Definition
Spark plugs whose firing tips have become coated with deposits — typically lead, carbon, or oil — that prevent them from producing a strong, consistent spark. Fouled plugs misfire or fail to fire at all, causing rough engine running, power loss, and a noticeable drop during the magneto check.
Plain English
Spark plugs that have gotten dirty enough on the inside tip that they can no longer spark properly, so the engine runs rough or loses power.
Context Anchor
Seen in piston-engine ignition discussions, especially during engine start, taxi, run-up, or when an engine runs rough.
Derivation
‘Foul’ comes from Old English fūl, meaning dirty or unclean. A fouled plug is simply a plug that has become dirtied to the point it no longer works correctly — the everyday meaning carries over directly.
Why Pilots Care
Fouled plugs cause engine roughness, misfires, power loss, and can prevent the engine from starting or running smoothly in flight.
Analogy
It is like trying to light a match with a wet or dirty match head: the part that should make the spark is there, but contamination keeps it from working well.
Intuition Check
Do not read fouled plugs as simply old or worn-out plugs. Here, fouled means the spark plugs are contaminated enough to interfere with the spark.
Example Sentence 1
After a long taxi at idle, the pilot noticed an excessive mag drop and suspected fouled plugs.
Example Sentence 2
After a long taxi at low power the engine ran rough until the pilot leaned the mixture to clear the fouled plugs.