Definition
Ignition sparks that occur later in the engine's compression and power cycle than the normal full-advance timing. In a piston aircraft engine, retarding the spark means firing the spark plug closer to top dead center rather than well before it, producing a slower, gentler combustion event used during engine starting.
Plain English
Sparks that fire later than usual in the engine cycle. The ignition system delays the spark on purpose so the engine starts smoothly without kicking back against the starter.
Context Anchor
Seen in reciprocating-engine ignition system discussions, especially during starting and magneto timing checks.
Derivation
Retard comes from the Latin retardare, meaning 'to slow down' or 'delay.' In ignition terms, the spark is delayed in the cycle — fired later than the normal advanced position.
Why Pilots Care
Prevents engine damage and injury from kickback when the engine is cranked.
Grounding Statement
A retarded spark is a spark made late on purpose so the engine starts in the correct direction without a hard kickback.
Intuition Check
Do not read retarded as meaning poor, broken, or less powerful here. It means delayed in timing: the spark happens later than the normal running spark.
Example Sentence 1
The impulse coupling on the left magneto delivers retarded sparks during cranking, then releases to provide normal timing once the engine starts.
Example Sentence 2
Retarded sparks are disabled once the engine reaches a set RPM and the advance mechanism takes over.