On May 21, 2014, about 1430 mountain daylight time, a Rockwell International S-2R, single engine, tailwheel-equipped, agricultural airplane, N702CB, registered to Boedeker Contracting Inc., of Childress, Texas, and operated by Clear Skies Aviation, LLC, of Medicine Lake, Montana, sustained substantial damage after colliding with terrain while maneuvering near Medicine Lake. The airline transport pilot, who was the sole occupant of the airplane, was not injured. Visual meteorological conditions prevailed and a flight plan was not filed for the 14 Code of Federal Regulations Part 137 aerial application flight. The local flight departed from Sher-Wood Airport, Plentywood, Montana, about 1315.
The pilot reported that while maneuvering into a left banking turn following the last aerial application pass on a field, he couldn't remember what happened until he saw the ground ahead of him. The pilot tried to correct the airplane's attitude, and subsequently collided with terrain. After the airplane came to rest upright in an open field, the pilot evacuated the airplane and noted that the propeller assembly was in a feathered position.
A review of the aircraft logbooks revealed that the airframe and engine underwent its most recent 100-hour inspection on March 1, 2014. At the time of the inspection, the engine had accumulated 12,833 hours since new and 58.4 hours since its hot section inspection. The airframe had accumulated a total of 6,653.8 hours of flight time.
An examination of the airplane, which was conducted by a representative from the Federal Aviation Administration (FAA), revealed that the rear fuselage was buckled, and the empennage and both wings had impact damage. It was further revealed that the outboard 4 feet of the left wing had separated. The propeller assembly revealed signatures on the hub and propeller blades that were consistent with ground impact. Flight control and engine control continuity was confirmed. The fuel system revealed no anomalies other than a small amount of debris found in the filter bowl.
The engine was examined at the Honeywell Product Integrity Laboratory, Phoenix, Arizona, by representatives from Honeywell, and the FAA under the supervision of the National Transportation Safety Board investigator-in-charge. The engine was disassembled. The examination of the shouldered main shaft displayed heat distress areas near four carbon bushing lands associated with the torsion shaft. The torsion shaft separated in a manor indicative of torsional overload. The torsion shaft and the shouldered main shaft overran each other, and thermal distress and rotational scoring was noted at the carbon bushing lands. According to Honeywell representatives, the separation signatures of the torsion shaft are consistent with a torsional overload due to sudden stoppage of a rotating engine at impact. In addition, both centrifugal compressor impellers and associated shrouds displayed rotational scoring through 360 degrees, which is indicative of a rotating engine at impact. Compressor shroud metal splatter was also found in the turbine section indicative of engine operation at the time of impact, although the power level could not be determined. The fuel control unit and propeller governor were further examined with no anomalies noted. The examination of the recovered engine and system components revealed no evidence of mechanical malfunctions or failures that would have precluded normal operation.