APU

Question 1

What is the primary purpose of the APU

Answer 1

Provide electrical power.

Question 2

What is the name of the computer than controls APU operation automatically?

Answer 2

Electronic Control Unit (ECU)

Question 3

When does the APU automatically discharge the bottle in event of a fire?

Answer 3

On the ground after 5 seconds

Question 4

What valve controls APU bleed output?

Answer 4

Load Control Valve (LCV)

Question 5

What computer shares control of the LCV with the ECU?

Answer 5

ACSC1 (Air Conditioning System Controller)

Question 6

What is the critical rotation speed of the APU and why is it important?

Answer 6

4%-30%

Below 4% is safe due to lack of heat that could cause bearing damage.

Above 30% allows sufficient cooling oil to lubricate bearings.

Question 7

What is the minimum battery voltage required to start the APU

Answer 7

22V

Question 8

What happens when the PWR/FUEL switchlight is selected on the ground?

Answer 8

PDIGS

Pump, door, in bite, gauges, SOV

Question 9

What happens when the PWR/FUEL switchlight is selected in the air?

Answer 9

PIGS

Pump, in bite, gauges, SOV

Question 10

What happens when you press START/STOP switchlight on APU start?

Answer 10

Start:
-DC starting motor engages
- APU START status msg
- White START switchlight on
- Fuel + ignition at 5%
- Starter disengages at 46-60% RPM depending on altitude
- 95% de-energize and SOV open removed
- 99% + 2 seconds green AVAIL illuminates

Question 11

What is the difference in APU air start vs ground start?

Answer 11

In air the door is opened when pressing START/STOP instead of PWR/FUEL

Question 12

What happens when you press START/STOP to turn off the APU?

Answer 12

ECU is sent an overspeed signal, Green AVAIL extinguishes, APU SOV closes to stop fuel, RPM/EGT decrease until 25% when intake door closes, Door close displayed on EICAS.

Question 13

When can you restart the APU following normal shutdown?

Answer 13

12% RPM or less.

Limitations: 2 minutes between cranks, no more than 3 starts per hour.

Question 14

Which tank does APU draw fuel from?

Answer 14

Left collector tank

Question 15

Which pump provides pressurized fuel to APU?

Answer 15

APU Pump

Question 16

If the APU pump were to fail, how does the APU continue operating?

Answer 16

Bypass valve allowing APU gearbox driven pump to provide fuel.

Question 17

What does green AVAIL mean on APU START/STOP push button?

Answer 17

APU is available to take electrical load.

Question 18

Where does the APU receive intake/cooling air from?

Answer 18

Eductor on tail pipe uses venturi effect to cool.

Question 19

3 times APU will shut down automatically in the air?

Answer 19

Fire, Overspeed, ECU failure.

Question 20

Which factors influence APU door position in flight?

Answer 20

APU RPM, Mach #, Weight on Wheels

Question 21

Which APU indication is always shown on status page?

Answer 21

Door position

Question 22

What are the ways the APU can be shut down from the flight deck? (3 ways)

Answer 22

1. START/STOP switch
2. Fire bottle
3. POWER/FUEL switch

Question 23

What are the ways the APU can be shut down from outside the aircraft? (2 ways)

Answer 23

1. External services panel
2. Shut off switch in APU compartment for maintenance

Question 24

Max altitude for APU start?

Answer 24

37,000’

Question 25

Max altitude for APU operations?

Answer 25

41,000’

Question 26

Max altitude for using APU bleed air to start engines?

Answer 26

21,000’

Question 27

What is the surge control valve on APU do?

Answer 27

Relieves pressure during compressor surges.

It is always open above 17,000’ because bleed air is not needed and APU is usually only used for electrical loads.