On July 20, 2003, about 0130 Pacific daylight time, a Bell 47G-2A, N4774S, experienced a mechanical malfunction while maneuvering during an agricultural spray flight about 10 miles west of Porterville, California. The helicopter descended 10 feet, impacted the cotton field, rolled over, and was substantially damaged. The commercial pilot was not injured. The helicopter was operated by John Slykerman Ag Aviation, Inc., Bakersfield, California, under the provisions of 14 CFR Part 137. Visual meteorological conditions prevailed. The flight originated from the field about midnight. The pilot reported to the National Transportation Safety Board investigator that during the flight he heard the engine's transmission make a loud noise and "disengage." Thereafter, the engine oversped as the helicopter suddenly yawed left. The pilot stated that he immediately attempted to land. The helicopter impacted the underlying field that had an uneven surface, and the helicopter rolled over. The helicopter rotor drive system was subsequently examined. The Federal Aviation Administration investigator verbally reported that the examination revealed 12 bolts on the planetary gear flange were found broken due to fatigue.