APU

Question 1

List the altitude limitations for bleed air usage and electrical Power usage?

Answer 1

Bleed air: 22’000ft
Electrical Power: 43’100 ft max certified alt

Question 2

What type of starter systems are there for the APU and which starter has priority?

Answer 2

The ATS (Air Turbine Starter), the electrical is secondary

Question 3

What supplies Power for the APU electrical starter?

Answer 3

A dedicated APU battery

Question 4

Can a ground cart be used to start the APU?

Answer 4

Yes

Question 5

Which AC fuel pump runs automatically for APU operation?

Answer 5

The one from left fuel manifold

Question 6

What fuel pump is activated if the AC is not working for APU start?

Answer 6

Left DC fuel pump

Question 7

When does the starter for the APU engage and disengage?

Answer 7

Engage: When inlet door reaches full open position
Disengage: APU reaches 50%

Question 8

When will the APU do an autostart?

Answer 8

If both AC transfer busses loses power

Question 9

What are the APU:s operating modes?

Answer 9

Attendend and Unattended

Question 10

What will cause an APU automatic shutdown in the Attended mode?

Answer 10

1. APU fire/inlet over temp
2. Overspeed, more than 106%
3. APU controller failiure
4. Speed droop, below 88% for 10 seconds

Question 11

Does the APU perform a cooldown during an automatic shutdown
?

Answer 11

No

Question 12

What does the APU message APU LIMIT indicate during attended mode operation?

Answer 12

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

Question 13

When is the APU operating in Unattended mode?

Answer 13

Basically on the ground when engines are not operating

Question 14

How is the APU shut down after an Auto start?

Answer 14

Position the APU selector to ON and then to OFF

Question 15

Describe the starting sequence for the APU?

Answer 15

BAT ON and APU GEN PUSH SWITCH ON
When you move the APU selector to ON the APU fuel shutoff valve and the APU air inlet door opens. The APU DC fuel pump turns on if AC power is not available. If AC power is available the left tank forward pump operates.

Fuel is sent to the APU fuel cluster where a boost pump and a gear pump increases the fuel pressure.
APU starter engages when the air inlet door is fully open and the RPM is less than 12%
During start, the output from the gear pump is sent directly to the pressurizing shutoff valve. This valve permits flow from the fuel metering valve to the fuel divider when the valve is open. The pressurizing shutoff valve opens at 7% rpm and the igniter plugs energizes immediately after that. The igniter plugs are turned off at 50% rpm and the air turbine starter control valve closes at 55% rpm. The APU continues to speed up and two seconds after the APU reaches 95% rpm, bleed air and electrical power becomes available for use. If during start the APU stops it’s acceleration for more than seven seconds, the APU controller(APUC) shuts down the APU.

Question 16

Why is Power from the main battery needed for APU start?

Answer 16

Power is needed to operate:

1. APU inlet door
2. Fuel valve
3. Fire detection

Question 17

What happens if there is not enough bleed air available for the ATS to start the APU?

Answer 17

An electrical starter is used that is powered by a dedicated APU battery

Question 18

What happens in flight with regards to the APU when main both Generators and back-up GEN goes off-line?

Answer 18

The APU starts automatically even with the switch to OFF

Question 19

Describe shutdown procedure for the APU below 14’000 ft?

Answer 19

Select switch to OFF
APU bleed load is removed.
APU is now in a 105 sec cooldown cycle. It runs at 100% for 15 seconds(90 sec), then decelerates down to 84,4% for 8 seconds(82 sec). Then it decelerates down to 70% for the remaining 82 seconds when the fuel metering valve stops fuel flow. Air inlet doors starts to close at 15%

Question 20

What does “Attended Mode” mean?

Answer 20

Means that there less faults that will cause automatic shutdown since at least one engine is running and someone probably is looking after the status of the aircraft

Question 21

Above what field elevation is APU to Pack Take-off prohibited?

Answer 21

6’900 ft

Question 22

How much fuel is needed to start and run the APU for the first hour?

Answer 22

430 kg. Means that it uses 100 kg for 15 min which then forms part of the 700 kg “Taxi Fuel” on the CFP

Question 23

What is the difference in how the APU is shutdown above and below 14’000 ft?

Answer 23

Above 14’000 ft it runs at 100% during the whole 105 seconds cool down cycle.

Question 24

What is max oil quantity for the APU?

Answer 24

8,2 quarts (7,8 liters)

Question 25

When does the EICAS “APU OIL QTY” show?

Answer 25

When oil quantity is below 3,8 quarts (3,5 liters)

Question 26

What is considered normal and low oil pressure for the APU?

Answer 26

Normal is around 70 psi
Low is 35 psi

Question 27

What is considered normal and high EGT for the APU?

Answer 27

Normal is around 400-500 deg C
High 1288 deg C

Question 28

What is considered normal and high oil temperature for the APU

Answer 28

Normal is around 80 deg C
High is 156 deg C

Question 29

How is starting of the APU assured at high altitude in extreme cold?

Answer 29

By three gearbox heater pads made of aluminum that heats up to about 232 deg C

Question 30

In Unattended mode, for what situations will the APU shut down automatically?

Answer 30

For all conditions in the Attended mode plus:

• Low oil pressure (below 35 psi)
• High EGT (above 1288 deg C)
• High oil temperature (above 156 deg C)
• Intake door failure
• No combustion/acceleration on start
• Generator scavenge filter is clogged

Question 31

What is the EICAS indication for a protective APU shutdown?

Answer 31

“APU SHUTDOWN” advisory message

Question 32

In attended mode, for what situations will the APU shut down automatically?

Answer 32

• APU fire/inlet over temperature
• Overspeed(>106% rpm)/loss of overspeed protection
• Speed droop, less than 88% rpm an no acceleration for 10 seconds
• APUC failiure