What happened
While cruising at approximately 15,900 feet, the aircraft experienced a partial loss of power from the left engine. Although a controlled airport with emergency services was located only 27 nautical miles to the southwest, the flight diverted to an uncontrolled airfield 93 nautical miles to the north. No emergency was declared during the flight. Upon arrival at the destination, a witness observed the aircraft approaching runway 3ered 34 at an excessive speed and altitude. The aircraft subsequently crashed, resulting in the destruction of the plane. The wreckage pattern indicated that the aircraft suffered a roll to the left, consistent with reaching the minimum controllable airspeed while the critical engine was inoperative.
Findings
Investigation into the left engine revealed a cylinder head separation on the number six cylinder assembly, which had 240 hours of time since installation. Similar fatigue fractures and unexpected material on cylinder barrel threads were noted on a comparable assembly from another aircraft. Furthermore, mechanical issues were identified within the engine's variable absolute pressure controller; safety wiring around the control arm prevented normal operation, and an unauthorized pressure relief valve was present. These modifications, part of a supplemental type certificate (STC), created a risk where maintenance following standard manuals might have inadvertently increased the Vmc speed. The pilot's recent multiengine experience was also noted as limited, having only participated in a few non-revenue flights in the preceding year.