Event Description
Approximately 35 minutes into flight, the helicopter experienced a sudden and violent right yaw accompanied by a slight nose tuck. The tail rotor pedals moved to near full right position. The copilot reduced collective and indicated airspeed, applied left pedal, and canceled the Flight Director. The aircraft oscillated twice at medium intensity with relatively small magnitude. Control was transferred to the pilot-in-command, who reestablished straight and level flight at reduced power, requiring full right pedal. Cruise power was about 60%.
In-Flight Troubleshooting
The crew reviewed emergency procedures and notified company dispatch. In-flight troubleshooting showed no improvement when securing one or both Helipilot SAS systems. Hydraulic systems were not cycled, but pressure was noted as normal on both systems, and hydraulic boost to the pedals appeared normal. A slight left pedal input confirmed tail rotor control input. Controlled slow flight was possible down to 40 knots, with increasing right yaw as speed decreased.
Approach and Landing
The 12,900-hour pilot executed a normal approach, touching down at 25 to 30 knots, and managed the yaw using collective and throttle inputs.
Examination Findings
Post-flight examination revealed that the upper/left bolt securing the tail rotor pitch control bellcrank support was bent and loose in the nutplate. The lower/left bolt had sheared just below the bolt head, and the lower/right bolt was sheared in the threaded area within the nutplate. Control movement was checked and showed no binding, lost motion, or unusual noises. No additional anomalies were noted in the tail rotor control system.