APU

Question 1

Up to which altitude can the APU be started and operated ?

Answer 1

Maximum certified altitude : 43 100 ft

Question 2

Can the APU be started if the left CCR is not online ?

Answer 2

NO

The APU controller will delay the start until the required common core applications are available.

Left CCR will take 2-3 minutes to start.

Question 3

What are the 2 operating modes of the APU ?

Answer 3

1. Attended : (Flight deck manned)
   
   • When either engine is starting or running,
   • When the airplane is in flight.
2. Unattended : (flight deck maybe unmanned) All other configurations on the ground.

Question 4

What is the difference between Attended and Unattended mode ?

Answer 4

In both modes, the APU will auto shutdown under different conditions (fire, overspeed, controller failure, speed droop…). BUT :

• in Attended mode, the 3 following conditions will NOT trigger an auto shutdown:
  
  • High EGT
  • High oil temperature
  • Low oil pressure

Question 5

When does the amber caution APU LIMIT appear on the EICAS ?

Answer 5

When the APU is in Attended mode, and one of the following faults occurred :

• High EGT
• High oil temperature
• Low oil pressure

Note : These conditions would trigger an Autoshutdown in Unattended mode.

Question 6

Is there a cool down period after an APU shutdown (manual or auto) ?

Answer 6

• YES (2 minutes) if normal manual shutdown,
• NO if Auto shutdown or through the APU FIRE handle.

Question 7

Can the APU be started with NO fuel pumps operating ?

Answer 7

YES :

On the ground or in-flight up to 14 000 ft

Question 8

If AC power is available, which fuel pump will feed the APU ?

Answer 8

The LEFT aft fuel pump

Question 9

If AC power is NOT available, which fuel pump will feed the APU ?

Answer 9

The Left dedicated DC pump

Question 10

How many starter/generators does the APU have ?

Answer 10

2

Only one at a time is used for starts.

Question 11

When starting on battery, how many start attempts are possible ?

Answer 11

2

The APU battery is designed to support 2 consecutive start attempts. (if the second attempt fails, a 5 minutes cooling period must be observed before a third attempt is made)

Question 12

Is there a cooling period to observe in case of failed battery start ?

Answer 12

YES, after the second failed attempt : 5 minutes

Question 13

Will the APU ever automatically start ?

Answer 13

YES

In flight, if three or more engine generators are off-line the APU automatically starts, regardless of the APU selector position.

Question 14

If an automatic APU start was triggered in-flight, can it be shutdown ?

Answer 14

YES

• While the automatic start condition(s) exist(s), the APU can only be shut down by pulling the APU fire switch.
• When the automatic start condition is no longer valid, the APU can be shut down by positioning the selector to ON, then OFF.

Question 15

When positioned to OFF, does the APU shutdown immediately ?

Answer 15

NO

There is a 2 minute cool down period during which APU COOLDOWN is displayed in white on the EICAS.

Question 16

Is the APU overspeed protection ever tested ?

Answer 16

Yes, at every normal shutdown !

After the 2 minute cooldown period is finished, an APU overspeed condition is simulated to trigger a fuel shutoff protection, thus shutting down the APU.

Question 17

What are the Attended and Unattended modes ?

Answer 17

• Unattended : On the ground, no engines running. The flight deck might be uncrewed (Unattended).
• Attended : On the ground with either engine starting or running and in flight. The flight deck is crewed (Attended).