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M15 > 15.18 APU > Flashcards

15.18 APU Flashcards

1
Q

What does the APU do?

A

Supplies bleed air and electrical power.

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2
Q

Where is the APU used?

A

On the ground

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3
Q

How are free turbines designed?

A

To run as constant speed without a CSDU.

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4
Q

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

A

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

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5
Q

What are the 3 sections of the APU?

A

Power.
Bleed air.
Access gearbox.

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6
Q

What is electrical and mechanical power required for ?

A

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

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7
Q

Up until what heights can the APU function?

A

40 000ft

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8
Q

What type of combustion chamber is used on modern APUs?

A

Annular type

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9
Q

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

A

Electrical Control Box (ECB)

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10
Q

Describe the APU fire extinguishing system?

A

Automatic.

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11
Q

What are the 3 operating modes of the APU?

A

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

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12
Q

How is the APU start sequence operated?

A

Manually.

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13
Q

What is the start sequence of the APU?

A
  1. Electrical power supplied by batteries or GPU.
  2. System test.
  3. Air inlet door opens.
  4. APU control unit monitors acceleration until 100%
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14
Q

Why is APU acceleration monitored Upto 100%?

A

To optimise APU start sequence.

Protect APU against damage.

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15
Q

What automatically controls the APU acceleration process?

A

Speed signal.

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16
Q

What controls the fuel metering of the APU?

A

APU control unit.

Electrically controlled for all acceleration.

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17
Q

What are the 2 tasks of The APU control unit?

A

Ensures generator drive remains at 100%

No excessive EGT is reached.

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18
Q

What are the 2 signals for APU constant speed control?

A

Speed signal

Torque motor signal.

19
Q

What is the purpose of the torque motor signal?

A

Adjusts fuel flow to maintain 100% speed.

20
Q

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

A

Slows rotor,
Uses more fuel
Increases EGT

21
Q

How is APU shutdown initiated?

A

Master switch is set to OFF.

22
Q

How long is the cool down period of the APU?

A

0-120s

23
Q

During APU shutdown, what occurs before fuel is removed?

A

Loads are removed and a cool down period.

24
Q

Where does the APU get its fuel from?

A

Main engine fuel feed supply.

25
Q

Which is the function of pressure switches?

A

Monitors fuel pressure.

26
Q

Why do we want positive fuel pressure?

A

To prevent cavitation at the HP pump.

27
Q

What are the 2 bleed air sub systems?

A

Cooling air.
Bleed air.

28
Q

What is cooling air used for?

A

For the oil cooler and ventilation.

29
Q

What are the tasks of bleed air?

A

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

30
Q

When does the APU start sequence end?

A

When speed is over 95%

31
Q

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

A

Bleed switch is ON
Start sequence is complete.

32
Q

When does the bleed valve open?

A

When the solenoid is energised

33
Q

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

A

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

34
Q

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

A

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

35
Q

Can the Load compressor be switched off?

A

No because it is fixed to the rotor shaft.

36
Q

Can the air inlet of the LC be closed?

A

Yes by IGVs

37
Q

What position are the IGVs during engine starting?

A

Fully open as maximum airflow is needed.

38
Q

When are compressors likely to surge?

A

When bleed loads change.

39
Q

What are the 2 surge protection systems?

A

Anti surge system= for aircraft without a LC

Surge protection= for aircraft with a LC.

40
Q

When does the anti surge valve operate?

A

Open during flight.

It always bleeds off air when surging is possible.

41
Q

When does the surge control valve opens?

A

When airflow behind the LC decreases or stops.

42
Q

How does an APU automatic shutdown differ from normal shutdown?

A

There is no cool-down period.

43
Q

How else can the APU be shutdown?

A

By external emergency shut down switches.

44
Q

What is the quickest way to shutdown the APU?

A

By activating the fire switch.

M15 (18 decks)

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