Incident Summary
A private pilot was conducting a cross-country flight when the engine rpm began to decrease. In response, the pilot applied carburetor heat. After a period, the rpm increased to 2500, and the pilot turned off the carburetor heat. The rpm then dropped immediately to 1000. Upon reapplying carburetor heat, the rpm rose to 2000 but subsequently decayed again. The pilot executed an emergency landing at an airport, but the airplane overran the end of the runway and collided with trees.
Inspection Findings
An examination of the airplane by an FAA inspector disclosed two issues: the carburetor heat control was not functioning properly, and the engine fuel mixture arm was slipping on the mixture control cable. The pilot stated in a written report to the NTSB that he was not aware of any mechanical problems with the airplane that could have contributed to the event.
Weather Conditions
A carburetor icing probability chart indicated that the temperature and humidity conditions in the vicinity of the accident were conducive to carburetor icing. The chart suggested that the ambient environment could have promoted ice formation in the carburetor.
Conclusion
The sequence of events involved engine power fluctuations, emergency landing, and runway overrun. The investigation identified mechanical discrepancies with the carburetor heat control and mixture control cable, but the pilot reported no prior awareness of such issues. The weather conditions were noted as favorable for carburetor icing.