What happened
During an enroute flight, the pilot notified air traffic control of a cylinder temperature irregularity and diverted to an alternative airport for a visual approach. After initially overflying the airfield, the pilot requested new vectors and subsequently requested a diversion to an airport located to the east. During this period, the pilot reported that the left engine had lost power and was directed back toward the original airport for another visual approach.
While attempting to land, the pilot transitioned from a planned landing on runway 14 to runway 32. An eyewitness observed the aircraft type performing a base-to-final turn for runway 32, noting that the wings leveled briefly before the plane executed a 60-degree left bank. This maneuver led the aircraft into a counterclockwise stall and spin. The aircraft type struck a trailer park approximately 0.38 miles from the approach end of runway 32, followed by a fire that consumed the wreckage.
Findings
Investigations into the left engine revealed that the starter adapter spline was worn, specifically featuring broken teeth on the crankshaft gear. While the crankshaft gear remained properly timed with the crankshaft cluster gear, it was unclear if the idler gear driving the magnetos had slipped timing. Such a timing error could potentially cause abnormal cylinder head temperatures. Additionally, improper hardware was found securing the intake valve for cylinder No. 6, which may have impacted valve performance.
Evidence of detonation was identified on the pistons of the left engine. Although the engine data monitor was destroyed in the fire, the investigation noted that the left engine failure was linked to mechanical issues within the starter adapter assembly and valve hardware. The aircraft had completed approximately 5.5 flight hours since its last annual inspection, which included the required inspection of the starter adapter assembly.