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

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.