Engine Starting Systems

Main Components and Functions

An aircraft engine starting system typically includes a starter (electric motor or pneumatic drive), starter relay or valve, ignition system, and associated controls. For piston engines, the electric starter engages the flywheel to rotate the crankshaft, while magnetos provide spark for ignition. Turbine engines use either electric or pneumatic starters to spin the compressor; ignition and fuel systems are sequenced to ensure safe combustion.

Types of Starters

• Electric Starters: Common in piston engines and some small turbines, these draw power from the aircraft battery to rotate the engine.
• Pneumatic Starters: Used in larger turbine engines, they use compressed air (from an APU, ground cart, or another engine) to drive a small turbine connected to the main engine shaft.
• Starter-Generators: Some aircraft combine the starter and generator into a single unit, especially in helicopters and smaller turboprops.

Turbine Engine Start Principle

Starting a turbine engine requires airflow through the core, fuel delivery, and ignition. The starter spins the compressor to a minimum speed (typically around 15% RPM), at which point fuel is introduced and ignition is activated. Once combustion is established, the engine accelerates to self-sustaining RPM (about 30% for most turbines), after which the starter and ignition disengage.

Typical Turbofan Start Sequence

1. Start switch engaged (starter motor begins rotation)
2. Ignition ON
3. At preset compressor speed, fuel ON
4. Light-up (EGT rise confirms combustion)
5. Engine accelerates to self-sustaining RPM
6. Starter and ignition OFF; engine stabilises at idle

Self-Sustaining RPM

This is the rotational speed at which the engine can continue to accelerate and run without assistance from the starter. For most turbine engines, it's around 30% of maximum RPM.

Common Start Malfunctions

• False (Dry or Wet) Start: No combustion (dry) or unburned fuel accumulates (wet); indicated by no EGT rise and low RPM.
• Tailpipe Fire (Torching): Unburned fuel ignites in the exhaust; can occur after a wet start.
• Hot Start: EGT rises excessively, usually due to too much fuel or insufficient airflow.
• Hung (Abortive) Start: Engine fails to reach self-sustaining RPM; RPM stagnates below normal.
• No N1 Rotation: Indicates a starter or mechanical failure; compressor does not turn.
• No FADEC Indications: Loss of electronic engine control signals; could indicate electrical or system fault.