Incident
On January 15, 2009, at approximately 0845 mountain standard time, an Air Tractor AT-301, registration N3170C, operated by Flying B Aviation, LLC, was conducting an aerial application flight about 10 miles southeast of Beaver, Utah. While maneuvering, the engine began to vibrate severely and lost partial power. The pilot performed a forced landing on a snow-covered field. During the rollout, the airplane nosed over, bending a wing and the rudder. The commercial certificated pilot sustained a minor injury, and the airplane was substantially damaged. Visual meteorological conditions prevailed, and no flight plan had been filed. The flight was conducted under 14 Code of Federal Regulations Part 137 and had originated from Beaver at about 0830.
Investigation
The pilot verbally reported to a National Transportation Safety Board investigator that while seeding the designated area, the engine started vibrating very badly. The Pratt & Whitney R-1340 engine had been overhauled several times, and the pilot suspected a mechanical malfunction had occurred.
The airplane was subsequently recovered from the accident site and examined under Federal Aviation Administration (FAA) supervision. The FAA inspector reported finding no evidence of internal engine malfunction when the crankshaft was rotated. No contaminating material was observed in the oil, which appeared clean.
Findings
The FAA inspector noted one observation that, in their opinion, may have related to the vibration reported by the pilot: one of the propeller's pitch change steel pins, which changes the beta angle of the propeller blade, was found sheared in two. No "Aircraft Accident Report" NTSB form 6120.1 was received by the Safety Board from the pilot or operator.