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.

Normally use Battery

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 main hot battery and the main battery bus

Answer 4

• APU Fire Ext (HOT)
• APU Fuel Valve SOV (HOT)
• APU Secondary Controller (MAIN)

(Fire - Fuel - Controller)

Question 5

What APU components are powered by APU battery?

Answer 5

• Inlet Door (Hot)
• APU Primary Controller (Hot)
• DC Fuel Pump (Main)
• APU Fire Detection System (Main)

(Fire - Fuel - Controller + Door)

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

APU Start Cycles

Answer 7

Between start through battery (atlas )

1 and 2 :—-> 1 minute
2 and 3 :—-> 1 minute
3 and 4 :—-> 75 minute

Question 8

What happens if TR overheats with start source in TR?

Answer 8

Start power is transferred to the battery.

Further attempts with overheated TR inhibited.

Question 9

How does a TR failure affect APU starting?

Answer 9

• 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 10

When does the EICAS memo message APU RUNNING display?

Answer 10

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

Question 11

Describe the APU shutdown sequence.

Answer 11

• 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 12

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

Answer 12

• It will shut down immediately as the DC pump is de-energized.
Remember, it’s on the APU MAIN BUS not APU HOT BUS.

Question 13

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

Answer 13

• 2 minutes

* Allows a complete shutdown sequence, with fire detection capability.

Question 14

What would cause the APU to shut down immediately?

Answer 14

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

Question 15

APU Selector: OFF

Answer 15

• APU BLEED CLOSED
• Initiates SHUTDOWN
• resets auto-shutdown fault logic except w/shutdown due to APU bleed air leak

Question 16

APU Selector: ON

Answer 16

• Opens APU fuel valve and inlet door
• Arms APU bleed air isolation valve
• Activates DC or two AC fuel pumps

Question 17

APU Cool down period?

Answer 17

Starts when switch placed in OFF and lasts for one minute (60 seconds)
Wait 2 minutes after shutdown before turning off battery switch. This ensures fire detection capability.

Question 18

APU auto shutdown items (3)?

Answer 18

• Limit Exceedence
• Fire
• Battery Switch Off

Question 19

What fuel pumps provide pressure for APU?

Answer 19

Main Tank 2 AFT AC PUMP
- otherwise -
Main Tank 2 DC

*Main Tank 3 AFT AC PUMP prevents xfer and is a backup.

Question 20

APU generators and load?

Answer 20

• 2 gens
• Capable of supplying entire load of A/C on ground
• Not avail in flight

Question 21

When is APU cool down cycle complete?

Answer 21

When fuel valve closes and APU decelerates.

Question 22

Is APU GEN power available during cool down cycle?

Answer 22

NO