On July 2, 2016, about 0230 Pacific daylight time, a Bell OH-58A, single-engine, agricultural helicopter, N6251G, experienced a partial loss of engine power while in the vicinity of Brawley Municipal Airport (BWC), Brawley, California. The commercial pilot was not injured. The helicopter was substantially damaged during the precautionary landing to a cropped field. The helicopter was registered to the Bank of Utah, and operated by Farm Aviation, Inc., Brawley, California, as a Title 14 Code of Federal Regulations Part 137 aerial application flight. Night visual meteorological conditions were reported about the time of the accident. A flight plan was not filed for the local flight which departed shortly before the accident. The pilot reported that after he departed BWC with a full chemical load, he maneuvered about 1/4 mile from the airport and made a 90° turn to line up on the field to be sprayed. Shortly after lining up, the engine started to surge, and the torque meter needle was "bouncing rapidly." The pilot then initiated a precautionary landing. The helicopter landed hard in an open field and came to rest in an upright position. Examination of the helicopter, which was conducted by a representative from the Federal Aviation Administration (FAA), revealed that the tailboom and tail rotor driveshaft had sustained impact damage consistent with a main rotor strike as a result of the hard landing. The onsite examination of the airframe and engine revealed no anomalies. Helicopter maintenance documentation was not available during the investigation. According to the pilot, the airframe had a total operating time of 13,688 hours at the time of the accident. The engine had a total operating time of 5,618 hours. The last 100-hour inspection was performed on June 15, 2016, about 74 hours prior to the accident flight. The helicopter was moved to a secure location and the engine was shipped to a facility for further examination. Examination of the engine was conducted at Aero Maritime in Phoenix, Arizona on July 28, 2016. An FAA inspector and a Rolls-Royce investigator were present for the examination. The engine was set up in a test cell and during the test run the engine's bleed valve did not close at any speed; the engine produced less than 250 horsepower. When the chip detectors were removed it was revealed that the lower detector had metal debris present and the upper detector had light amounts of metal debris present. When the bleed valve was replaced with an overhauled bleed valve, an additional test run resulted in increased performance rating of about 380 hp being produced (about 7% below minimum allowable power for a serviceable engine) before reaching the maximum specified operating temperature. After the engine test run, the engine was revealed to be unserviceable due to its low power production. No component log card or historical information for the bleed valve was found. According to the operator, the compressor, turbine and bleed valve assemblies were overhauled in July 2014, and assembled as an engine. According to Rolls-Royce, the overhaul period for the bleed valve assembly is 1,500 hours. No log cards were available for the bleed valve installed. The engine had 766 hours since overhaul at the time of the accident. When the accident engine's bleed valve was disassembled it was found that the diaphragm had several holes and cracks. The poppet valve located in the bleed valve assembly, had radial play consistent with worn bushings. According to Rolls-Royce, the non-functioning bleed valve exhibited wear that was not consistent with a bleed valve with comparable service time.