Incident Details
A pilot and passenger were traveling on a night cross-country flight when the engine experienced a partial power loss. The pilot engaged the carburetor heat for approximately 10 seconds. Observing no change in engine operation, he returned the carburetor heat to the cold setting and informed the passenger that he believed the engine was suffering from vapor lock. Despite attempts to restore full power, the engine did not recover. The pilot executed a forced landing off the airport, during which the aircraft struck trees.
Post-Accident Examination
Examination of the accident site revealed evidence of a fuel spill. The engine was subsequently run and no mechanical problems were identified.
Weather Conditions
Weather data along the route of flight indicated that the temperature and dewpoint spreads were decreasing as the flight progressed toward its destination. At the accident site, the temperature dewpoint spread was recorded as 3°C. According to FAA publications, a temperature dewpoint spread of 11°C corresponds to a relative humidity of approximately 50 percent, and under such conditions there is an increased likelihood of carburetor icing. The FAA further stated that if carburetor ice is detected, full carburetor heat should be applied immediately and left in the on position until the pilot is certain that all ice has been removed. An FAA carburetor icing chart indicated that the airplane was approaching conditions associated with serious icing at cruise power.
Toxicology Results
Toxicological examination detected an over-the-counter antihistamine in the pilot's blood. The adverse effects of this antihistamine include drowsiness.