Sequence of Events
The pilot received a weather briefing from the Flight Service Station the evening before departure. A friend at the destination informed him that the area had been free of fog for several days. Upon descending to 1,500 feet at the destination, the pilot could not spot the airport due to a fog layer and decided to divert to his alternate airport. After turning toward the alternate, the engine began to run roughly. The pilot attempted to remedy the power loss by applying carburetor heat, switching fuel tanks, leaning the mixture, and checking the magnetos in the both position, but was unable to resolve the issue. As he turned back toward his original destination airport, the engine continued to run rough, and he could not arrest the airplane's descent. He was just above the fog layer and saw the runway through the fog, then turned back toward the runway. During the turn, he entered the fog, and the airplane collided with treetops and became lodged in branches. The occupants noticed fire in the floorboard area, exited through the pilot's door, and jumped to the ground. The fuselage was subsequently consumed by fire.
Weather and Carburetor Icing Information
The temperature and dew point were in an area of a carburetor icing probability chart annotated, 'serious carburetor icing with cruise or climb power.' The FAA publication, 'Tips on Winter Flying,' recommends that carburetor heat be applied prior to reducing power.
Witness Observations
Witnesses on the ground heard the airplane circle the area twice at low altitude but did not hear any change in engine sound.