APU

Question 1

What are the different modes and when do they occur?

Answer 1

APU Attended Mode - When any engine is running or starting or when airborne

APU Unattended Mode - Operates all other times the aircraft is on the ground with no engines running or starting

Question 2

When will the APU shutdown in Attended Mode?

Answer 2

APU fire/inlet over temperature
overspeed/loss of overspeed protection
APU controller failure
Speed Droop

Question 3

When will the API shutdown in Unattended Mode

Answer 3

Generator oil filter approaching bypass
intake door failure
no combustion on start
no acceleration on start

Question 4

When does APU Limit displace on EICAS

Answer 4

High EGT
High Oil Temperature
Low Oil Pressure

Question 5

When would the APU automatically start?

Answer 5

Loss of both Main and Backup generators

Question 6

Explain APU Shutdown

Answer 6

Rotate APU selector to OFF begins shutdown cycle of 105 seconds by closing the APU BLEED AIR VALVE. APU continues running for a cooldown period. EICAS memo APU COOLDOWN during this time. After cooldown, FUEL VALVE closes and APU shuts down.

Question 7

What causes the APU bleed valve to open automatically?

Answer 7

It is armed to open whenever the APU is running and the altitude is less than 22,000 ft

Question 8

What other fuel pump source is available to the APU?

Answer 8

There is a DC fuel pump in the left main for APU start when no AC is available.

Question 9

How many fire warning loops does the APU have?

Answer 9

2

Question 10

Does the APU have overheat protection?

Answer 10

No, only fire detection…no overheat detection.

Question 11

How many loops are required to sense a fire for auto shutdown of the APU?

Answer 11

On the ground - only one loop

In the Air - both loops must be functioning and sensing fire.

Question 12

What bus does the APU automatically connect to when started on the ground?

Answer 12

Left main AC bus.

Question 13

What will cause an automatic shutdown of the APU on the ground?

Answer 13

1. Fire warning
2. high oil temp
3. low oil pressure
4. High EGT
   * *On the ground, APU fire warning also discharges the bottle.

Question 14

What will cause an automatic shutdown of the APU in flight?

Answer 14

APU fire

Question 15

If the APU auto shuts down in flight because of a fire warning, will the bottle discharge automatically?

Answer 15

No, the crew will have to discharge it.

Question 16

What happens when you pull the APU fire switch?

Answer 16

1. Shuts off fuel, air, electric
2. Closes the APU door
3. Arms the bottle for discharge

Question 17

When is the APU fire switch lock automatically released?

Answer 17

When a fire is detected.

Question 18

What does the APU FAULT light mean?

Answer 18

1. High EGT
2. High oil temp
3. Low oil pressure

Question 19

When does the APU start automatically?

Answer 19

In flight if both AC transfer busses lose power.

Question 20

What is the altitude limit for APU starting?

Answer 20

43,100 ft.