16.5 Piston engine starting and ignition systems

Scope

A level 1, B1/B3 level 2: starting systems and preheat systems; magneto types, construction and principles; ignition harnesses and spark plugs; low- and high-tension systems.

Starting systems

• Direct-cranking electric starter (12 V or 24 V DC series motor): a starter solenoid (relay) switches the heavy current; the pinion engages the ring gear via a Bendix drive (inertia engagement) or a starter adapter with an overrunning clutch (on the accessory case).
• Starter duty cycle: limited cranking time followed by cooling (FAA example: after 1 minute of cranking, cool at least 1 minute; after a second or subsequent attempt, cool 5 minutes — follow the manual).
• Starter-engaged warning light on many aircraft: illuminates if the starter remains engaged after start (relay stuck) — shut down to prevent starter damage.
• Older systems: inertia starters (hand- or electrically-wound flywheel), hand propping (with strict safety procedures).

Preheat systems

In cold weather the oil is very viscous and fuel vaporises poorly: cranking is slow, oil does not reach bearings, and cylinders may be damaged. Preheating with hot-air heaters (ground units ducted into the cowling) or engine-mounted electric heaters (oil sump pads, cylinder band heaters) is recommended below manufacturer-specified temperatures. Do not leave unattended open-flame heaters; avoid heating only part of the engine.