Auxiliary Power Units

The function of an APU air inlet plenum is to

• stabilize the pressure of the air before it enters the compressor.

• increase the velocity of the air before entering the compressor.

• decrease the pressure of the air before entering the compressor.

Fuel is normally supplied to an APU from

• the airplane's reserve fuel supply.

• its own independent fuel supply.

• the airplane's main fuel supply.

An APU is usually rotated during start by

• a pneumatic starter.

• a turbine impingement system.

• an electric starter.

Fuel scheduling during APU start and 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.

Generally, when maximum APU shaft output power is being used in conjunction with pneumatic power

• temperature limits and loads must be carefully monitored by the operator to maintain a safe EGT.

• pneumatic loading will be automatically modulated to maintain a safe EGT.

• electrical loading will be automatically modulated to maintain a safe EGT.

When necessary, APU engine cooling before shutdown may be accomplished by

• opening the bleed air valve.

• closing the bleed air valve.

• unloading the generator(s).

Frequently, an aircraft's auxiliary power unit (APU) generator

• supplements the aircraft's engine-driven generators during peak loads.

• has a higher load capacity than the engine-driven generators.

• is identical to the engine-driven generators.

When in operation, the speed of an APU

• remains at or near rated speed regardless of the load condition.

• remains at idle and automatically accelerates to rated speed when placed under load.

• is controlled by a cockpit power lever.