What happened
During an IFR flight conducted in IMC conditions, the right engine of the aircraft began to shudder and emit black smoke. Following this malfunction, the propeller entered a feathered position without any actuation from the propeller control. Due to the loss of power, the pilot was unable to maintain altitude, resulting in a forced landing in an open field.
The investigation
The investigation focused on the cause of the engine malfunction and the subsequent propeller behavior. It was determined that gasoline had diluted the right engine oil. This dilution was caused by a leak in the carburetor's accelerator pump, which resulted in the engine running excessively rich. Both engines were noted to have high total time and high time since their last overhaul. Additionally, personnel from Hartzell Propellers verified that oil dilution is a known factor that can cause a propeller to enter an uncommanded feather state.