While on a visual approach to the runway during the instructional flight, the engine lost partial power. The flight instructor reported that the engine was operating normally and then the engine rpm abruptly decreased from 2,200 to about 1,300. The carburetor heat was in the OFF position for the approach. The flight instructor's efforts to regain engine power were unsuccessful. Unable to make a landing at the airport, the instructor executed a forced landing to a field. The airplane touched down short of the airport, nosed over, and came to rest inverted. Postaccident examination of the airplane revealed that the vertical stabilizer and rudder were bent; no mechanical anomalies that would have precluded normal operation were identified. The engine was test run at various power settings with no anomalies noted. Although a carburetor icing probability chart revealed that the ambient temperature and dew point at the time of the accident favored serious carburetor icing at glide power, the pilot reported that the engine was not at glide power when the power loss occurred. Accordingly, the investigation was not able to identify the reason for the power loss.