Section 16.5

Starting & Ignition Systems

Magneto ignition and its independence, dual ignition, the hot-mag hazard and impulse coupling.

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Main ideas

Aircraft piston engines use MAGNETO ignition: an engine-driven magneto generates its own high-tension spark, completely independent of the aircraft battery and electrical system — so the engine keeps running with the master switch off.
Dual ignition — two magnetos, each firing one of two spark plugs per cylinder — gives redundancy AND more complete, faster combustion (better power and smoothness); the run-up 'mag check' confirms each system.
The ignition switch works by GROUNDING (shorting) the magneto's primary to stop it. If the ground (P-)lead breaks or is disconnected, the magneto stays 'live' even with the switch OFF — a hot mag, which can fire the engine if the propeller is turned by hand.
At low cranking speed a magneto spark is weak, so an impulse coupling (or starting booster) retards and intensifies the spark for starting; normal running spark is advanced for efficiency.
The system includes spark plugs, HT leads, a distributor and a starter motor; magneto timing to the engine must be correct.
⚠ Exam trap: the ignition switch STOPS a magneto by grounding it — a broken P-lead leaves a HOT magneto that can fire even with the switch OFF, so always treat a propeller as live; magnetos are independent of the aircraft electrical supply.

Solved examples

During a mag check the engine keeps running normally when the ignition is switched fully OFF momentarily. Why is this dangerous and what does it indicate?
It indicates a broken or disconnected P-lead, so the magneto is not being grounded and stays 'hot' regardless of the switch. The engine could start if the propeller is turned by hand — a serious hazard. The aircraft must be treated as live and the fault rectified before further operation.

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