Accident Overview
On September 11, 2013, at about 0715 Pacific daylight time, a Bell 47G5 helicopter, registration N7836S, sustained substantial damage during a precautionary landing approximately 6 miles southeast of King City, California. The pilot, the sole occupant, was not injured. The helicopter's lower center frame and tailboom sustained substantial damage. The helicopter was registered to and operated by Wilbur-Ellis Corporation as an agricultural spray flight under 14 Code of Federal Regulations Part 137. Visual meteorological conditions prevailed, and no flight plan was filed.
Sequence of Events
The pilot reported that he was spraying a field about 3-4 feet above the ground when he heard a loud bang. He elected to make a precautionary landing. During the landing flare, the helicopter started to spin. The pilot lowered the collective, and the helicopter landed hard onto a road. Subsequently, the helicopter's skids spread, and the lower center frame and tailboom were bent.
Postaccident Examination
Postaccident examination by a Federal Aviation Administration inspector revealed that one of the helicopter's center frame tubes, located just aft of the cabin, had fracture separated. A senior metallurgist at the National Transportation Safety Board materials laboratory reviewed photographs of the fractured surfaces and reported that a fatigue fracture initiated in the tube at the root of a fillet weld associated with a reinforcing filet welded axially along the tube. The fatigue crack propagated through the thickness of the tube and circumferentially around the tube from both sides of the weld root. Darker corrosion product on the fracture face is indicative of a pre-existing fatigue crack that had been exposed to atmospheric elements. The fracture in the tube occurred in single-sided bending consistent with inflight airframe loads.
On August 8, 2013, the helicopter underwent a 100-hour inspection. According to manufacturer's guidance, the 100-hour inspection requires, in part, to "Inspect all structural tubing and fittings for cracks, cuts, bends, corrosion, distortion and damage."