What happened
During a cross-country flight, the pilot made the decision to return to the departure airport because of adverse weather conditions. While the aircraft was approximately 10 miles south of the field and on final approach for landing, the pilot communicated with tower personnel to report his position. After receiving standard landing information, the pilot reported an unexpected loss of engine revolutions per minute (RPM) that he could not correct.
The situation deteriorated rapidly, forcing the pilot to ditch the aircraft in nearby waters. Fortunately, a Coast Guard vessel located and rescued the pilot from the water. At the time of the incident, visual meteorological conditions prevailed, but visibility was reduced to five miles due to the presence of haze and fog. The temperature was 67 degrees Fahrenheit with a dewpoint of 66 degrees Fahrenheit.
The investigation
The aircraft has not been recovered from the water, preventing a physical examination of the engine or airframe. However, weather data indicates conditions conducive to carburetor ice formation. Advisory circulars note that when temperatures are between 20 and 70 degrees Fahrenheit with visible moisture or high humidity, pilots should be alert for this issue. The initial sign of carburetor ice is typically a loss of RPM.
Findings
The combination of high humidity and a narrow temperature-dewpoint spread created an environment where carburetor ice was a significant risk. The pilot's report of uncorrectable RPM loss aligns with the classic symptoms of this mechanical failure mode. The lack of aircraft recovery means no further mechanical evidence is available to confirm the specific cause, but the weather data strongly supports the likelihood of icing.