Definition
A condition in which deposits of lead, carbon, oil, or other combustion residue accumulate on the firing end of a spark plug, reducing or preventing its ability to produce a strong spark. Fouling commonly occurs during prolonged low-power operations, extended idling, or descents at low engine RPM with a rich mixture, where combustion temperatures stay too low to burn off these deposits.
Plain English
It is when gunk builds up on a spark plug so it cannot spark properly. This usually happens when the engine runs too cool for too long, often during low-power descents.
Context Anchor
Seen in engine operation discussions, especially during long low-power descents such as a steep spiral, extended taxi, or other periods when the engine may run cooler than normal.
Derivation
Foul' comes from Old English 'ful', meaning dirty or impure. A fouled spark plug is literally a dirty one — coated with deposits that stop it from working cleanly.
Why Pilots Care
It can cause engine roughness, power loss, or sudden misfires that reduce climb performance or force an emergency landing.
Grounding Statement
During a long idle descent, the engine can run cool enough that deposits stay on the spark plugs instead of burning away.
Intuition Check
Spark plug fouling does not mean the spark plug is necessarily broken. It means the plug may be dirty or coated enough that the spark is weak or unreliable.
Example Sentence 1
During the steep spiral, the pilot advanced the throttle every few hundred feet to keep the engine warm and prevent spark plug fouling.
Example Sentence 2
Leaning the mixture properly during cruise helps prevent spark plug fouling on long flights.