NTSB investigators either traveled in support of this investigation or conducted a significant amount of investigative work without any travel, and used data obtained from various sources to prepare this aircraft accident report.
The pilot and passenger were en route on a night cross-country flight when the engine experienced a partial power loss. The pilot pulled on carburetor heat for about 10 seconds and then with no change in engine operation, returned the carburetor heat to cold and told the passenger he thought the engine was experiencing vapor lock. The pilot was unable to restore full power and struck trees during the execution of an off airport, forced landing. Examination revealed evidence of a fuel spill at the accident site. The engine was run and no problems were noted. A check of weather along the route of flight revealed the temperature and dewpoint spreads were getting closer together as the flight continued towards its destination. The spread at the accident site was 3 degrees C. According to FAA publications a temperature spread of 11 degrees C corresponds to a relative humidity of about 50 percent, and there is an increased likelihood of carburetor icing. Further, the FAA stated, "If detected, full carburetor heat should be applied immediately, and it should be left in the on position until the pilot is certain that all the ice has been removed." The FAA carburetor icing chart revealed the airplane nearing conditions of serious icing at cruise power. The toxicological examination found an over-the-counter antihistamine in the pilot's blood. Adverse affects for the antihistamine included, drowsiness.