APU

Question 1

APU start sequence

Answer 1

1. ) Battery Master on
2. ) Wait 30 seconds then test the fire system
   
   • All 9 fire lights should illuminate and 2 master warning lights illuminate
   • FIRE SYS OK advisory message
3. ) Anti-ice off
4. ) Batteries (main and APU) above 22volts
5. ) Press the APU PWR/FUEL switchlight and check for the following messages
   
   • APU DOOR OPEN
   • APU IN BITE status message
   • APU EGT and RPM gauges are displayed
   • APU SOV OPEN status message
6. ) Wait for APU IN BITE status message to disappear
7. ) Press the APU START/STOP switchlight and check for the following
   
   • Start switch illuminates
   • APU START message displayed
   • APU start light extinguishes (46-60% RPM)
   • APU START message disappears
   • APU AVAIL switchlight turns green (99%rpm + 2 seconds)

Question 2

APU shutdown sequence

Answer 2

1. ) Press the START/STOP which deselects it
2. ) AVAIL light should extinguish
3. ) Wait for for the APU DOOR CLOSE indication to appear
4. ) press the PWR/FUEL switchlight which deselects it
5. ) APU RPM and EGT gauges should disappear after approximately 10 seconds

Question 3

How does the GCU command the the APU to stop?

Answer 3

It commands an APU over speed condition which tells the APU ECU to shut down

Question 4

What is the APU priority?

Answer 4

Electrical first, then pneumatics

Question 5

What is the APU rating?

Answer 5

40KVA, 115VAC, 400HZ, 3 Phase

Question 6

What is the APU ECU?

Answer 6

Electronic control unit. It controls and monitors all facets of APU operation.

Question 7

How is an APU compressor surge prevented?

Answer 7

By venting some compressor air overboard via a surge control valve

Question 8

What’s the APU’s maximum operating altitude?

Answer 8

41,000’

Question 9

What’s the maximum altitude for APU starting?

Answer 9

37,000’

Question 10

Maximum altitude for APU bleed air extraction?

Answer 10

25,000’

Question 11

Where can you get a textual description of the position of the APU door?

Answer 11

EICAS primary page

Question 12

What does it mean when the APU door position on the status page is a sequences of dashes?

Answer 12

The APU door position is unknown

Question 13

Is there a hazard area for the APU exhaust?

Answer 13

No, due to the height of the exhaust.

Question 14

Does the APU share oil with the engines?

Answer 14

No, it has its own self-contained oil system

Question 15

How is the APU oil cooled?

Answer 15

It’s cooled via a heat exchanger. APU intake air passes directly over the cooling fins of the heat exchanger. This air is then used to ventilate the APU compartment and then exhausted out the tail pipe.

Question 16

What happens on the ground when the APU ECU sends high oil temperature or low oil pressure?

Answer 16

ECU automatically shuts down the APU

Question 17

What happens in flight if the ECU senses a loss of APU oil pressure or high APU oil temperature?

Answer 17

You’ll receive a caution message on the EICAS primary page, but the APU will continue to operate.

Question 18

Where does the APU get fuel from?

Answer 18

The left collector tank

Question 19

What provides fuel pressure for APU start and operation?

Answer 19

A dedicated APU fuel pump, which takes fuel from the left collector tank

Question 20

What valve allows/disallows fuel to flow to the APU?

Answer 20

APU fuel shut off valve (SOV)

Question 21

If the APU fuel pump fails while the APU is running, what will happen?

Answer 21

An internal bypass valve will open, and the high-pressure, gear-driven fuel pump draws in fuel, allowing the APU to keep running.

Question 22

Which batteries are required for APU start?

Answer 22

Both APU and main batteries

Question 23

What is the minimum battery voltage for APU start?

Answer 23

22V minimum on both batteries

Question 24

Which battery supplies power to crank the APU starter?

Answer 24

APU battery

Question 25

Which battery supplies power to the APU ECU?

Answer 25

MAIN battery

Question 26

What will happen during an APU flameout?

Answer 26

The high-energy ignition unit, controlled by the ECU, will energize the igniter and attempt to auto relight.

Question 27

Maximum APU RPM

Answer 27

106%

Question 28

Maximum APU EGT

Answer 28

806ºC

Question 29

Minimum ambient temperature for APU starting

Answer 29

-40ºC

Question 30

APU starter motor limitations

Answer 30

- A two minute delay must be observed between APU starter cranking attempts to allow for APU starter/contactor cooling, and APU fuel drainage. - No more than three start attempts in a one hour period

Question 31

When is the APU surge control valve command open automatically by the APU ECU?

Answer 31

When the APU is supplying electrical loads above 17,000’. This reduces APU surge potential.

Question 32

What controls the APU load control valve?

Answer 32

Both the ECU and the ACSC. Generally the load control valve is modulated to provide 45psi to the common bleed air manifold.

Question 33

How can the APU be shutoff externally?

Answer 33

2 ways. Via an emergency stop switch in the APU compartment on the forward firewall. Also via a guarded APU shutoff switch inside the external service panel on the right side of the nose.

Question 34

Where can you pictorially see the position of the LCV?

Answer 34

ECS synoptic page

Question 35

Can the APU supply bleed air to the anti-ice system?

Answer 35

No. The system logic prevents this.

Question 36

What does it mean if “PUMP/FAIL” is indicated on the APU PWR FUEL switchlight?

Answer 36

That the APU fuel pump has failed

Question 37

What does it mean if “SOV FAILED” is illuminated on the APU PWR FUEL switchlight?

Answer 37

That the APU fuel shutoff valve is in an uncommanded position.

Question 38

What are the for small exposed tubes on the left side of the aft fuselage near the tail cone?

Answer 38

APU drains

Question 39

What happens when you press the APU fire switch?

Answer 39

In no particular order… 1. ) The ECU closes the APU fuel SOV (if not already automatically done) 2. ) The APU fire bottle squib is armed and the BOTTLE ARMED PUSH TO DISCH switchlight is illuminated 3. ) APU fuel solenoid valve is closed 4. ) APU load control valve (LCV) is closed 5. ) APU generator is tripped offline

Question 40

Will the APU fire extinguisher automatically go off in flight?

Answer 40

No

Question 41

Will the APU fire extinguisher bottle automatically go off on the ground?

Answer 41

Yes, 5 seconds after the APU fire is detected