On June 23, 2000, about 0430 hours Pacific daylight time, a Bell, UH-1B, N807SB, collided with terrain near Arbuckle, California. The helicopter, operated by Pacific Valley Aviation under 14 CFR Part 137 as an aerial application flight, was destroyed. The commercial pilot was fatally injured. The operator reported visual meteorological conditions for the operation and a company flight plan was filed. The flight originated from a chemical loading site about 0415. A worker reported that the pilot took off from the loading truck, made a left turn, and departed to the sunflower field that they were working . The wreckage was located 1/2 mile west of the field. None of the workers in the field reported hearing or seeing the helicopter go down, nor did they hear any distress calls on the company radio. A weather observation was obtained from Chico, California, about 46 miles north of the accident site. At the time of the occurrence (0430), the observation was reporting visibility 2 miles; temperature 59 degrees Fahrenheit; dew point 57 degrees Fahrenheit; and calm wind conditions. A postaccident examination of the helicopter was conducted. The anti-torque control system examination revealed a tail rotor pitch change shaft's AN320 castellated shear nut stripped from the rotor end of the shaft. The threads were still intact on the shaft but missing from the nut. The shaft cotter pin hole retained the sheared portions of the cotter pin. The shearing force came from an unknown source. An ongoing wear imprint from the shaft end into the AN320 castle shear nut was noted by a laboratory report. Nut damage to the interior of the pitch change cap was also found during the wreckage examination. According to the Federal Aviation Administration inspector, it appeared the helicopter collided with level farm terrain while in a gradual level descent. The horizontal and vertical stabilizers and tail rotor assembly appeared to be undamaged. The main rotor mast had rotated about 45 degrees forward, and symmetrical upward crushing of the cabin area occurred. A laboratory report provided by AIG Aviation Insurance is attached. The last documented 100-hour inspection occurred on April 22, 2000.