What happened
During the flight, the pilot reported smoke in the cockpit to Air Traffic Control (ATC), prompting a diversion to an alternate airport. During the approach, the aircraft entered a downwind pattern and overshot the runway. Witnesses observed that the pilot did not shut down the engine prior to the impact. The aircraft sustained an in-flight fire and ultimately impacted the terrain in a nose-low attitude during the forced landing.
The investigation
The investigation into the mechanical failures revealed that the fuel line servicing the No. 5 cylinder had separated due to cyclic stresses that propagated a fatigue crack. It was determined that the maintenance installation of this line was improperly routed and the line had not been clamped. Notably, manufacturer information regarding the correct routing and clamping of this line was unavailable in existing service bulletins.
Additionally, the exhaust pipe showed evidence of mechanical abrasion and thermal oxidation prior to the accident, despite the exhaust system having undergone a recent maintenance check. Investigators also found that a 'B' nut was missing from the top spark plug for the No. 5 cylinder, which was located in an area of intense heat during the fire. Furthermore, surface cracks penetrating the inside of the V-band coupling used to secure the exhaust pipe exhibited oxidation damage that had been present for some time before the accident.