Bell 206B N90146 accident near Chualar, California

Probable cause

Maintenance personnel's 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.

NTSB analysis

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.

Text from the NTSB. Open the NTSB docket for WPR13LA358 for the full record.