT
Over-speed
Low oil pressure
High oil temperature and compressor surging
Fire
Where are the external emergency shut-down switches placed?
In areas where they areas where they are easily accessible for ground personnel
What are 3 examples of where external shut-down switches are?
At the nose landing gear
In the main wheel well
On the refuelling station depending on aircraft type or size
When is an automatic APU emergency shut-down activated in a fire condition?
When the aircraft is on ground
What does the APU do when there is a fire on ground?
Automatically extinguish itself
How can an APU extinguisher be discharged?
Manually by the crew or automatically be the APU controller
