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M15 > APU > Flashcards

APU Flashcards

1
Q

Why is positive fuel pressure extremely important?

A

Prevent cavitation at high pressure fuel pump.

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2
Q

What is an advantage for maintenance of a vent push button?

A

Bypasses APU master switch.

Purge the APU fuel feed line without someone in cockpit.

(APU fuel boost pump started and APU fire shut off valve opens)

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3
Q

What is the APU air system divided into?

A

Cooling air system

Bleed air system.

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4
Q

What 3 tasks does the bleed air system have?

A
  • Bleed supply
  • Surge protection
  • Load compressor control
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5
Q

Why do APU’s have load compressors?

A

Reduce fuel consumption by APU.

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6
Q

Can APU’s supply electrical power and bleed air at the same time?

A

Yes, but normally only one used.

When bleed air switched off, EGT decreases.

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7
Q

How is the air inlet to the load compressor closed off?

A

Inlet guide vanes. (Mechanically actuated)

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8
Q

When are APU’s most likely to surge?

A

When bleed load changes.

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9
Q

How is compressor surging prevented?

A

Bleeding off air that is blocking compressor discharge.

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10
Q

Where are anti-surge systems installed?

A

APU’s with no load compressor.

Normally opened during flight, not very fuel efficient

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11
Q

What is the best method of preventing surging?

A

Preventing surging on APU’s with load compressors, where airflow is monitored and only bled if compressor surging occurs.

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12
Q

What is the airflow in the compressor discharge duct equal to?

A

Difference between static and total pressure.

Used to open and close surge control valve

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13
Q

Why does the oil system have a de-oiling valve?

A

Improve APU start sequence.

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14
Q

What does the a APU system cool?

A

APU generator.

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15
Q

Where is the APU oil reservoir?

A

Lower part of the APU gearbox.

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16
Q

What are the 3 types of supply pump for the APU oil system? (Always positive displacement pump)

A

Gerotor type
Gear type
Cane type

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17
Q

How is air removed from the APU oil system?

A

Carried by scavenged oil.

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18
Q

How does the air/oil separator work?

A

Has vanes Mounted on a drive gear which rotates and separates the air/oil mist.

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19
Q

After an air/oil separator where does the oil go?

A

Returns to the sump (gearbox)

by gravity

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20
Q

What would an additional vent valve installed in the APU exhaust duct help?

A

Pressurise the gearbox. (Approx 4-5 psi)

Important at high altitudes.

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21
Q

What are the advantages of a pressurised gearbox?

A

Supports the oil supply pump and minimises oil foaming.

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22
Q

What happens if the oil temperature becomes too high?

A

APU automatically shuts down.

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23
Q

What does the master switch for the APU do?

A

Supplies electrical power to the APU control unit which starts its power up test.

After successful test, fuel shut off valve opens.

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24
Q

Where is the APU start relay usually located?

A

Electronic equipment compartment.

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25
Q

How are APU’s usually started?

A

DC motors on APU gearbox.

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26
Q

What type of DC motor is the starter motor?

A

Brush type.

Brush wear indicators show if the brushes on the starter motor are worn

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27
Q

How do you check the brush wear indicators?

A

Small window, with an indicator pin which shows the length of the brushes.

Or electrical sensors which send wear levels to cockpit.

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28
Q

What is the voltage feedback signal on the APU starter for?

A

Feedback signal for troubleshooting to show what component has caused APU start problem.

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29
Q

What happens when the starter relay unit is opened at starter cut off speed?

A

Battery is disconnected and APU continues to accelerate without starter support.

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30
Q

Why does the battery and starter motor become very hot?

A

Very high currents are needed to accelerate the APU rotor.

After each APU start must be cooled before using again

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31
Q

How is the APU ignition system usually powered?

A

Direct current. (From APU control unit)

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32
Q

What does the ignition exciter do?

A

Converts low voltage DC power into high voltage for igniter plugs.

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33
Q

Is the ignition system activated in parallel with the APU start system?

A

Yes.

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34
Q

When is the APU ignition switched off?

A

Happens either at cut out speed or when the APU reaches operational speed.

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35
Q

What does the APU generator push button do?

A

Connects the APU to the aircrafts electrical circuits.

36
Q

During a fire warning test for the APU, what also occurs when external power is available?

A

Aural warning

As well as red light illuminating and squib and discharge lights illuminating

37
Q

What would a decrease in battery voltage show on start indications for the APU?

A

APU starter is accelerating the APU.

After starter cut out, battery voltage increases again as it draws energy from APU

38
Q

When are the ECAM indications only available?

A

If AC power is switched on.

39
Q

What is the quickest way of shutting down the APU?

A

Activate the fire switch.

40
Q

What is the purpose of the APU?

A

Supply pneumatic bleed air and electrical power for different systems.

41
Q

In flight, in the event of bleed air and Engine AC generator failure, what can done?

A

Run the APU to supply pneumatics and electrical power

42
Q

What section of the APU drives the compressor and gearbox?

A

The power section.

43
Q

What accessories will the power section of the APU drive?

A
  • Fuel pump
  • oil pump
  • cooling fan
  • ac generator
44
Q

What is the load compressor?

