Detonation & pre-ignition

Two abnormal-combustion faults, do not swap them (definitions per the POH/AFM and the engine manufacturer's manual):

• Detonation: the fuel/air charge explodes violently instead of burning smoothly, after normal ignition. It overheats and can damage the engine.
• Pre-ignition: the charge is lit too early by a hot spot (a glowing carbon deposit or an overheated spark plug), before the spark plug fires.

The difference is timing: pre-ignition happens before the spark, from a hot spot; detonation is the violent burn after the spark.

Causes TP 1102 names for detonation:

• A mixture too lean at high power (a too-lean fuel/air ratio "may damage the engine due to detonation", TP 1102).
• Using carburettor heat during take-off (TP 1102).

Other classic causes come from the POH/AFM / engine manufacturer's manual: a fuel grade lower than specified (TP 1102 says never use a lower grade) and a high cylinder-head temperature.

Trap: detonation and pre-ignition are different faults. Pre-ignition fires before the plug; detonation is the explosive burn after it.

Practice