Flight Events
The aircraft was on an instrument flight rules (IFR) flight in instrument meteorological conditions (IMC). During the flight, the right engine began to shudder and produce black smoke. Subsequently, the propeller of the right engine went into feather without any actuation of the propeller control by the pilot. The pilot was unable to maintain altitude and executed a forced landing in an open field.
Engine and Propeller Examination
Investigation revealed that the right engine oil was diluted by gasoline. This was attributed to a leak in the accelerator pump of the carburetor, which caused the engine to run rich. Hartzell Propellers personnel verified that diluted oil could cause a propeller to go into uncommanded feather.
Pilot Actions
Interviews disclosed that the pilot exhibited denial regarding the engine problem. The pilot did not secure the engine and did not declare an emergency with ARC.
Additional Details
Both engines were noted to be high time with high time since overhaul.