Definition
A small auxiliary magneto, usually hand-cranked or electrically driven, used to produce a strong ignition spark during engine starting when the main magnetos are turning too slowly to generate sufficient voltage on their own.
Plain English
An extra spark-maker used only at startup. When the engine is being cranked it turns slowly, and the regular spark system is too weak to fire the cylinders. The booster magneto supplies a strong spark long enough to get the engine running, then drops out of the picture.
Context Anchor
Seen in piston-engine starting systems, maintenance descriptions, and troubleshooting for hard starting.
Derivation
Magneto comes from the same root as magnet — it is a small generator that uses spinning magnets to create electricity for the spark plugs. Booster simply means something that gives an extra push. So a booster magneto is a small magnet-driven spark generator that gives the ignition system an extra push during starting.
Why Pilots Care
Ensures reliable engine starts in cold conditions or with marginal battery power, reducing the risk of flooded engines or failed starts.
Grounding Statement
When a piston engine is only being turned by the starter, it is moving slowly, so the booster magneto supplies the extra starting spark needed until normal ignition takes over.
Intuition Check
Booster does not mean it adds power to the engine in flight. Here it means it boosts the ignition spark during engine starting.
Example Sentence 1
On the old radial, the pilot energized the booster magneto while the starter cranked the engine, and the cylinders fired almost immediately.
Example Sentence 2
Older training manuals note that the booster magneto supplies ignition until the engine reaches idle RPM.