Accident Overview
On January 22, 2010, at approximately 0909 Pacific standard time, a Bell 47G-4A helicopter, registration N28938, sustained substantial damage in a forced landing near Shafter, California. The commercial pilot, who was the sole occupant, received minor injuries. The helicopter was operated by Inland Crop Dusters, Inc., under 14 Code of Federal Regulations Part 137 for an aerial application flight. Visual meteorological conditions prevailed, and no flight plan was filed.
Sequence of Events
The pilot reported that while maneuvering at an altitude of 80 feet to conduct an aerial application run, he heard a muffled "crack/bang." The helicopter immediately yawed, and the pilot experienced lateral vibrations in the cyclic control. He initiated a forced landing to the field below, during which the shaking became more violent. Although the engine was still running, the helicopter touched down hard. The impact caused the main rotor blades to sever the tail boom, and the engine separated from its mounts.
Examination Findings
A post-accident examination by a Federal Aviation Administration inspector revealed that a main rotor blade counterweight was missing. A search for the missing part was conducted but was unsuccessful. Examination of the broken counterweight rod end showed evidence of corrosion over approximately 45 percent of the diameter of the rod end. Maintenance records indicated that the rod had not been inspected for corrosion pits in compliance with Bell Helicopter Textron Service Bulletin 47-78-2 Rev. B, dated April 13, 1981. That service bulletin requires an initial inspection for corrosion pits on the counterweight rods, followed by recurrent inspections every 300 hours of time in service.