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APU Flashcards

1
Q
  1. What’s the purpose of APU?
A

Supply flight crew with auxiliary source of bleed air and electrical power during ground operations. In-flight, APU is used for backup electrical power or used for air for main engine start. APU air cannot be used for pressurization.

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2
Q
  1. What are APUs two on-speed modes?
A

Non-essential if on ground and essential in-flight.

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3
Q
  1. What’s the indication that air inlet door has opened and APU is ready to start?
A

Cyan “APU Ready” annunciation illuminates on IRS / APU / Batt page of OHPTSs.

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4
Q
  1. When should you check APU oil level?
A

5 mins to 24 hours after shutdown to prevent over-servicing.

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5
Q
  1. What’s the purpose of APU load control valve?
A

It opens to provide bleed air from APU while aircraft is on ground, as well as in-flight if needed, for main engine start. It’s controlled by APU BLEED AIR switch.

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6
Q
  1. When is APU bleed air available?
A

After APU reaches operating speed. If APU was started under cold conditions (i.e., EGT < 149°C), bleed air loading is delayed for 60 seconds unless attempting main engine start inflight.

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7
Q
  1. When selecting APU BLEED AIR switch to ON during ground operations, what valve opens in addition to the load control valve?
A

Isolation valve

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8
Q
  1. Regarding APU, two things occur with right engine lower cowl door open. What are they?
A

Amber Cowl Door Open annunciation appears on APU panel and ECU inhibits start while on ground in order to protect engine cowl.

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9
Q
  1. What function does ECU perform if APU limits are exceeded?
A

It automatically shuts down APU on ground (non-essential mode) and in some cases, in flight (essential mode).

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10
Q
  1. Name things that occur when APU MASTER switch is selected to ON.
A

Power-up Built-In Test (PBIT),
Inlet door open,
Fuel shutoff valve open.
Nav lights
Left main fuel pump ON,
TROV opens.

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11
Q
  1. What happens when APU MASTER switch is selected to OFF?
A

Switch legend extinguishes, APU fuel shutoff valve closes and APU, if running, immediately shuts down. Inlet door also begins to close.

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12
Q
  1. What’s the recommended action if amber FAULT light illuminates?
A

Cycle MASTER and give it another try.

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13
Q
  1. What’s the significance of ON legend in APU START / STOP switch?
A

It indicates that starter is engaged or APU is running.

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14
Q
  1. What happens when APU START / STOP switch is selected when APU is running?
A

APU load sheds and APU enters cool-down mode. If on ground and up to 20,000 feet, ECU decreases APU RPM on linear scale (1/2% per second) for 60 seconds (to 70% RPM), then shuts down APU. If above 20,000 feet, ECU maintains APU RPM for 60 seconds, then shuts down APU.

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15
Q
  1. What happens if APU START switch is depressed at any time during 60 second cool down period?
A

APU is commanded to 100% RPM.

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16
Q
  1. What’s the quickest way to shutdown APU?
A

Any time APU MASTER switch is selected off, APU immediately shuts down (APU fuel shutoff valve commanded closed).

17
Q
  1. When does APU Generator come on line?
A

When APU reaches operating speed.

18
Q
  1. Why does APU GEN switch indicate amber ON?
A

It represents abnormal in-flight condition.

19
Q
  1. What’s the difference between essential and non-essential?
A

In non-essential mode, there are many conditions which would cause ECU to shutdown the APU. If this were to occur, cool down mode is bypassed, APU’s fuel shutoff valve is closed, and APU shuts down immediately. When in essential mode (in-flight), some protective shutdowns are disabled to allow continued operation.

20
Q
  1. What happens if APU is operating in essential mode after landing?
A

APU continues to assume aircraft is in essential mode until APU MASTER switch is selected off (or 15 min has elapsed).

21
Q
  1. What’s an amber APU Essential CAS message indicate?
A

It means it hasn’t shutdown for one of the malfunctions that would have caused shutdown on ground. Don’t shut it down if you really need it because a subsequent start may be inhibited. If you don’t need it, shut it down promptly.

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