15.18 APU

Question 1

What does the APU do?

Answer 1

Supplies bleed air and electrical power.

Question 2

Where is the APU used?

Answer 2

On the ground

Question 3

How are free turbines designed?

Answer 3

To run as constant speed without a CSDU.

Question 4

What are the 2 air supply’s from the APU?

Answer 4

From compressor directly.
Separate Load compressor (LC) = only supplies air

Question 5

What are the 3 sections of the APU?

Answer 5

Power.
Bleed air.
Access gearbox.

Question 6

What is electrical and mechanical power required for ?

Answer 6

Engine starting
Pneumatics for cabin air conditioning system.
Shaft power for other pneumatic and hydraulic systems
Back up power.
Ground operation power.

Question 7

Up until what heights can the APU function?

Answer 7

40 000ft

Question 8

What type of combustion chamber is used on modern APUs?

Answer 8

Annular type

Question 9

What monitors the start sequence and turbine speed of the APU?

Answer 9

Electrical Control Box (ECB)

Question 10

Describe the APU fire extinguishing system?

Answer 10

Automatic.

Question 11

What are the 3 operating modes of the APU?

Answer 11

Start= 0-100%
Normal= Maintains 100% speed
Shut down= Cool down

Question 12

How is the APU start sequence operated?

Answer 12

Manually.

Question 13

What is the start sequence of the APU?

Answer 13

1. Electrical power supplied by batteries or GPU.
2. System test.
3. Air inlet door opens.
4. APU control unit monitors acceleration until 100%

Question 14

Why is APU acceleration monitored Upto 100%?

Answer 14

To optimise APU start sequence.

Protect APU against damage.

Question 15

What automatically controls the APU acceleration process?

Answer 15

Speed signal.

Question 16

What controls the fuel metering of the APU?

Answer 16

APU control unit.

Electrically controlled for all acceleration.

Question 17

What are the 2 tasks of The APU control unit?

Answer 17

Ensures generator drive remains at 100%

No excessive EGT is reached.

Question 18

What are the 2 signals for APU constant speed control?

Answer 18

Speed signal

Torque motor signal.

Question 19

What is the purpose of the torque motor signal?

Answer 19

Adjusts fuel flow to maintain 100% speed.

Question 20

What is the effect of high bleed loads on the APU?

Answer 20

Slows rotor,
Uses more fuel
Increases EGT

Question 21

How is APU shutdown initiated?

Answer 21

Master switch is set to OFF.

Question 22

How long is the cool down period of the APU?

Answer 22

0-120s

Question 23

During APU shutdown, what occurs before fuel is removed?

Answer 23

Loads are removed and a cool down period.

Question 24

Where does the APU get its fuel from?

Answer 24

Main engine fuel feed supply.

Question 25

Which is the function of pressure switches?

Answer 25

Monitors fuel pressure.

Question 26

Why do we want positive fuel pressure?

Answer 26

To prevent cavitation at the HP pump.

Question 27

What are the 2 bleed air sub systems?

Answer 27

Cooling air. Bleed air.

Question 28

What is cooling air used for?

Answer 28

For the oil cooler and ventilation.

Question 29

What are the tasks of bleed air?

Answer 29

Bleed supply. Surge protection = prevents surge conditions of APU compressors. Load compressor control.

Question 30

When does the APU start sequence end?

Answer 30

When speed is over 95%

Question 31

What are the 2 conditions to open the APU bleed valve?

Answer 31

Bleed switch is ON Start sequence is complete.

Question 32

When does the bleed valve open?

Answer 32

When the solenoid is energised

Question 33

Bleed valve closes when master switch is set to OFF. When else does the APU bleed valve close?

Answer 33

When pneumatic controllers detect an overheating condition near the bleed air ducts.

Question 34

What can be said about aircraft with Load compressors and why?

Answer 34

They are more fuel efficient because the LC can be adapted to the bleed air demand.

Question 35

Can the Load compressor be switched off?

Answer 35

No because it is fixed to the rotor shaft.

Question 36

Can the air inlet of the LC be closed?

Answer 36

Yes by IGVs

Question 37

What position are the IGVs during engine starting?

Answer 37

Fully open as maximum airflow is needed.

Question 38

When are compressors likely to surge?

Answer 38

When bleed loads change.

Question 39

What are the 2 surge protection systems?

Answer 39

Anti surge system= for aircraft without a LC Surge protection= for aircraft with a LC.

Question 40

When does the anti surge valve operate?

Answer 40

Open during flight. It always bleeds off air when surging is possible.

Question 41

When does the surge control valve opens?

Answer 41

When airflow behind the LC decreases or stops.

Question 42

How does an APU automatic shutdown differ from normal shutdown?

Answer 42

There is no cool-down period.

Question 43

How else can the APU be shutdown?

Answer 43

By external emergency shut down switches.

Question 44

What is the quickest way to shutdown the APU?

Answer 44

By activating the fire switch.