APU Flashcards
What is the primary purpose of the APU?
To provide electrical power to the aircraft buses as a backup to the engine generators.
What is the secondary purpose of the APU?
To provide pneumatic pressure for engine start and air conditioning.
What kind of generator is connected to the APU?
A 40kva AC generator.
What is the maximum operating altitude of the APU?
41 000 ft.
What is the maximum height to start the APU?
37 000 ft.
What is the APU ECS bleed altitude limit?
25 000 ft.
What is the maximum altitude for an APU ground start.
15000 ft.
Air from the APU inlet door is ducted to where?
The APU compressor and the APU oil cooler.
Where is the APU air inlet door?
The upper right side of the fuselage.
What components are mounted onthe APU accessory gearbox?
The generator, oil pump and fuel pump.
ECU speed sensor, starter motor and fuel control unit.
What variables control the amount the APU air inlet door is opened?
APU speed, airspeed (MACH number) and weight on wheels (WOW) sensor.
If the inlet door position fails (or the door position is unknown), what limitations apply in flight?
Airspeed is limited to 220 kt unless the APU is running.
Where does the fuel for the APU come from?
The left collector box.
What does the APU load control valve safeguard against?
Excessive EGTs.
Is the APU able to supply anti-ice bleed air?
No.
Prior to APU start, what should the pilot consider?
Both battery voltages at least 22V.
Altitude limits.
When is the APU able to supply electrical power after start?
99% plus 2 seconds - green AVAIL illuminates.
How is the inflight APU start sequence different from the ground APU start sequence?
On the ground the APU air inlet door opens when the pump light comes on, inflight the air inlet door doesn’t open until the start/stop switch is selected.
When will the APU ECU prevent an APU start?
If the APU is rotating at more than 5% on the ground or 12% in flight.
If an APU fire is detected, what happens?
The APU will immediately shutdown, and if on the ground the APU fire bottles will be discharged 5 seconds after shutdown.
How is the APU shutdown?
Start/stop selector switch followed by the fuel switch.
At the external services panel on the right forward section of the fuselage.
Within the APU housing.
What limitations apply if the APU door is stuck open, or position is uncertain, in flight?
APU must be running or reduce speed to 220 kt.
When will the APU starter inhibit APU start?
Rotation above 12% in flight or 5% on the ground.
What are the APU starter limits?
No more than 3 starts in an hour. 2 minutes between each start attempt to allow cooling and fuel drainage.
