15.18 APUs

Question 1

Whats the main function of an APU?

Answer 1

To provide electrical and pneumatic power for ground and backup in-flight operations.

Question 2

What are the three main sections of an APU?

Answer 2

Power section, bleed section, accessory gearbox.

Question 3

What does the APU Electrical Control Box (ECB) do?

Answer 3

Monitors operation and triggers auto shutdown on fault.

Question 4

What are the three APU control modes?

Answer 4

Start Mode (to 100%)
Normal Speed Mode (maintains 100%)
Shut-down Mode

Question 5

What conditions must be met for the APU bleed valve to open?

Answer 5

APU bleed switch ON and speed >95%.

Question 6

What is the role of Inlet Guide Vanes (IGVs) in load compressors?

Answer 6

Control air intake to adapt to aircraft’s bleed air demand.

Question 7

What triggers automatic APU shutdown?

Answer 7

A: Overspeed, high EGT, oil issues, surge, fire.

Question 8

How does the surge control valve protect the APU?

Answer 8

Dumps airflow when load compressor discharge drops.

Question 9

What happens during APU cool-down period?

Answer 9

APU runs without load to avoid thermal stress before shutdown.

Question 10

Where are emergency shutdown switches located?

Answer 10

Nose gear, main gear well, or refuelling station.

Question 11

Where the emergency APU shutdown switches?

Answer 11

Nose landing gear

Main wheel well

Refuelling station

Question 12

What are the signals used by the APU control unit to maintain constant speed?

Answer 12

The APU control unit uses the APU speed signal and the torque motor signal to maintain constant speed by adjusting fuel metering.