On January 22, 2010, approximately 0909 Pacific standard time, a Bell 47G-4A helicopter, N28938, sustained substantial damage following a forced landing near Shafter, California. The commercial pilot, the sole occupant, received minor injuries. Inland Crop Dusters, Inc., was operating the helicopter under the provisions of 14 Code of Federal Regulations Part 137. Visual meteorological conditions prevailed for the aerial application flight. A flight plan had not been filed. The pilot reported that while maneuvering at 80 feet to make an aerial application run, he heard a muffled "crack/bang." The helicopter immediately yawed and the pilot experienced lateral vibrations in the cyclic. He initiated a forced landing to the field below and the shaking became more "violent." He said the engine was still running, but the aircraft landed hard. The main rotor blades contacted and severed the tail boom, and the engine separated from its mounts. A post accident examination of the helicopter by a Federal Aviation Administration inspector revealed that a main rotor blade counterweight was missing. There was a search for the missing part, but it was never located. Examination of the broken counterweight rod end showed evidence of corrosion over approximately 45 percent of the diameter of the rod end. Maintenance records indicate the rod was not inspected for corrosion pits to comply with Bell Helicopter Textron Service Bulletin 47-78-2 Rev. B, April 13, 1981. The Service Bulletin states that after an initial inspection for corrosion pits on the counterweight rods, recurrent inspections will be accomplished every 300 hours of time in service.