APU - Question Bank

Question 1

What are the start sources for the APU?

Which one do we normally use?

Answer 1

• TR (Transformer-Rectifier Unit), when Utility Bus 4 is powered.
• APU Battery, when Utility Bus 4 is unpowered.
• TR is the normal switch position.

Question 2

Can the APU be operated in flight?

Answer 2

Yes, when left running for takeoff is can be operated in flight up to 20,000’ ft

Question 3

Is electrical power available from the APU in flight?

Answer 3

No

APU electrical power is not available in flight.

Question 4

Is APU bleed air available in flight?

Answer 4

Yes

APU bleed air is available in flight for one pack up to 15,000’ft

Question 5

What APU components are powered by the APU battery?

Answer 5

• Inlet door
• APU controller
• DC fuel pump
• APU fire detection system

Question 6

What APU components are powered by the main battery?

Answer 6

• APU fire extinguisher
• APU fuel valve
• Standby power for the APU controller

Question 7

What powers the APU starter during a battery start sequence?

Answer 7

-The APU battery

All other APU components are powered by the main battery while the APU starter is engaged

Question 8

What happens if the TR should overheat with the start source in TR?

Answer 8

• Start power is transferred to the battery

- Further start attempts with an overheated TR are inhibited

Question 9

How does a TR failure affect APU starting?

Answer 9

• Automatic switching to APU battery is not provided

- Moving the start switch to BATTERY removes the TR from the starting circuit and allows APU starting on battery power.

Question 10

Describe the APU shutdown sequence.

Answer 10

• Rotating the APU selector switch to OFF begins the shutdown cycle
• APU bleed air valve closes
• APU continues running unloaded for a 60-sec cooldown period
• When the cooldown period finishes, the APU shuts down.

Question 11

What would happen if the battery switch is positioned off prior to completion of the APU shutdown cooldown period?

Answer 11

The APU shuts down immediately

Question 12

How long should you wait before turning the battery switch OFF prior to completion of the APU shutdown cooldown period?

Answer 12

• 2 mins

- Allows a complete shutdown sequence with fire detection capability

Question 13

What would cause the APU to shutdown immediately?

Answer 13

• A fire is detected
• A limit is exceeded
• Battery switch is turned off during APU shutdown sequence