A

Ensures sufficient pneumatics are supplied to the aircraft pneumatics system

45
Q

What comes from the APU compressor for the aircraft?

A

Pneumatic bleed air

46
Q

What are centrifugal flow compressors resistant from?

A

FOD

47
Q

What type of combustion chamber are found on modern APU’s?

A

Annular

Can-type on older

48
Q

What type of turbines will you find on small and larger APU’s?

A

Centrifugal on small

Axial on larger with two or more stages

49
Q

What is the air pressure usually delivered by the APU?

A

30 to 45 psi

50
Q

What 2 ways can methods of bleed air are there on APU’s?

A

Compressor

Individual load compressor

51
Q

Why is fuel efficiency and life time of an APU with an load compressors increased?

A

Compressors can be switched off I’d bleed air is not required by the aircraft.
(EGT limits not reached as easy as air is not taken from combustor)

52
Q

How can the airflow be changed on demand from the pneumatic system?

A

Inlet guide vanes which can modulate in any position between open and closed.

53
Q

Why do APU generators not need a constant speed drive?

A

Is always controlled at a constant speed by the APU fuel control unit

54
Q

What compartments is the APU and it’s components enclosed by?

A

APU compartment,
Equipment compartment
Muffler compartment

55
Q

What equipment does the APU equipment compartment house?

A

Air-intake actuator
APU extinguisher bottle
Fuel supply line
Bleed air duct

56
Q

How many mounts support the APU?

A

3

Right/left Aft and forward mounts

57
Q

What connect the APU mounts on large aircraft?

A

APU mount links

Links and rods which transmit forces to structural points

58
Q

What is each APU mount designed to take?

A

2 for flexibility allowing for thermal expansion/limited movement
1 for lateral loads from APU to the aircraft

59
Q

What loads are all 3 APU mounts designed to take?

A

Lateral loads

60
Q

What is the name for the hoist equipment used to remove an APU for overhaul?

A

Fish pole hoist and a support stand

61
Q

What aircraft is an intake flap usually found on?

A

Airbus

62
Q

Where is the APU intake door usually found for Boeing aircraft?

A

Right hand upper side of the fuselage

63
Q

What does the APU intake door prevent?

A

FOD, birds and aerodynamic drag in flight

64
Q

How is the APU inlet door operated?

A

With an electrical Actuator

Operated by 28 volts dc power

65
Q

If closed, what will stop the APU for operating?

A

The intake door, must be open to operate.

66
Q

What is the APU intake nose?

A

Forms optimum aerodynamic surface for lower APU air intake

67
Q

What does a diffuser do?

A

Increases static pressure of airflow passing through it

APU inlet duct

68
Q

What does the APU intake elbow form?

A

Air duct between the air inlet duct and the APU air inlet

69
Q

What are the two types of APU drain?

A

Direct (catch fluids and drain overboard)

Indirect (catch fluids and drain to special drain tank connected to drain mast)

70
Q

During flight, what does airflow at the drain mast cause?

A

Suction in the suction line.

71
Q

What modes does the APU control unit operate?

A

Start (until reaches 100% speed)
Normal speed mode (remains at 100% speed)
APU shut down mode

72
Q

For APU start, what is the start initiation?

A

Electrical power supplied to APU control unit and carries out pre-start checks

73
Q

What does the APU control unit carry pre-start checks on?

A

Electrical circuits and condition of the sensors

74
Q

Once pre-start checks have been completed, what should happen next?

A

APU air inlet door is opened

75
Q

What 2 signals will the APU control unit use to ensure it remains at 100% speed?

A

APU speed signal
Torque motor signal
(Maintain speed by comparing actual speed with necessary speed)

76
Q

What are the 3 methods of APU shut down?

A
  • manual by the master switch
  • automatically when APU malfunctions
  • emergency (fire switch or emergency shut down switch)
77
Q

When will an APU shut down?

A

Once it has operated for a sufficient time without electrical or pneumatic load.
(Between 0-120s can be adjusted)

78
Q

In automatic/emergency shutdown mode, how is the APU stopped?

A

Imminently without cool down

79
Q

Where are the APU emergency shut down buttons on the aircraft?

A

Nose wheel
Main wheels
Refuelling station depending on type or size of Ac.

80
Q

How is the APU extinguished of a fire on modern aircraft?

A

Automatically by a special extinguisher

81
Q

What are the primary and secondary fuel systems on the APU?

A

Primary - LP fuel system (tanks to fuel control unit)

Secondary - HP fuel system (fuel control unit to fuel nozzles)

82
Q

What system will allow fuel for the APU to be taken from any tank?

A

Cross-feed manifold

83
Q

What is the purpose of the APU boost pump?

A

Gives positive fuel supply to the APU during the start sequence when the fuel pressure is too low.

84
Q

What is the APU fuel shut off valve for?

A

Serves as an isolation valve. Will be shut when the APU is not running.

85
Q

What’s the purpose of fuel feed line?

A

Supply’s fuel from the tanks to the APU via the side walls in the bulk cargo compartment.

86
Q

What is the minimum number of pressure switches In the APU fuel system?

A

1

Second switch operates boost pump if pressure in cross feed is too low

M15 (18 decks)

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