Incident Summary
While in cruise flight at 9,000 feet above mean sea level, the left engine began to run rough and lose power. The pilot interpreted the problem as carburetor icing and applied carburetor heat. However, the engine continued to run rough. The pilot then feathered the left propeller and shut down the engine. Subsequent restart attempts were unsuccessful.
Emergency Descent and Forced Landing
Unable to maintain altitude, the pilot requested to land at a nearby airport. During the descent, the aircraft passed through instrument meteorological conditions. The pilot realized that reaching the airport was not possible and executed a forced landing to rough and uneven terrain.
Investigation Findings
Examination of the engines revealed that the alternate air doors were missing on both the right and left engines. The hinges for the doors were attached to both carburetors and showed no evidence of distortion or impact damage. The doors themselves were not found at the wreckage site. A missing alternate air door would allow ambient air to enter the carburetor, rendering the carburetor heating system ineffective. According to carburetor icing probability charts, the reported temperature and dew point values at the time were favorable to the formation of induction system icing.
Probable Cause
The probable cause was determined to be inadequate maintenance, which resulted in diminished carburetor heat effectiveness due to missing alternate air doors. Contributing factors included conducive carburetor icing weather conditions, low ceilings during the emergency descent, and the lack of suitable terrain for the forced landing.