Definition
A condition in which one or more spark plugs in a piston aircraft engine become coated with deposits — typically lead, carbon, or oil — that prevent them from producing a strong, consistent spark across the electrode gap. Fouled plugs cause weak ignition, rough running, misfiring, or a failed magneto check during run-up.
Plain English
The spark plugs have gunk on them, so they can't spark properly and the engine runs poorly.
Context Anchor
Commonly encountered during engine start, taxi, run-up, or troubleshooting a rough-running piston aircraft engine.
Derivation
Foul' comes from the Old English fūl, meaning 'dirty' or 'unclean.' A fouled spark plug is literally a dirtied spark plug — coated with something that stops it from working as designed.
Why Pilots Care
Fouled plugs cause engine roughness, power loss, or hard starting and must be cleared before safe flight.
Grounding Statement
A spark plug has to fire cleanly inside the engine; if its tip is coated with residue, the spark may be weak or may not happen at all.
Intuition Check
“Fouled” does not mean the spark plug has violated a rule or is permanently ruined. Here it means the plug is dirty or coated with residue enough to interfere with normal sparking.
Example Sentence 1
During run-up, the right magneto showed a 200-RPM drop, so the pilot suspected fouled spark plugs and leaned the mixture to clear them.
Example Sentence 2
The mechanic cleared the fouled spark plugs by advancing the throttle to high power for several minutes.