NTSB Factual Report

On April 21, 2002, approximately 1030 Pacific daylight time, an experimental Osprey II amphibious airplane, N56GB, sustained substantial damage during an attempted forced landing on the Clearwater River, near Orofino, Idaho. The commercial pilot, who was the sole occupant, was not injured, but the aircraft, which was owned and operated by the pilot, sustained substantial damage. The 14 CFR Part 91 pleasure flight, which departed Orofino Municipal Airport about 60 minutes earlier, was being operated in visual meteorological conditions. No flight plan had been filed. The ELT, which was activated by the impact, was turned off after the aircraft was recovered.

According to the pilot, he recently purchased the experimental aircraft, which had accumulated about 80 hours total time since it was manufactured, and this was his first flight in it as sole manipulator of the controls. He performed high-speed taxi tests the previous day, and also prior to takeoff on the day of the accident. After departing the airport, he proceeded to an area about five miles to the west, where he practiced various flight maneuvers in order to become comfortable with the aircraft's feel and response. After completing these maneuvers, he established a straight-in descending final approach to the easterly runway. As he reduced the engine power below 2,000 rpm, he applied full carburetor heat. About one mile from the airport, the aircraft's engine began to run rough, and soon thereafter stopped producing power. At that point, the pilot turned on the electric fuel pump, checked the mixture, and tried pumping the throttle. Being unable to restart the engine, and believing that he could not stretch his glide to the end of the runway, he attempted a forced landing on the nearby river. During the landing, the pilot, who does not hold a seaplane rating, flared too high and during his attempt to correct for the misjudged height, one wing impacted the surface of the water.

An FAA Airworthiness Inspector conducted a post-accident inspection of the engine and its subsystems, and discovered that the aircraft's carburetor heat system was inadequate to supply sufficient warm air to the carburetor throat. In addition to the fact that the hot air collection muff was approximately half the size of those found on type-certificated aircraft with the same series Lycoming O-320 engine, the carburetor heat box butterfly valve fit loosely and did not have sealing material on the edge that separated cold air from hot. The inspector also found that some of the support wire in the scat tubing that delivered the hot air to the carburetor throat had collapsed.

It was also determined that according to the FAA/DOT Carburetor Icing Probability Chart, the ambient conditions at the time of the accident were conducive to moderate icing at cruise power and severe icing at glide power.

NTSB Probable Cause

The aircraft's inadequate carburetor heat air box system, leading to an accumulation of ice in the carburetor throat during final approach to landing. Factors include carburetor icing conditions, the pilot's failure to correctly judge the height of the aircraft above the landing surface, and the pilot's total lack of experience in executing water landings.