What happened
While operating in cruise flight, the aircraft began to accumulate light rime ice on the airframe. In response to the icing conditions, the pilot elected to return to the departure point and received clearance to descend to a lower altitude.
Following the descent to the lower altitude, the engine was unable to maintain full power and began running rough. Unable to maintain sufficient altitude to reach the airport, the pilot performed an emergency landing in an open field. The aircraft, which underwent a gear-up landing into the soft field, collided with a berm and a barbed wire fence during the sequence.
The investigation
Upon recovery of the aircraft, investigators determined that the engine remained in a condition suitable for a test run. After installing a new propeller, connecting a battery, and rigging a fuel supply, the engine was started and operated normally at idle power through the warm-up period. The engine was subsequently run to 1700 RPM, and a magneto check was completed; oil temperature, oil pressure, EGT, and suction were all found to be within normal parameters.
Investigation of the air intake system revealed that the inlet air box was severely damaged during the accident. While the hinge for the alternate air door was clear and free, the door would only operate partially open. The foam air filter was inspected and found to be in good condition with no signs of blockage or deterioration.