Turbine Auxillary Power Unit Flashcards

1
Q

Name the main components of a typical APU:

A

A small power turbine for power and bleed air, and an electrical generator.

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2
Q

When is the greatest demand placed on an APU?

A

When supplying bleed air.

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3
Q

How is an APU generally started?

A

With its own electrical starter and battery power.

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4
Q

Where does the APU get its fuel supply?

A

From the aircraft’s main fuel tanks.

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5
Q

At what speed does a gas turbine APU operate and how is this speed maintained?

A

->At or near its rated speed (100%)

regardless of electrical or pneumatic loads imposed. The APU fuel control automatically adjusts the fuel flow to maintain the rated speed.

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6
Q

What is used to prevent a heavily loaded APU from exceeding its maximum EGT?

A

A load control valve modulates the pneumatic load to maintain EGT’s within limits.

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7
Q

How is an APU shut down and why is this procedure used?

A

After the APU is unloaded by closing the bleed air valve, it is run for a specified amount of time to allow the EGT to cool and stabilize. It takes around 3 minutes. If a heavily loaded APU is abruptly shut down without any cooling period, damage may occur as result of thermal shock.

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8
Q

What powers the variable inlet guide vanes (IGV) used on some APU’s to regulate compressor intake airflow?

A

Fuel Pressure

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9
Q

What is the function of an APU on modern transport category aircraft?

A

To supply ground electrical and pneumatic power to aircraft systems when the engines are not operating, and in some aircraft as a backup source of inflight electrical power.

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10
Q

Name at least two uses for APU produced pneumatic power.

A

(1)Engine Starting (2) Ground air conditioning (3) Anti-icing.

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11
Q

How is the cool-down for many APU’s accomplished prior to shutdown?

A

By running the APU for at least 3 minutes , period of time after closing the bleed air valve.

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12
Q

What is the operating effect of following a required cool-down period prior:

A

The APU’s EGT is lowered and stabilized.

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13
Q

How is an APU’s operating speed maintened despite variations in the loads imposed?

A

By the APU’s fuel control automatically adjusting the fuel flow.

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