Bell 206L 3 accident near Dickinson, Alabama, October 15, 2015

The NTSB's narrative final · quoted from the NTSB record

During an aerial application flight, the helicopter was flying between 70 and 120 ft above trees, and the pilot heard a "pop" sound followed by an immediate loss of tail rotor thrust. He lowered collective and descended into trees, and the helicopter came to rest on its left side. Examination of the helicopter determined that the loss of tail rotor thrust was precipitated by a disconnect in the tail rotor drive system. A self-locking nut securing one of the two bolts that attached the aft end of the first tail rotor driveshaft (shaft S1) aft of the oil cooler blower to the disc pack coupling was missing. The nut and the bolt that it secured were not recovered at the accident site. The separated hardware allowed misalignment of the driveshaft with the axis of rotation, which led to the fracture of shaft S1 and of the second tail rotor driveshaft (shaft S2) aft of the oil cooler blower. The prevailing or tare torque on two out of the remaining three self-locking nuts securing the forward and aft ends of shaft S1 to the disc pack couplings were below the minimum torque specified in the helicopter manufacturer's standard practices manual. The prevailing torque is a measure of the turning resistance of a lock nut, and the turning resistance generally decreases as nuts are reused. About 490 hours earlier, the operator had performed a modification to the helicopter that required in part, removal of shaft S1 and the tailboom assembly. Since this modification, the helicopter had been subjected to four 100-hour inspections. Three lots of defective nuts of the type used to secure components of the tail rotor drive system had been identified; however, the operator could not determine whether they had previously had any of the suspect nuts in stock. Although the possibility of a defective nut resulting in the driveshaft disconnect could not be eliminated, given the insufficient prevailing torque on 2 of the remaining 3 self-locking nuts securing shaft S1 to the disc pack couplings, it is more likely that the separated nut also had insufficient prevailing torque. And it was that insufficient prevailing torque of the missing self-locking nut that had been removed and improperly re-installed by the operator's maintenance personnel, which allowed the self-locking nut to back-off. This accident and six previous accidents involving the same make and model of helicopter illustrate an in-service issue with hardware used to secure the tail rotor driveshafts. As a result of these accidents, Transport Canada issued a safety alert notifying maintenance personnel of the need to check the prevailing or tare torque of hardware securing the tail rotor drive system.

The complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.