What happened
During a local instructional flight, a two-seat airplane was performing an extended, low-power descending approach toward a remote airstrip. During this phase of flight, the engine lost power. The pilot attempted remedial actions to restore power, which included activating the carburetor heat, but these efforts were unsuccessful.
The flight instructor, seated in the rear seat of the tandem aircraft, determined that the airplane was at an altitude too low to reach the intended airstrip. The instructor took control of the aircraft from the front-seat pilot and performed an emergency landing in soft terrain. The impact caused the aircraft to nose over. There were no injuries reported.
The investigation
A post-accident inspection of the aircraft revealed that there was ample fuel remaining and no mechanical problems were identified prior to the impact. The front-seat pilot stated that the carburetor heat had not been activated until after the loss of engine power occurred, at which point the instructor directed him to turn it on.
Weather conditions at the time included low, wispy clouds. Local weather reports indicated a temperature of 57F and a dew point of 52F.
Findings
Analysis of the temperature and dew point values using a carburetor icing probability chart placed the conditions in the "Serious Icing" category. It is concluded that carburetor ice likely formed within the carburetor, leading to the loss of engine power. Additionally, it was noted that carburetor heat becomes ineffective once the engine has already lost power.