Piper PA-28-181 N8129M accident near Manassas, Virginia

Probable cause

An engine fire caused by chafing and subsequent electrical arcing of the alternator wire near a fuel source.

NTSB analysis

In preparation for the instructional flight, the flight instructor primed the engine with fuel several times and attempted to start it; each time the engine would get close to starting but would not run. On the fourth start attempt, he did not prime the engine and white smoke started to emanate from the engine compartment. The flight instructor then shut off the fuel valve and electrical equipment and then exited the airplane to investigate. He discharged a fire extinguisher into the air intake without effect. The smoke increased in intensity before a fire broke out and within minutes engulfed the engine compartment and cockpit area before eventually being extinguished by airport fire rescue.

Postaccident examination of the airplane revealed electrical arcing damage to an alternator wire and surrounding structure. Pitting consistent with electrical arcing was discovered on the engine mounts and the alternator wire displayed features consistent with electrical arcing damage that was located near a fuel line. The clamp that affixed the wire was severely damaged and melted. Based on this information, the wire likely chafed over time until metal-to-metal contact was made as the wire was energized, which resulted in a fire that was fed by fuel from a nearby fuel line.

Text from the NTSB. Open the NTSB docket for ERA23LA065 for the full record.