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.