15.18 - Auxiliary Power Unit Flashcards
What is the purpose of the APU?
Provide electrical power
Provide pneumatic duct pressure for air conditioning and engine starting
What are the main parts of the APU?
The power section and the load compressor. The load compressor produces pneumatic power and also drives the accessory gearbox which contains the electrical generator
How many compressors does a modern APU have?
2 centrifugal load compressors
What is the ‘hotel mode’ on a turbo prop?
The free turbine and propeller shaft is mechanically stopped from turning using a brake. The gas turbine then runs, supplying power like an APU.
It is costly to run, but saves weight from having no APU required
What controls and monitors the APU?
The electronic control box (ECB)
What altitude is bleed air shut off at from an APU?
25,000 ft
What type of combustion chamber is installed in an APU?
A reverse-flow annular design
When is an APU start cycle completed?
95% RPM
What is the purpose of the surge valve on an APU?
It bleeds off air to the exhaust that could surge the compressor
What is the lube module?
It supplies pressurised oil supply to the APU components and has a scavenge system
When will an APU automatically shut down?
Over speed shutdown (106% +)
ECB failure
HOT shutdown
Emergency shutdown triggered - fire, overheat, emergency shut down switch on fuselage
What needs to be installed if the APU inlet is manually deactivated and placed into the open position?
A jumper lead to close the circuit allowing the APU to start
What are the three electronic speed control switches set at?
35%, 95% and 100%
What is the purpose of the centrifugal switch on the APU gearbox?
It cancels the ignition circuits and arms the overspeed protection circuits
