APU

Question 1

What are the start sources for the APU, and 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.

Note:FCOM, VOL 2; 7.14.2; 7.30.1

Question 2

Can the APU be operated in flight?

Answer 2

• Yes, when left running for takeoff it can be operated in flight up to 20,000 feet

Note:FCOM, VOL 2; 7.30.1

Question 3

Is electrical power available from the APU in flight? How about APU bleed air in flight?

Answer 3

• No, APU electrical power is not available in flight
• Yes, APU bleed air is available in flight for one pack up to 15,000 feet

Note:FCOM, VOL 2; 7.30.1

Question 4

What APU components are powered by the APU battery bus and APU Hott battery bus?

Answer 4

C-F-F-I: Control the fuel fire in the door

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

Note:FCOM, VOL 2; 7.30.1

Question 5

What APU components are powered by the main battery and main hot battery bus?

Answer 5

F-F-S: Fuel Fire Standby

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

Note:FCOM, VOL 2; 7.30.1

Question 6

What powers the APU starter during a battery start sequence?

Answer 6

• The APU battery
• (All other APU components are powered by the main battery while the APU starter is engaged)

Note:FCOM, VOL 2; 7.30.1

Question 7

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

Answer 7

• Start power is transferred to the battery
• Further start attempts with an overheated TR are inhibited

Note:FCOM, VOL 2; 7.30.2

Question 8

How does a TR failure affect APU starting?

Answer 8

• Automatic switching to the 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.

Note:FCOM, VOL 2; 7.30.2

Question 9

When does the EICAS memo message APU RUNNING display?

Answer 9

• When the APU selector is ON and APU N1 RPM exceeds 95% N1.

Note:FCOM, VOL 2; 7.30.1; 7.40.3

Question 10

Describe the APU shutdown sequence.

Answer 10

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

Note:FCOM, VOL 2; 7.30.2

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.

Note:FCOM, VOL 2; 7.30.2

Question 12

How long should you wait before turning the battery switch OFF after the APU selector is rotated to OFF, and why?

Answer 12

• 2 minutes
• Allows a complete shutdown sequence, with fire detection capability.

Note:FCOM, VOL 2; 7.30.2

Question 13

What would cause the APU to shut down immediately?

Answer 13

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

Note:FCOM, VOL 2; 7.30.2