Gas Turbine Additional Systems: Accessory Gearbox, Ignition, Starting, and FADEC

021-11-03Pablo Asensio Martínez2026-03-272 min

Accessory Gearbox

The accessory gearbox drives essential engine and aircraft systems.

  • Drive Source: Usually driven by the HP Compressor shaft (High speed) via an internal gearbox.
  • Driven Components:
    • Fuel Pumps (High Pressure).
    • Oil Pumps.
    • Hydraulic Pumps.
    • IDG/CSD (AC Generator).
  • Lubrication: Lubricated by the engine oil system.
  • Shear Neck: A weak point in the drive shaft designed to break if a component seizes (e.g., a generator jams). This protects the main gearbox and engine from damage, allowing the engine to continue running (minus the failed accessory).

Ignition System

Gas turbine engines use high-energy ignition systems.

  • Components:
    • Ignition Exciter/Unit: Converts low voltage (DC/AC) to high voltage (up to 25,000 V).
    • Igniters: Spark plugs designed for high energy discharge. Two per engine.
  • Operation:
    • Start: High energy spark to ignite the mixture.
    • Continuous Ignition: Low energy spark used to prevent flameout. Selected automatically (by FADEC) or manually during:
      • Take-off / Landing.
      • Heavy precipitation / Turbulence.
      • Anti-ice operation.
      • Stall warning or engine surge.
  • Ignition Type: High intensity capacitor discharge system.

Engine Starting

  • Sequence:
    1. Rotation: Starter spins HP compressor (N2).
    2. Ignition: Spark initiated.
    3. Fuel: Fuel introduced.
  • Starter Types:
    • Electric: Used on small engines and APUs (battery powered).
    • Pneumatic (Air Starter): Used on most large turbofans. Powered by high-pressure air from:
      • APU (Auxiliary Power Unit).
      • Ground Cart (GPU/ASU).
      • Cross-bleed (from the other running engine).

FADEC (Full Authority Digital Engine Control)

A digital computer system that manages all engine functions.

  • Architecture:
    • Dual Channel: Two independent ECUs (Electronic Control Units) for redundancy (Active/Standby).
    • Power Supply: Dedicated Permanent Magnet Alternator (PMA) on the engine provides independent power.
  • Redundancy:
    • Single Fault Tolerant: If one channel fails, the other takes over immediately with no loss of performance.
    • Total Failure: If both channels fail (and no hydromechanical backup exists), the engine shuts down (fuel valve closes).
  • Functions:
    • Fuel metering.
    • Engine limit protection (Overspeed, Over-temp).
    • Auto-start.
    • Thrust Reverser control.
    • Engine health monitoring.

Auxiliary Power Unit (APU)

A small gas turbine located in the tail.

  • Purpose: Provides Electrical Power (AC) and Pneumatic Air (for air conditioning and engine starting) on the ground and in flight.
  • Starting: Usually started electrically using the aircraft battery.