The pilot reported that about one hour into the flight the engine rpm began to decrease. The rpm quickly worsened, and he applied carburetor heat, but the engine did not respond before losing all power. The pilot conducted a forced landing to rough terrain, during which the airplane struck a ditch, substantially damaging the fuselage. The pilot reported that there were no preaccident mechanical malfunctions or failures with the airplane that would have precluded normal operation and that loss of engine power was likely the result of carburetor icing. The weather reported at an airport about 13 nautical miles from of the accident site included a temperature of 14°C and a dew point 3°C. The calculated relative humidity at this temperature and dewpoint was 47 percent. Review of the icing probability chart contained within Federal Aviation Administration Special Airworthiness Information Bulletin CE-09-35 revealed the atmospheric conditions at the time of the accident were "conducive to serious icing at glide power."