What happened
Prior to departing on an IFR flight, the pilot received a full weather briefing that included an advisory regarding icing conditions north of the intended route. During the flight, the pilot contacted Flight Watch for an updated weather briefing and was informed that the destination airport was reporting rain and snow.
In response to the weather, the pilot requested clearance to an alternate airport. While waiting for a response to that request, the pilot contacted the approach control for the alternate airport to request clearance. During the approach, the pilot noticed the engine running rough and applied carburetor heat. After the engine smoothed out, the heat was removed; however, the engine began running rough again, prompting the pilot to apply carburetor heat a second time. While being vectored toward the airport, the engine stopped running, resulting in a forced landing in a muddy corn field.
The investigation
A post-accident inspection of the aircraft revealed no mechanical failure or malfunction. Environmental conditions at the time indicated an outside air temperature of 35 degrees and a dewpoint of 34 degrees. According to icing probability charts, these conditions are conducive to the formation of serious carburetor ice at cruise power.