15.18 Auxiliary Power Units Flashcards
What is the primary purpose of an Auxiliary Power Unit (APU)?
To supply pneumatic bleed air and electrical power for different aircraft systems
The APU allows aircraft operation without ground support equipment and serves as a backup source in flight.
Where is the APU typically located in large commercial aircraft?
In an unpressurized, fireproof compartment within the rear fuselage in the tail cone
This location provides autonomous electrical and mechanical power.
What are the main sections of an APU?
The power section, the bleed section, and the accessory gearbox section
Each section has specific functions, such as driving accessories and supplying bleed air.
What does the power section of the APU include?
A compressor, a combustion chamber, and a turbine
The compressor provides air for combustion and can be single-stage or multiple-stage.
What is the function of the APU control unit?
To control the APU during different operating modes such as start, normal speed, and shut-down
It monitors various parameters like turbine speed and exhaust gas temperature.
Fill in the blank: The APU uses _______ to monitor the start sequence and other operational parameters.
Electrical Control Box (ECB)
The ECB controls automatic shutdown in case of failures.
What happens during the APU start sequence if the pre-start test fails?
The APU will not start
This is a safety measure to prevent damage.
What is the maximum altitude at which in-flight APUs can operate?
Up to 40,000 ft
They typically require a lower starting altitude to ensure sufficient airflow.
True or False: The APU can provide backup electrical and pneumatic power during emergencies in flight.
True
This capability is critical for safety in case of engine failure.
What are the main tasks of the bleed air system in an APU?
- Bleed supply
- Surge protection
- Load compressor control (if applicable)
The bleed air system controls the supply of air to the aircraft’s pneumatic system.
What components are involved in the APU fuel feed system?
- APU fuel boost pump
- APU fuel shut-off valve
- APU fuel feed line
- Pressure switches
These components ensure proper fuel supply and pressure to the APU.
What are the protective systems in place for the 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
These systems help prevent damage and ensure safe operation.
What does the APU bleed valve do?
Controls the supply of bleed air to the aircraft’s pneumatic system
It opens when the APU bleed switch is selected to ‘ON’ and the start sequence is complete.
Fill in the blank: The APU’s surge protection system prevents compressor surging by _______.
bleeding off the air that is blocking the compressor discharge
This is crucial for maintaining stable operation.
What type of turbine is commonly found on larger APUs?
Axial turbines with two or more stages
Smaller APUs may use centrifugal turbines.
What is the role of the variable inlet guide vanes in the load compressor?
Control the quantity of airflow into the load compressor
They adapt airflow to meet the aircraft’s bleed air demands.
True or False: The load compressor can be switched off independently from the APU.
False
The load compressor is fixed to the rotor shaft and cannot be turned off.
What is the purpose of the cooling air system in the APU?
Supplies air for the oil cooler and APU compartment ventilation
This system is essential for maintaining optimal operating conditions.
Describe the APU shutdown process.
The APU is only allowed to shut down after operating without a load for a sufficient cool-down time
The cool-down time varies between 0 and 120 seconds.
What happens if the APU control unit detects a low acceleration rate during start?
The start sequence is stopped to prevent thermal stress
This protects the APU from damage during start.
Fill in the blank: The APU’s fuel metering is controlled _______ to ensure safe operation.
electronically
This allows for optimal acceleration while adhering to EGT limits.
What is the function of the APU fuel boost pump?
To provide positive fuel supply if the tank boost pumps are not available
This prevents cavitation at the high-pressure fuel pump.
What type of combustion chamber is typically used in APUs?
Annular type combustion chambers
Older APUs may use can-type combustion chambers.
What is the purpose of the anti-surge valve?
To bleed off air when surging is possible
When is the anti-surge valve normally opened?
During flight to prevent surging
Why is the anti-surge method not very fuel efficient?
Air is also bled off when there is no surging
What is a better surge protection method used on APUs with load compressors?
A surge protection system that monitors airflow and opens a surge control valve if the compressor surges
What happens to excessive airflow when the surge control valve opens?
It is discharged to atmosphere through the APU’s exhaust duct
Under what condition does the surge control valve open?
If the airflow behind the load compressor decreases or stops
What triggers an automatic APU shutdown?
The APU control unit
In which operational modes can an automatic shutdown be activated?
In any operational mode
What does the automatic shutdown protect the APU from?
Damage if operating limits are exceeded or important APU components are defective
How quickly is the stop command activated during automatic shutdown?
Immediately without any cool-down period
List the reasons for initiating an automatic shutdown.
- Excessive EGT
- Over-speed
- Low oil pressure
- High oil temperature and compressor surging
- Fire
What must be done in case of an APU emergency shut-down?
The APU must be switched off immediately without any cooling-down period
Where are the external emergency shut-down switches typically located?
- At the nose landing gear
- In the main wheel well
- On the refueling station depending on aircraft type or size
When is an automatic APU emergency shutdown activated?
In a fire condition when the aircraft is on the ground
How is the APU fire extinguished during an emergency?
Automatically by its fire extinguishing system
What is the quickest way to shut down the APU in an emergency?
Activate the fire switch
How can the extinguisher system for the APU be discharged?
Manually by the crew and autonomously by the APU controller if a fire is detected
