On August 31, 2019, about 2217 Pacific daylight time, a Hughes 369D helicopter, N54528, was substantially damaged when it was involved in an accident near Asotin, Washington. The pilot and passenger were not injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that he and his passenger departed from a private residence near the Snake River in dark night conditions. The pilot recalled that shortly after lifting off, the helicopter yawed severely to the left. He applied full right anti-torque pedal and the helicopter subsequently impacted the water. He and the passenger egressed the helicopter, which subsequently sank, and were rescued shortly thereafter. The pilot stated that the helicopter had experienced a loss of tail rotor effectiveness, and that the accident was not due to anything he did incorrectly. The mechanic who was repairing the helicopter after the accident reported that the aft tail rotor aft drive flex coupling had sheared. He stated that none of the four tail rotor blades were damaged during the accident sequence. No follow-up examination of the helicopter was performed, and the reason for the loss of control could not be determined.