What happened
The pilot was operating at 10,000 feet mean sea level in conditions involving intermittent rain. During the flight, the pilot activated the alternate air for both engines, noting an outside air temperature of 43 degrees Fahrenheit. Following this action, a loss of RPM was observed on the left engine. The pilot chose not to feather the propeller, believing that minimal RPM performance was still being maintained.
As the aircraft descended through 8,000 feet, the outside air temperature dropped to between 39 and 40 degrees Fahrenheit. At this altitude, the right engine also experienced a drop of 200 to 300 RPM. Consequently, the pilot executed an off-airport forced landing with the landing gear in the retracted position. There were no fatalities or injuries reported.
The investigation
An examination of the wreckage confirmed that fuel was available on the aircraft at the time of the accident. Subsequent testing of the left engine showed it started without issue and functioned normally up to a manifold pressure of 25 inches of mercury.