Introduction
On June 8, 2013, about 1900 central daylight time, an Air Tractor AT-301 airplane, registration N4421S, was involved in a forced landing that resulted in substantial damage near Farnsworth, Texas. The aircraft, operated by Terhune Flying Service under 14 CFR Part 137 as an aerial application flight, was not on a flight plan and operated under visual meteorological conditions.
Flight History
The pilot reported completing two flights earlier that morning but halting operations due to adverse weather. When conditions improved, flight operations resumed, and the accident flight was the first after this break.
Accident Sequence
While starting the first spray pass, the engine experienced a partial loss of power, and the pilot could not maintain altitude. The pilot attempted to jettison the chemical load, but the airplane continued descending. A forced landing in a field was attempted; during the landing, the right main landing gear collapsed, and the aircraft came to rest on its nose and right wing. The pilot stated that the engine ran smoothly but at a reduced power output insufficient for level flight.
Postaccident Examination
A postaccident inspection revealed no anomalies that could explain the partial loss of engine power. The engine was intact with no external pre-impact damage. Internal examination of pistons, cylinders, valves, and the fuel system showed no defects.
Environmental Conditions
At a nearby airport around the time of the accident, the reported temperature was 26 degrees Celsius and the dew point was 15 degrees Celsius. According to a carburetor icing probability chart, these conditions fell within the range of susceptibility for carburetor icing at glide and cruise power settings.
Conclusion
The circumstances of the partial power loss remain undetermined, as no mechanical or system anomalies were found. The environmental conditions were conducive to carburetor icing, though no direct link was established.