15.18 Auxiliary Power Units Flashcards
What type of engine is the APU?
Small gas turbine turboshaft used for bleed air and electrical power
What is the APU used for?
On the ground power, also provides bleed air and electrical pose in flights as a backup
Where is the APU usually located in an aircraft?
in an unpressurised, fireproof compartment within the rear fuselage in the tail cone of an aircraft
What does the APU do
Starting power for the main engines.
Pneumatic power for cabin air conditioning systems.
Shaft power for other pneumatic and hydraulic systems.
Backup electrical and pneumatic power for in-flight operations and emergencies (see ETOPS information below).
Electric and pneumatic power for ground operations with the engines shut down.
What can the APU do in flight if an engine shuts down?
Should an in-flight main engine shutdown occur, the APU can then provide the electrical and pneumatic requirements from the lost engine
When are in flight APUs usually started?
Usually started during an aircraft’s landing phase. This unloads the main engine’s pneumatic burden and allows the engine to keep compressor air to use for reserve thrust should an aborted landing or a go-around emergency event occurs
Does the APU need a constant speed drive?
No because it runs at a constant speed
How can you get air off an APU?
A compressor or a separate load compressor
What are the 3 main sections of an APU?
The power section
The bleed section
The accessory gearbox section
What does the power section of the APU drive?
The compressor and the gearbox
This APU gearbox drives all APU accessories what are they?
The fuel pump
The oil pumps
The cooling fan
The AC-generator
What are the power section in all gas turbine engines?
A compressor
A combustion chamber
A turbine
The compressor of the power section is mainly used to provide the air for combustion, what can it be?
It can be either a single-stage or a multiple-stage compressor
What is the in flight APU starting altitude?
In-flight APUs usually require a lower starting altitude (around 15 000 to 17 000 ft) to ensure enough primary/combustion air and secondary/cooling air for start but are then capable of operation up to 40 000 ft. altitude
What is the combustion chamber on an APU?
usually an annular type, but on older APUs, you can also find can-type combustion chambers
The APU has its Electrical Control Box (ECB), what does it monitor?
It monitors the start sequence, turbine speed, Exhaust Gas Temperature (EGT), the oil and fuel supply and the bleed air outlet
What does the electrical control box control in case of failure?
Am automatic shutdown to prevent further damage
What fire protection does the APU have?
Automatic fire extinguisher
How does the APU supply the AC generator?
At a constant speed
An electronic APU control unit controls the APU in three separate operating modes what are they?
The start mode controls the APU from start activation until it has reached 100% speed
The Normal Speed Mode keeps the APU at 100% speed regardless of load changes
The APU shut-down mode monitors and controls APU shut-down
What is the APU start sequence?
Electrical power must be supplied to the APU, APU carries out a system test to ensure circuits and sensors are functioning correctly, the APU fuel supply is prepared, the fuel shutoff valve is opened, the APU fuel boost pump is switched on and the APU control unit activates the APU starter motor
What happens if the pre start test fails?
The APU will not start
What does the APU do until 100% speed is reached?
APU control unit monitors and controls the correct acceleration
Why does the APU control unit monitor the acceleration?
To optimize the APU start sequence and to protect the APU against damage