Dual magneto ignition system
Light-aircraft piston engines have two independent magnetos per engine; each fires one of two spark plugs per cylinder.
| Property | Detail |
|---|---|
| Drive | Engine-driven (gear or splined shaft) — independent of the electrical system |
| Spark voltage | High voltage 15 000–25 000 V via own coil unit |
| Behaviour on electrical failure | Continues to operate — engine runs even with total battery/alternator failure |
| Two-spark advantage | Redundancy and better combustion via two flame fronts — shorter burn time, less detonation risk |
Magneto switch (ignition switch)
Five typical positions:
- OFF — both magnetos grounded, no spark
- R — only right magneto active
- L — only left magneto active
- BOTH — both magnetos active (normal)
- START — starter engages; depending on configuration, only the impulse-coupled magneto provides spark
Mag check before flight
At run-up RPM (per AFM, typically 1 700–2 000 RPM):
- BOTH to R — RPM drops slightly.
- Back to BOTH — RPM stabilises.
- BOTH to L — RPM drops slightly.
- Back to BOTH.
Limits (type-specific — AFM governs):
- Maximum RPM drop: often ≤ 175 RPM per side
- Maximum difference between L and R: often ≤ 50 RPM
If exceeded: return to ramp, contact maintenance. Common causes: fouled spark plug, defective magneto, mis-timed ignition.
Impulse coupling
On at least one magneto: a spring mechanism that retards the spark at low cranking RPM (closer to TDC instead of before TDC) and intensifies it (higher voltage). Necessary because at cranking speed the normal magneto generates insufficient voltage.