Boeing 777 Auxiliary Power Unit (APU) – Startup and Components

Overview of the Boeing 777 APU

• Tail-cone–mounted Auxiliary Power Unit (APU) is a small gas-turbine engine.
  • Provides electric power and pneumatic (bleed-air) power so the aircraft can operate independently on the ground.
  • Starts the main engines.
  • Supplies emergency backup power in flight.
• Series goal: Explain how the APU works and why it is critical to aircraft operations.

Primary Functions & Importance

• Electrical independence on ramp: no need for ground power unit (GPU).
• Pneumatic source for environmental control system (ECS) and engine air-start valves.
• Redundancy: ensures flight continues safely if main engine or generator fails.

Initial Power Availability

• With engines OFF and no external power connected, only two onboard batteries are available:
  • Main battery → energises the DC Battery Bus via the Main Battery switch and internal contactors.
  • Dedicated APU start battery (separate, higher-capacity) reserved solely for cranking the APU.

Cockpit Switchology Sequence

• Pilot selects MAIN BATTERY switch → ON
  • Closes contactors; DC Battery Bus becomes live.
• Pilot moves APU switch → ON
  • Electrical Load Management System (ELMS) performs multiple automatic actions:
  • Sends signal to APU fuel pump (boost pump) to run.
  • Commands APU fuel feed valve to OPEN.
  • Powers the APU Electronic Controller (APU-ECU).
  • Drives the APU air-inlet door via electric actuator to the full-open position.

APU Controller Logic – Selecting a Start Method

• Controller checks for available energy sources:
  • No pneumatic air available (engines & bleed manifold un-pressurised).
  • Therefore controller selects electric start.

Electric Start System Hardware

• Dedicated APU battery → APU Power Distribution Panel (PDP).
• APU-ECU closes PDP contactors → energises DC electric starter motor mounted on APU front frame.
• Starter drive train:
  • Motor → ratchet wheel → pawl clutch → drive gear → accessory gearbox (AGB) → main engine shaft.
• Items on AGB include: fuel pump/cluster, lube pump, generators, etc.

Compressor Section Details

• Two-stage centrifugal compressor mounted on the single spool shaft.
  • Each stage = impeller + diffuser.
  • Impeller accelerates air radially outward (velocity ↑).
  • Diffuser slows flow to convert velocity to static pressure.
• Result: progressive rise in total pressure before combustion.

Fuel System Path & Components

• Fuel sourced from wing tank via APU fuel feed line.
• Within the fuel cluster (mounted on the AGB):
  • Mechanical fuel pump (positive-displacement).
  • Pressure regulating & shut-off valves.
  • Fine filter element.
  • Primary & secondary distribution manifolds.

Ignition & Light-Off Sequence (Around 7\% RPM)

• At N = 7\% of rated speed, APU-ECU commands:
  • Fuel Shut-Off Valve (FSOV) OPEN → fuel flows through primary manifold to 14 simplex fuel nozzles.
  • Ignition unit ON:
  • 28\,\text{V DC} supplied → solid-state exciter converts to high-voltage pulses.
  • Pulses travel via shielded leads to 2 igniter plugs (sparkers) in combustor.
• Air from compressors enters via swirlers → intense vortex ensures homogeneous fuel–air mix.
• Spark ignites mixture → stable flame begins.

Combustor Architecture & Airflow Management

• Reverse-flow annular combustor: inlet air turns opposite to turbine flow, shortening engine length.
• Peak gas temperature can reach 2000^{\circ}\text{C}.
• Cooling & stability features:
  • Multiple dilution, liner, and film-cooling holes admit secondary air.
  • Protects liner, reduces NOx, keeps turbine inlet temperature within limits.

Turbine & Exhaust Section

• Three-stage axial turbine extracts work:
  • Each stage = stationary stator (nozzle guide vane) + rotating rotor wheel.
  • Drives single shaft → spins compressors & accessories.
• Spent gases exit via bent duct, leaving aircraft on left side of tail cone.

Automatic Sequencing Beyond Light-Off

• Combustion is self-sustaining once ignited.
• Ignition unit remains energised until N \approx 50\% (½ rated speed) to prevent flameout.
• At N = 50\%:
  • ECU opens PDP contactors → de-energises electric starter.
  • Pawl clutch disengages when starter speed decays; isolates starter from running gearbox.
  • Fuel pump discharge pressure now high; internal flow-divider valve opens, sending fuel through secondary manifold → increases fuel flow & thrust.
• Engine accelerates autonomously to governed rated speed (≈ 100\% N).

Availability of On-Line Power

• Near N = 100\%, ECU signals:
  • APU generator(s) ready → aircraft AC buses can be powered.
  • APU bleed air valve ready → pneumatic manifold pressurisation available for packs & engine start.

Key Numbers & Parameters (Quick Reference)

• 7\%\,N → fuel & igniters on.
• 28\,\text{V DC} → ignition unit input.
• 50\%\,N → starter cut-out, ignition off, flow-divider opens.
• \text{Peak }T_{comb} \approx 2000^{\circ}C.
• 100\%\,N → electrical + pneumatic power available.

Practical & Safety Implications

• Separate APU battery ensures start capability even if main battery low or failed.
• Electric start is quieter and independent of external pneumatic carts.
• In-flight APU starts are possible (within certified envelopes) to recover electrical power.
• Proper cooling airflow through combustor critical: liner damage or turbine over-temperature can occur if cooling holes clog.
• Automatic disengagement of starter prevents mechanical over-speed or gearbox damage.

Links to Future Procedures (Foreshadowing)

• Next video: using APU-supplied bleed air to motor and start the GE90/Trent 800 main engines.
• Demonstrations of how ELMS prioritises loads when both APU & external power present.