Accident Overview
An airplane experienced an in-flight fire and subsequently conducted a forced landing, during which it impacted the terrain in a nose-low attitude. According to air traffic control, the pilot diverted to an alternate airport after reporting smoke in the cockpit. Witnesses on the ground reported that the pilot did not shut down the engine during the emergency. The aircraft entered a downwind pattern and, after overshooting the runway during the approach, impacted the terrain.
Investigation Findings
Post-accident investigation revealed that the fuel line to the No. 5 cylinder had separated over time due to cyclic stresses that propagated a fatigue crack. The maintenance installation of the fuel line had been improper: the line was routed incorrectly and was not clamped as required. Furthermore, manufacturer information regarding the correct routing and clamping of the line had not been available in the service bulletins.
Examination of the exhaust system showed that the exhaust pipe exhibited mechanical abrasion and thermal oxidation prior to the accident, even though the system had undergone a recent maintenance check. A 'B' nut was missing on the top spark plug for the No. 5 cylinder, a location that experienced intense heat during the in-flight fire. Additionally, surface cracks penetrating the inside surface of the V-band coupling that secures the exhaust pipe were found, with surface oxidation damage determined to have been in existence for some time prior to the accident.