Plain English Meaning

A break in the white ceramic part of a spark plug. When that ceramic cracks, the spark can escape through the crack instead of jumping the gap where it's supposed to, so the cylinder doesn't fire properly.

Full Definition

A fracture in the ceramic insulator surrounding the center electrode of an aircraft spark plug, typically caused by thermal shock from rapid cooling, mechanical stress during installation, or sustained operation outside the plug's intended heat range. The crack creates an unintended electrical path that can allow the high-voltage spark to leak away from the firing tip, resulting in a weak spark, a missed firing, or ignition of fuel-air mixture at the wrong location.