Starting & Ignition Systems
Magneto ignition and its independence, dual ignition, the hot-mag hazard and impulse coupling.
Starting & ignition, summary notes
- 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.
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.
Ignition & starting concept map
Starting & Ignition
Starting & ignition quiz
Starting & Ignition Systems, quiz
1. An aircraft piston engine uses dual magnetos mainly to:
Provide redundancy and improve combustion with two plugs per cylinderIncrease the effective compression ratio by burning the charge more completely each cycleReduce the weight of the ignition installation compared with a single high-tension system2. A magneto is independent of the aircraft electrical system because it:
Generates its own high tension supplyRuns from the batteryUses the generator output3. In the magneto shown, the contact breaker points are at X. Opening the points causes:
The primary field to collapse and induce a high voltage in the secondaryThe magnet to stop rotatingThe capacitor to discharge to earth4. An engine fires at 24° before top dead centre and runs at 2400 rev/min. The crankshaft turns 24° in approximately:
1.7 milliseconds17 milliseconds0.17 milliseconds5. A magneto ignition system is:
Independent of the aircraft battery/electrical systemPowered by the batteryDriven by the exhaust6. Dual ignition (two mags, two plugs) provides:
Redundancy and more complete combustionOnly redundancyOnly cost saving7. The ignition switch stops a magneto by:
Grounding (shorting) its primaryCutting the fuelOpening the exhaust valve8. A broken P-lead (ground lead) results in a:
'Hot' magneto that can fire with the switch OFFDead magnetoRicher mixture