MF

Boeing 7 Series – APU Electrical & Pneumatic Operation Study Notes

Chapter 1 – Introduction: APU Startup & Initial Electrical Supply

  • Scenario & Initial Conditions

    • Aircraft batteries supply only limited electrical power.
    • No pneumatic power available.
    • No external (ground) electrical power connected.
    • APU (Auxiliary Power Unit) gas-turbine engine is running at full (governed) speed.
    • Objective: use the APU to provide both electrical and pneumatic power to aircraft systems.

  • Primary Electrical Source: APU‐Driven AC Generator

    • Converts mechanical shaft power (from gearbox) into 3-phase AC electrical power.
    • Three concentric machines share the same shaft, each containing stator + rotor:
    • Permanent-Magnet Generator (PMG)
    • Exciter
    • Main Generator

  • Permanent-Magnet Generator (PMG)

    • Rotor carries permanent magnets → establishes a constant magnetic field.
    • Gearbox drive spins rotor → rotating magnetic field cuts stator windings.
    • By Faraday’s law of electromagnetic induction, a small-power, 3-phase AC output is produced.
    • PMG output is routed to the Generator Control Unit (GCU).

  • Generator Control Unit (GCU)

    • Functions as:
    • Voltage regulator (controls generator delivered voltage).
    • Rectifier (converts PMG AC → regulated DC).
    • Regulated DC is fed into the exciter stator windings.

  • Exciter

    • DC in exciter stator ➔ stationary magnetic field.
    • Exciter rotor spins through that field, inducing AC in rotor windings.
    • Rotating rectifier (mounted on the rotor) converts that rotor AC → DC.
    • Resulting DC energises the main generator rotor (field windings).

  • Main Generator

    • Now-energised rotor (DC field) spins, producing a rotating magnetic field.
    • Main stator cuts that field → generates aircraft-quality, 3-phase AC (typically 115\,\text{V}_{\text{L–N}},\ 400\,\text{Hz}).

Chapter 2 – APU Generator Output & Electrical Load Management

  • Power Transfer Path

    • Main generator output → Auxiliary Power Panel (APP), part of the Electrical Load Management System (ELMS).
    • When cockpit APU GEN switch is selected:
    • GCU energises + closes a contactor in the APP.
    • AC power becomes available to aircraft distribution busses.

  • AC → DC Conversion & Bus Activation

    • Generator feeds Transformer Rectifier Units (TRUs).
    • TRUs produce regulated DC, energising the aircraft DC busses.

  • Two Main Electrical Distribution Channels

    • Left channel (Left AC + DC busses).
    • Right channel (Right AC + DC busses).
    • With only one APU generator online:
    • ELMS powers both channels but sheds non-essential loads if demand exceeds single-generator capacity.
    • Shedding logic preserves essential/flight-critical loads.

  • Need for Pneumatic Power

    • To start a main engine, pneumatic (bleed-air) pressure is mandatory for the air-starter.
    • Electrical system readiness alone is insufficient.
    • Pneumatic system is supervised by the Air Supply & Cabin Pressure Controller (ASCPC).

Chapter 3 – APU Bleed-Air System & Left-Engine Start Sequence

  • APU Bleed Control Architecture

    • Dedicated APU Controller modulates bleed output using fuel-pressure-driven actuator.
    • Actuator rotation ➔ drives a ring gear that meshes with sector gears connected to Inlet Guide Vanes (IGVs) at the APU load compressor inlet.

  • IGV Control Logic

    • During APU start: IGVs held nearly closed → minimal airflow, ensuring reliable, acceleration without compressor stall.
    • At governed speed: Controller progressively opens IGVs to satisfy pneumatic demand.

  • Bleed-Air Flow Path

    1. Centrifugal load compressor → pressurised bleed air.
    2. Check valve (prevents back-flow).
    3. APU shut-off valve (SOV).
    4. Downstream pneumatic ducts managed by ASCPC.

  • System Logic (APU BLEED switch in AUTO)

    • ASCPC commands:
    • APU SOV OPEN.
    • Isolation valve(s) OPEN ➔ pressurise both left/right pneumatic manifolds.
    • Primary uses: air-conditioning packs + engine start.

  • Left-Engine Start with A/C Packs Running

    • Start button pressed while packs ON ➔ ASCPC temporarily shuts both packs (load-sheds them).
    • Ensures full bleed supply to the left engine air-starter.
    • After N2 reaches self-sustaining speed & left engine bleed becomes available:
    • ASCPC closes APU SOV.
    • Opens left engine bleed valve → duct pressurisation resumes.
    • Reactivates A/C packs using left engine bleed.
    • Left engine gearbox-driven generator now produces AC → powers left electrical busses.

Chapter 4 – Right-Engine Start & System Reconfiguration

  • Post Left-Engine Start State

    • Electrical: Left engine generator ➔ left busses; APU generator ➔ right busses.
    • Both essential + non-essential busses active (two independent sources available).

  • Right-Engine Start Sequence

    1. Pilot selects RIGHT ENG START.
    2. ASCPC again stops A/C packs + closes left isolation valve (to prioritise starter air on right side).
    3. Commands APU SOV OPEN → APU bleed flows to right air-starter.
    4. Once right engine reaches idle & bleed available, pilot turns R ENG BLEED ON.
    5. ASCPC:
    • Closes APU SOV (APU bleed no longer required).
    • Configures isolation valves:
      • Left engine bleed ➔ left A/C pack.
      • Right engine bleed ➔ right A/C pack.

  • Electrical Transition

    • Right engine generator comes online ➔ ELMS connects it to right electrical busses.
    • APU generator is automatically disconnected.

  • End State

    • Both main engines provide full pneumatic + electrical supply.
    • All aircraft systems powered; non-essential loads restored.
    • APU can be shut down or remain available as standby.

Additional Technical & Operational Notes

  • Electromagnetic Induction Basics

    • Voltage induced in a conductor proportional to \dfrac{d\Phi}{dt} (rate of change of magnetic flux \Phi).
    • Three-phase system provides constant power transfer & reduced conductor size vs single-phase.

  • Frequency Regulation

    • APU speed governor maintains constant RPM so that AC output frequency remains at industry standard 400\,\text{Hz}.

  • Load-Shedding Hierarchy (typical)

    • Stage 1: Galley & utility outlets.
    • Stage 2: Passenger entertainment.
    • Stage 3: Selected environmental control components.
    • Ensures avionics, flight controls, lighting, pumps stay powered.

  • Safety / Check-Valve Function

    • Prevents reverse flow from engines into APU during dual-bleed conditions.

  • Fuel-Pressure Actuation Advantage

    • Avoids separate bleed or electrical actuation sources → simplifies design.

  • Real-World Relevance & Cockpit Implications

    • Pilots monitor ELEC synoptic page for bus status, frequency, voltage, current.
    • Monitor AIR synoptic for bleed pressures, valve positions.
    • Mis-management can lead to:
    • Hot starts (excess fuel, insufficient airflow).
    • Duct over-pressurisation.
    • Electrical load trips.

  • Ethical / Operational Considerations

    • Proper APU management reduces fuel burn & emissions on ground.
    • Unnecessary APU use may violate noise or environmental regulations at certain airports.

  • Preview of Next Lecture

    • In-flight APU use (backup electrical source, high-altitude bleed limits).
    • Automatic load-sharing & protective shutdown logic.