APU Study Notes on Garrett Model GTCP331-2506

APU Overview

  • APU: Auxiliary Power Unit, specifically the Garrett model GTCP331-2506.
  • Functionality:
    • Supplies pneumatic and electrical power.
    • Provides bleed air for various systems.

Components of APU

  • Main Components:

    • Compressor:
    • Powers the main engine starting and cabin air.
    • Power Section:
    • Contains the gearbox and load compressor.

  • Location:

    • Located in the right wheel pod mounted to a firewall using vibration isolation mounts.

  • Inlet Door:

    • Prevents airflow during flight which is crucial for safety.

Operational Theory

  • Starting Process:

    • The starter motor engages when it receives a signal from the Flight Control Officer (FCO) indicating that the APU door is open.
    • If the APU door is not fully open, an error message "APU DOOR DISAGREE" will appear on the WAP (Warning Alert Panel).

  • ECU Command:

    • Upon starting, the Engine Control Unit (ECU) requires confirmation from:
    • The landing gear ground sensing wow sensor.
    • This system confirms that the wheels are on the ground allowing the APU to start.

  • Fuel Supply:

    • Initially supplied by gravity feed from the No. 3 fuel tank.
    • Once the aircraft's power is available, the fuel supply switches to:
    • Forward boost pump or aft boost pump.

  • Starting Sequence:

    • Fuel is delivered to the APU fuel fire shutoff valve and the inlet door opens, enabling the ignition process.
    • The gear train begins rotating as force is applied, initiating:
    • A 2-stage centrifugal compressor impeller.
    • A 3-stage axial flow turbine assembly.
    • A centrifugal load compressor rotor, which starts rotation to provide necessary pressure.

Control of Load Compressor Output

  • The output of the load compressor is controlled during this operational cycle to ensure efficiency and operational safety.