APU

Question 1

What are the start sources for the APU, and which one do we normally use?

Answer 1

• APU Battery, when Utility Bus 4 is unpowered.
• APU Battery is the normal switch position.
TR when switch is selected and Util bis 4 is powered

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

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

Question 4

What APU components are powered by the APU battery?

Answer 4

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

Question 5

What APU components are powered by the main battery?

Answer 5

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

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)

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

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.

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.

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.

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 after the APU selector is rotated to OFF, and why?

Answer 12

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

Question 13

What would cause the APU to shut down automatically?

Answer 13

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