Powerplant APU

On APU’s equipped with a free turbine and load compressor, the primary function of the load compressor is to

• Provide air for combustion and cooling in the engine gas path

• Provide bleed air for aircraft pneumatic system

• Supply the turning force for operation of the APU generator

The function of an APU air inlet plenum is to

• Increase the velocity of the air before entering

  the compressor.

• Decrease the pressure of the air before entering

  the compressor.

• Stabilize the pressure of the air before it

  enters the compressor.

Frequently, an aircraft’s auxiliary power unit (APU)

generator

• Is identical to the engine-driven generators.

• Supplements the aircraft’s engine- driven

  generators during peak loads.

• Has a higher load capacity than the engine-

  driven generators.

An APU is usually rotated during start by

• A turbine impingement system.

• A pneumatic starter.

• An electric starter.

When in operation, the speed of an APU

• Is controlled by a cockpit power lever.

• Remains at idle and automatically accelerates to

  rated speed when placed under load.

• Remains at or near rated speed regardless of

  the load condition

Generally, when maximum APU shaft output power

is being used in conjunction with pneumatic power

• Pneumatic loading will be automatically

  modulated to maintain a safe EGT.

• Electrical loading will be automatically

  modulated to maintain a safe EGT.

• Temperature limits and loads must be carefully

  monitored by the operator to maintain a safe

  EGT.

Fuel scheduling during APU start under varying

pneumatic bleed and electrical loads is maintained

• Manually through power control lever position.

• Automatically by the APU fuel control

  system.

• Automatically by an aircraft main engine fuel

  control unit.

When necessary, APU engine cooling before

shutdown may be accomplished by

• Unloading the generator(s).

• Closing the bleed air valve.

• Opening the bleed air valve.

Usually, most of the load placed on an APU occurs

when

• An electrical load is placed on the generator(s).

• The bleed air valve is opened.

• The bleed air valve is closed.

Fuel is normally supplied to an APU form

• Its own independent fuel supply

• The airplane’s reserve fuel supply

• The airplane’s main fuel system