Accident Details
On August 14, 2011, at approximately 1230 central daylight time, an Air Tractor AT-301, registration N73071, experienced a partial loss of engine power and executed a forced landing to a bean field near Edgeley, North Dakota. The aircraft sustained substantial damage to its fuselage. The commercial pilot, who was the sole occupant, was not injured. The airplane was operated by Semper Fi Aviation under 14 Code of Federal Regulations Part 137 as an aerial application flight. Visual meteorological conditions prevailed for the local flight, which was not operating on a flight plan. The flight originated from Edgeley Municipal Airport (51D) at about 1225.
Flight Events
According to the pilot, he performed a normal takeoff, climbed to about 200 feet above ground level, and established level flight. He then reduced engine power to 30 inches of manifold air pressure (MAP) and 2,000 rpm. After several seconds of normal flight, he noticed a descent developing and increased power to 32 inches MAP, but received no response. The pilot added one notch of flaps and dumped the chemical load using the emergency dump handle, but the descent continued. He subsequently performed a forced landing in a bean field.
Post-Accident Examination
On-scene examination by Federal Aviation Administration (FAA) inspectors did not reveal any preimpact mechanical failures. The engine was separated from the airplane, and firewall damage had occurred. There were no outward signs of a mechanical failure of the engine. At the request of the local landowner, the pilot agreed to leave the airplane in the field until after the crops were harvested. No further examination of the airplane was conducted after removal from the field.
The airplane was powered by a supercharged Pratt & Whitney R-1340-AN-1 engine, serial number 5684, rated to produce 600 horsepower for takeoff and 550 horsepower continuously. Maintenance records indicated that the airplane had received an annual inspection completed on April 6, 2011, which noted all cylinder compressions were 75/80 or better, and the oil filter was removed and cut open with no anomalies found.
Environmental Conditions
The temperature and dew point recorded at Oakes Municipal Airport, about 29 nautical miles east of the accident, were 24°C and 17°C, respectively. According to FAA Special Airworthiness Information Bulletin (SAIB) CE-09-35, these readings were in a range susceptible to the formation of carburetor ice at glide and cruise power settings. The SAIB noted that carburetor icing can be detected by a drop in manifold pressure, though roughness in engine operation is typical. The pilot made no mention of roughness in operation.