Background
A local instructional flight was conducted in a tandem two-seat airplane with a flight instructor in the rear seat and a pilot in the front seat. The flight involved an extended, low power descending approach to a remote airstrip.
Sequence of Events
During the approach, the engine lost power. The front-seat pilot attempted to restore power, including turning on the carburetor heat upon the instructor's direction, but these actions were unsuccessful. The instructor, realizing the airplane was too low to reach the airstrip, took control from the front-seat pilot and executed an off-airport emergency landing in soft terrain. During the landing, the airplane nosed over.
Investigation Findings
Post-accident inspection revealed ample fuel and no pre-impact mechanical problems. The front-seat pilot reported that he did not have the carburetor heat on before the engine lost power, and that there were low, wispy clouds in the area. A local weather report indicated a temperature of 57°F and a dew point of 52°F. When these values are entered into a carburetor icing probability chart, the results fall under the "Serious Icing" category.
Probable Cause
The investigation concluded that carburetor ice likely formed inside the carburetor, resulting in a loss of engine power. It was noted that carburetor heat becomes ineffective once the engine has lost power.