Accident Details
On August 4, 2013, at approximately 1030 Pacific daylight time, a Bell 206B-III helicopter, registration N90146, was substantially damaged during a forced landing near Chualar, California. The commercial pilot, who was the sole occupant, sustained minor injuries. The helicopter was operated by R and B Helicopters, Inc., of Salinas, California, under 14 Code of Federal Regulations Part 137 for aerial application. Visual meteorological conditions prevailed, and the flight was local, originating from a nearby remote supply truck. The helicopter had been airborne for about 10 minutes and was returning to the truck for more applicant when the accident occurred. No flight plan had been filed.
The pilot reported hearing a loud boom followed by a decrease in main rotor rpm. He stated that he had minimal flight controllability before ground impact seconds later. During the subsequent run-on landing, the tail boom separated, and the fuselage rolled onto its right side.
Postaccident Examination
A postaccident examination conducted by a Federal Aviation Administration (FAA) inspector and the operator revealed that the main driveshaft, which connects the engine to the main transmission, had disconnected. The driveshaft was sent to Bell Helicopter's Metallurgical Laboratory in Fort Worth, Texas, where it was examined under FAA supervision. The examination found evidence indicating that the bolts attaching the forward coupling to the transmission had failed.
Fractured threaded remnants (with nuts attached) of two of the four bolts were recovered and examined; the other two bolts were not recovered. Both remnants exhibited high cycle fatigue fractures. One remnant fractured through its entire cross-section via fatigue, while the other fractured through about 75% of its cross-section via fatigue before final overload failure. Multiple fatigue origins around approximately one-quarter of the circumferences indicated relatively high stresses. No mechanical damage was noted at the fatigue origin locations. One remnant showed smearing on the faying surface, smearing of adjacent threads, and decreased nut height, suggesting this bolt had experienced a relatively large amount of play.
Maintenance History
On May 6, 2013, the helicopter had undergone an overhaul of most major components, including the transmission and engine. At that time, the main driveshaft was removed and replaced. The helicopter had accumulated 197 flight hours since the completion of that work.