Auxiliary Power Unit

Question 1

Explain APU fuel supply.

Answer 1

DC power only? DC fuel pump in right wing tank. AC power, but engine not running? AC fuel pump. Engine running? Ejector fuel pump from the right wing tank (also possible from the left wing tank via XFEED valve).

Question 2

What does APU bleed supply?

Answer 2

ECS and engine starting.

Question 3

What is considered the brain of the APU and what does it do? (3)

Answer 3

FADEC monitors and controls the start/shutdown sequence, fault detection and APU status.

Question 4

What can the APU supply in flight and up to what altitude can it do that?

Answer 4

Electrical AC power: 33,000 ft. Bleed for engine start: 21,000 ft. Bleed for ECS: 15,000 ft.

Question 5

APU Max start altitude?

Answer 5

30,000 ft.

Question 6

When starting up the APU, what does battery 1 power? Battery 2?

Answer 6

Battery 1: Powers FADEC and EICAS for monitoring. Battery 2: Powers the starter.

Question 7

What happens when you select the APU start switch to OFF? (3)

Answer 7

1 Minute cool down period begins. Bleed air stops immediately but generator keeps working until shutdown.

Question 8

When does the FADEC auto-shutdown the APU on the ground? (8)

Answer 8

UFOSHELF! Underspeed, FADEC fault, Overspeed, Sensor fail, High oil temperature, Excessive EGT, Low oil pressure, Fire (after 10 seconds).

Question 9

When does FADEC auto-shutdown the APU in-flight? (3)

Answer 9

UFO! Underspeed, FADEC fault, Overspeed.

Question 10

How can you shutdown the APU immediately?

Answer 10

APU Emergency Stop button; No 1-minute cool-down.

Question 11

What will happen if the batteries are turned OFF during the 1-minute cool-down?

Answer 11

APU will shut down immediately and an APU Fault may occur.

Do NOT turn the batteries off until the APU shows OFF on EICAS.