Chualar, CA, USA
The pilot stated that he was operating the helicopter at a low altitude when he heard a loud boom and then noted a decrease in main rotor rpm. During the ensuing emergency landing, the helicopter impacted the ground, the tail boom separated from the fuselage, and the helicopter rolled onto its right side. Postaccident examination revealed that the main driveshaft had disconnected and that the bolts attaching the forward coupling to the transmission had failed. Portions of two of the four bolts that attached the forward coupling to the transmission were found, and the fractured threaded remnants (with nuts attached) were examined. One of the remnants was fractured through its entire cross-section due to fatigue, and the other remnant fractured through about 75% of its cross-section due to fatigue; the remainder of the failure on this remnant occurred in overload. Both fracture faces had multiple fatigue origins around about 1/4 of their circumferences, indicating relatively high stresses. No mechanical damage was noted at the fatigue origin locations. One of the remnants displayed smearing of adjacent threads and decreased height of the nut, suggesting this bolt had a relatively large amount of play. It is likely that the large amount of play was due to improper torqueing of the bolts when the driveshaft was reinstalled after extensive maintenance that was performed on the helicopter about 3 months and 197 flight hours before the accident.
On August 4, 2013, about 1030 Pacific daylight time, a Bell 206B-III helicopter, N90146, was substantially damaged when it landed hard during a forced landing near Chualar, California. The commercial pilot, the sole occupant on board, received minor injuries. R and B Helicopters, Inc., Salinas, California, was operating the helicopter under the provisions of 14 Code of Federal Regulations Part 137. Visual meteorological conditions prevailed for the local aerial application flight, which was operating from a remote supply truck nearby. The helicopter had been airborne for about 10 minutes, and was returning to the truck for more applicant at the time of the accident. A flight plan had not been filed. The pilot reported hearing a loud boom followed by a decrease in main rotor rpm. He reported having minimal flight controllability to ground impact seconds later. The tail boom separated during the run-on landing, and the fuselage subsequently rolled onto its right side. Postaccident examination of the wreckage by a Federal Aviation Administration (FAA) inspector and the operator found that the main driveshaft connecting the engine to the main transmission had disconnected. The part was sent to Bell Helicopters Metallurgical Laboratory, Fort Worth, Texas, where it was examined under the supervision of an FAA inspector. Examination of the driveshaft revealed 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 that attached the forward coupling to the transmission were examined; no remnants of the other two bolts were recovered. The two remnants both fractured in a relatively high cycle fatigue mode. One of the remnants fractured through its entire cross-section via fatigue, and the other remnant fractured through about 75% of its cross-section via fatigue before it completed the failure in overload. Both fracture faces had multiple fatigue origins around approximately 1/4 of their circumferences indicating relatively high stresses. No mechanical damage was noted at the fatigue origin locations. One of the remnants displayed smearing on the faying surface, smearing of adjacent threads, and decreased height of the nut, suggesting this bolt had a relatively large amount of play. On May 6, 2013, the helicopter had undergone an overhaul of most of its major components, including the transmission and engine. At this time, the main driveshaft was removed and replaced. The helicopter had flown 197 hours since completion of this work.
Maintenance personnels failure to properly tighten the bolts securing the main driveshaft to the transmission, which resulted in fatigue failure of the bolts and disconnection of the driveshaft during low-altitude flight.
Source: NTSB Aviation Accident Database
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