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CAA Module 15 > Module 15.18 Auxiliary power units > Flashcards

Module 15.18 Auxiliary power units Flashcards

1
Q

What are the two functions of the APU?

A
  • Supplies pneumatic bleed
  • Supplies electrical power
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2
Q

When is the APU primarily in use?

A

On the ground

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3
Q

APU’s are what type of engine?

A

Turboshaft

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4
Q

The free turbine in an APU is designed to run at what speed?

A

Constant speed

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5
Q

Does the APU require a CSDU?

A

No

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6
Q

What are the two methods of getting bleed air from the APU?

A
  • Compressor section of the APU
  • Separate load compressor driven by the free turbine
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7
Q

What are the 3 main sections of the APU?

A
  • Accessory gearbox section
  • Power section
  • Bleed section
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8
Q

Where is the APU usually located?

A

In an unpressurised, fireproof compartment in the empanage

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9
Q

Having an APU is one of the primary requirements to achieve what?

A

ETOPS certification

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10
Q

When are inflight APU’s often started?

A

Landing phase

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11
Q

What is the purpose of APU use during landing phase?

A

Unloads the main engines pneumatic burden and allows the engine to keep compressor air to reserve thrust for TOGA

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12
Q

Depending on the design the compressor can be?

A

Single or multi stage

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13
Q

What type of combustion chambers are APU’s?

A

Annular

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14
Q

What does the Electrical Control Box (ECB) monitor?

A
  • Start sequence
  • Turbine speed
  • EGT
  • Oil and fuel supply
  • Bleed air outlet
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15
Q

In the case of a failure what does the ECB do automatically?

A

Shutdown to prevent further damage

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16
Q

How is the APU fire extinguishing system activated?

A

Automatically

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17
Q

What are the 3 modes of operation?

A
  • Start mode, from start to 100%
  • Normal speed, keeps APU at 100%
  • APU shutdown
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18
Q

How is the APU start sequence operated?

A

Manually by flight deck switches

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19
Q

Once start has been selected the APU control unit carries out what?

A
  • System test to ensure circuits and sensors are functioning correct
  • APU inlet door commanded to be opened
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20
Q

The APU control unit monitors the acceleration of the APU for what two reasons?

A
  • Optimise APU start sequence
  • Protect APU from damage
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21
Q

High thermal stress on the APU is caused by what?

A

Acceleration rate is too low

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22
Q

If the acceleration rate is too low what does the ECB do?

A

Stops the start sequence

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23
Q

The APU control unit uses what two signals for constant speed control?

A
  • APU signal speed
  • Torque motor signal
24
Q

In the normal mode what must the APU control unit ensure?

A

Generator drive remains constant at 100%

25
Q

What does the ECB do when comparing APU actual speed with necessary speed?

A

Changes the torque motor signal to change fuel metering

26
Q

When will the torque motor signal be changed?

A

Changes in load

27
Q

What increase when the bleed load increases?

A

APU EGT

28
Q

What occurs when the master APU switch is set too off?

A

APU prepared for safe shutdown

29
Q

What occurs in a safe shutdown?

A

Loads are removed. A cool down period of between 0 and 120 second begins. After the cool down period the fuel supply stops

30
Q

Where is the APU fuel taken from?

A

Main engine fuel ffed system

31
Q

Where does the APU fuel ffed line run?

A

Along the inner wall of the fuselage, behind the sidewall linings of the aft cargo compartments

32
Q

There is a minimum of how many pressure switches in the primary APU fuel system?

A

1

33
Q

What does the fuel pressure switch trigger a warning for?

A

Low pressure

34
Q

Positive fuel pressure is required for what reason?

A

To prevent pump cavitation

35
Q

The APU air system is divided into what two sub-systems?

A
  • Cooling air system
  • Bleed air system
36
Q

The cooling air system provides air for what?

A

Oil cooler and the APU compartment

37
Q

What are the 3 main tasks of the APU bleed air system?

A
  • Bleed supply
  • surge protection
  • Load compressor control (if it has it)
38
Q

What two conditions must be met to open the APU bleed valve?

A
  • APU bleed switch selected to be on
  • APU start sequence must be complete
39
Q

When does the APU start sequence end?

A

When the APU speed becomes greater than 95%

40
Q

What de-energises the APU bleed valve solenoid?

A

Shut-down signal

41
Q

What are the main components of the load compressor system?

A
  • Variable inlet guide vanes
  • Inlet guide vane actuators
  • APU control unit (ECB)
42
Q

When would the APU bleed valve also close?

A

Overheat detected

43
Q

The APU surge control system actuates what valve?

A

Surge control valve

44
Q

Can the load compressor be switched off?

A

No, as it is fixed to the rotor shaft

45
Q

How can air to the load compressor be closed off?

A

Closing inlet guide vanes

46
Q

What position do the IGV’s move to when the APU bleed switch is set to on?

A

Partially open

47
Q

What position do the IGV’s move to during engine start?

A

Fully open

48
Q

When can APU compressors surge?

A

Bleed load changes

49
Q

Anti-surge system is fitted on what APU’s?

A

APU’s without a load compressor

50
Q

When does the anti surge valve bleed air?

A

Conditions when surging is possible and in flight

51
Q

When would surge protection be used?

A

APU’s with load compressors

52
Q

When does the surge control valve open in the surge protection?

A

When the airflow behind the load compressor decreases or stops

53
Q

In automatic shutdown is there a cool down period?

A

No

54
Q

Where are the external emergency APU shutdown switches located?

A
  • NLG wheel well
  • Main wheel well
  • On the refuelling station (depends on aircraft size and type)
55
Q

What are the 3 APU shutdown modes?

A
  • Manual (operated the switch)
  • Automatic
  • Emergency
56
Q
A

CAA Module 15 (18 decks)

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