Definition
A coating of conductive carbon deposits that forms on the white ceramic insulator of a spark plug, creating an electrical leakage path that allows the high-voltage current to bleed away to ground instead of jumping the plug's gap to produce a spark. It is a common cause of spark plug fouling in piston aircraft engines, particularly after extended low-power ground operations or running with an over-rich mixture.
Plain English
Black soot builds up on the white ceramic part inside a spark plug. Because soot conducts electricity, the spark sneaks down the soot instead of jumping across the gap, so the plug stops sparking properly and the engine runs rough.
Context Anchor
Seen in aircraft electrical and avionics maintenance when identifying or describing carbon-type resistors.
Derivation
The ceramic refers to the white porcelain-like insulator that surrounds the centre electrode of a spark plug. Carbon here means soot — the black residue left when fuel does not burn completely. The term simply describes the visible result: black carbon deposited on white ceramic.
Why Pilots Care
Signals possible rich mixture, incomplete combustion, or the need for leaning adjustments and cleaning to avoid power loss or fouling.
Grounding Statement
Picture a small hard white tube with a thin dark coating around it; the dark coating controls the electricity passing through the part.
Intuition Check
Do not read “cylinder” here as an engine cylinder. In this term, it means the small tube-shaped ceramic body inside an electrical component.
Example Sentence 1
The right magneto dropped 200 RPM during run-up, so the instructor suspected carbon on a ceramic cylinder from taxiing too long at idle with a rich mixture.
Example Sentence 2
After the flight, the mechanic checked for carbon on a ceramic cylinder before approving the aircraft for service.