On January 15, 2009, about 0845 mountain standard time, an Air Tractor AT-301, N3170C, operated by Flying B Aviation, LLC, experienced an engine vibration and partial loss of power while maneuvering during an aerial application flight about 10 miles southeast of Beaver, Utah. The pilot made a forced landing on a snow-covered field. During roll out the airplane nosed over, bending a wing and rudder. The commercial certificated pilot sustained a minor injury, and the airplane was substantially damaged. Visual meteorological conditions prevailed at the time, and no flight plan had been filed. The flight was performed under the provisions of 14 Code of Federal Regulations Part 137, and it originated from Beaver about 0830. The pilot verbally reported to the National Transportation Safety Board investigator that, while he was seeding the designated area, the engine started vibrating very badly. The Pratt & Whitney R-1340 engine had been overhauled several times, and he suspected a mechanical malfunction had occurred. The airplane was subsequently recovered from the accident site and examined under Federal Aviation Administration (FAA) supervision. In summary, 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 looked clean. The FAA inspector reported one observation that, in his opinion, may have related to the vibration reported by the pilot. The FAA inspector stated that 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/operator.