Definition
A condition in a piston aircraft engine's ignition system where a high-voltage spark intended for one cylinder jumps across to the wrong ignition lead, firing a different cylinder at the wrong time in its combustion cycle. It is usually caused by insulation breakdown, moisture, or improper routing of ignition leads inside the harness or distributor block.
Plain English
The spark meant for one cylinder accidentally jumps over to the wrong cylinder, making it fire when it shouldn't.
Context Anchor
Seen in reciprocating-engine ignition troubleshooting, especially when checking spark plug wires, ignition shielding, or rough engine operation.
Derivation
From 'cross' (going across or to the wrong place) and 'firing' (the spark igniting the fuel-air mixture). The name describes exactly what happens: the firing event crosses over to a cylinder it wasn't meant for.
Why Pilots Care
It produces rough running, power loss, and possible engine damage if the fault is not corrected before flight.
Analogy
It is like a light switch that is supposed to turn on one lamp but accidentally sends power to a different lamp instead.
Intuition Check
Cross-firing is not the same as a normal spark crossing the spark plug gap. It means the spark has crossed over into the wrong ignition path.
Example Sentence 1
The mechanic suspected cross-firing in the ignition harness after the engine ran rough during the magneto check.
Example Sentence 2
A hairline crack in the distributor block allowed cross-firing between the number two and number five cylinders.