Auxiliary Power Unit (APU)

Question 1

Primary purpose of the APU?

Answer 1

Provide pneumatic and electrical AC power - for engine starting and supply bleed air to the ECS. Supplies 115V 40 kVA to the electrical system.

Question 2

What does the APU EMER STOP BUTTON (GUARDED) do? What happens when an APU fire is detected?

Answer 2

• Closes APU shutoff valve
• Shutting down APU with no cooldown period

When APU fire detected, upper half of button illuminates red

Question 3

What supplies fuel when only DC power is available or AC power?

What about when the engine is running?

Answer 3

If DC - DC Fuel pump provides fuel

If AC power is available and the engine is not running, fuel will be provided by the AC fuel pump.

With the engine running, the ejector fuel pump provides fuel to the APU from the right wing tank. It is possible to provide from the left via a crossfeed valve.

Question 4

The APU can supply:
Electrical power up to:
Bleed air for engine start up to:
Bleed air for air conditioning and pressurisation up to:

Answer 4

Electrical - 33,000ft
Bleed air engine start - 21,000ft
Bleed air for air conditioning and pressurisation - 15,000ft

Question 5

Max alt for APU start?

Answer 5

30,000ft

Question 6

How many seconds after APU speed has reached 95% is electrical and pneumatic loading available?

What happens if the APU does not reach proper speed or acceleration rate within the starting cycle time?

Answer 6

3 seconds

It automatically shutsdown

Question 7

During the normal shutdown sequence how long does it take for electrical and pneumatic power to be removed?

Answer 7

Electrical - after a 1 minute cooldown period
Pneumatic - immediately

Question 8

In the event of the APU emergency stop button being pressed what happens?

Answer 8

APU fuel shutoff valve closes and the APU shuts down without a cooldown period

Question 9

In flight - what does the APU automatically shutdown for?

Answer 9

• Overspeed
• Underspeed
• FADEC critical fault

Question 10

To avoid unsuccessful attempt to start it is recommended to wait how long after the EICAS is energised to start the APU?

Answer 10

30 Seconds

Question 11

APU fuel supplied by which pumps? No engines running, engines running and no DC?

Answer 11

NO: DC fuel pump
Engines: ejector right wing
no DC: AC PUMP 2