APU

Question 1

What is the primary purpose of the APU?

Answer 1

To provide electrical power to the aircraft buses as a backup to the engine generators.

Question 2

What is the secondary purpose of the APU?

Answer 2

To provide pneumatic pressure for engine start and air conditioning.

Question 3

What kind of generator is connected to the APU?

Answer 3

A 40kva AC generator.

Question 4

What is the maximum operating altitude of the APU?

Answer 4

41 000 ft.

Question 5

What is the maximum height to start the APU?

Answer 5

37 000 ft.

Question 6

What is the APU ECS bleed altitude limit?

Answer 6

25 000 ft.

Question 7

What is the maximum altitude for an APU ground start.

Answer 7

15000 ft.

Question 8

Air from the APU inlet door is ducted to where?

Answer 8

The APU compressor and the APU oil cooler.

Question 9

Where is the APU air inlet door?

Answer 9

The upper right side of the fuselage.

Question 10

What components are mounted onthe APU accessory gearbox?

Answer 10

The generator, oil pump and fuel pump.

ECU speed sensor, starter motor and fuel control unit.

Question 11

What variables control the amount the APU air inlet door is opened?

Answer 11

APU speed, airspeed (MACH number) and weight on wheels (WOW) sensor.

Question 12

If the inlet door position fails (or the door position is unknown), what limitations apply in flight?

Answer 12

Airspeed is limited to 220 kt unless the APU is running.

Question 13

Where does the fuel for the APU come from?

Answer 13

The left collector box.

Question 14

What does the APU load control valve safeguard against?

Answer 14

Excessive EGTs.

Question 15

Is the APU able to supply anti-ice bleed air?

Answer 15

No.

Question 16

Prior to APU start, what should the pilot consider?

Answer 16

Both battery voltages at least 22V.

Altitude limits.

Question 17

When is the APU able to supply electrical power after start?

Answer 17

99% plus 2 seconds - green AVAIL illuminates.

Question 18

How is the inflight APU start sequence different from the ground APU start sequence?

Answer 18

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.

Question 19

When will the APU ECU prevent an APU start?

Answer 19

If the APU is rotating at more than 5% on the ground or 12% in flight.

Question 20

If an APU fire is detected, what happens?

Answer 20

The APU will immediately shutdown, and if on the ground the APU fire bottles will be discharged 5 seconds after shutdown.

Question 21

How is the APU shutdown?

Answer 21

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.

Question 22

What limitations apply if the APU door is stuck open, or position is uncertain, in flight?

Answer 22

APU must be running or reduce speed to 220 kt.

Question 23

When will the APU starter inhibit APU start?

Answer 23

Rotation above 12% in flight or 5% on the ground.

Question 24

What are the APU starter limits?

Answer 24

No more than 3 starts in an hour. 2 minutes between each start attempt to allow cooling and fuel drainage.