M15.18 Auxiliary Power Units Flashcards
What is the Auxiliary Power Unit (APU)?
A small gas turbine turboshaft engine that supplies pneumatic bleed air and electrical power for aircraft systems.
What is the primary function of the APU on the ground?
It allows the aircraft to operate without ground support equipment and provides bleed air for engine starting.
What is a free turbine arrangement in an APU?
An arrangement where the compressor is not affected by changes in load on the free turbine, providing flexibility.
Where is the APU typically located in large commercial aircraft?
In an unpressurized, fireproof compartment within the rear fuselage in the tail cone.
List the main functions of the APU.
- Starting power for main engines
- Pneumatic power for cabin air conditioning
- Shaft power for pneumatic and hydraulic systems
- Backup electrical and pneumatic power for emergencies
- Power for ground operations with engines shut down
What is the significance of ETOPS in relation to the APU?
ETOPS certification requires an in-flight APU to provide electrical and pneumatic power in case of an engine shutdown.
What are the three main sections of an APU?
- The power section
- The bleed section
- The accessory gearbox section
What is the role of the power section in an APU?
It drives the compressor and the gearbox, which powers APU accessories.
What components are included in the power section of the APU?
- A compressor
- A combustion chamber
- A turbine
What type of combustion chamber is commonly found in APUs?
An annular type, though can-type combustion chambers may be found in older models.
What does the APU Electrical Control Box (ECB) monitor?
- Start sequence
- Turbine speed
- Exhaust Gas Temperature (EGT)
- Oil and fuel supply
- Bleed air outlet
What are the three operating modes of the APU control unit?
- Start mode
- Normal Speed Mode
- Shut-down mode
Fill in the blank: The APU control unit monitors the acceleration rate to protect the APU from _______.
damage
What happens when the APU master switch is set to OFF?
The APU is prepared for a safe shutdown after operating without a load.
What is the purpose of the APU fuel boost pump?
To supply fuel from the aircraft’s main-engine fuel feed system to the APU.
What are the two sub-systems of the APU air system?
- The cooling air system
- The bleed air system
What are the main tasks of the bleed air system?
- Bleed supply
- Surge protection
- Load compressor control (if applicable)
What conditions must be met to open the APU bleed valve?
- APU bleed switch must be ON
- APU start sequence must be complete (speed > 95%)
What is the function of the load compressor in an APU?
To adapt airflow to the aircraft’s bleed air demands, improving fuel efficiency.
List the protective systems present in an APU.
- Turbine Overspeed
- Excessive EGT
- Excessive inlet air temperature
- Low oil pressure
- High oil pressure
- Fire detection
- Bleed air delivery duct overheating
- Bleed air overpressure
- APU Surge protection
What is the purpose of the anti-surge system in an APU?
To prevent compressor surging by bleeding off air when surging is possible.
What triggers an automatic APU shutdown?
- Excessive EGT
- Over-speed
- Low oil pressure
- High oil temperature
- Compressor surging
- Fire
Where are external emergency shut-down switches typically located on an aircraft?
- At the nose landing gear
- In the main wheel well
- On the refueling station
True or False: The APU can be shut down immediately without a cool-down period in case of an emergency.
True
