Incident Overview
A pilot reported that the engine began to run rough and lose power as the airplane climbed through 9,000 feet mean sea level. In response, she switched fuel tanks and moved the mixture control to full rich, but the engine continued to lose power. The carburetor heat was not used at any point during the flight.
Forced Landing and Collision
A forced landing was subsequently made in a field. During the landing, the airplane collided with a utility pole and a ditch.
Post-Crash Examination
A post-crash examination of the engine revealed no evidence of preexisting mechanical failure or malfunction. Further examination of an icing probability chart indicated that the reported weather conditions in the area were favorable for the formation of moderate carburetor icing at cruise power.
Manual Guidance
The Cessna 172M owner's manual notes that a gradual loss in RPM and eventual engine roughness may result from the formation of carburetor ice, and prescribes the use of carburetor heat to clear the ice.