What happened
During a normal descent for landing, a MD 520N (NOTAR) helicopter entered an uncommand and uncontrollable right spin. The incident occurred as the aircraft was passing over a group of trees, approximately 15 to 30 feet above ground level and 60 feet from the intended touchdown point.
In response to the rapid, uncommanded right yaw, the pilot applied full left pedal pressure to stop the rotation; however, these control inputs had no effect on the spin rate. The pilot subsequently retarded the left engine throttle and maneuvered the aircraft away from the trees during the continued descent. Upon touchdown, the helicopter rolled onto its left side, resulting in the main rotor blades impacting the terrain. There were no injuries reported.
The investigation
The investigation focused on the NOTAR anti-torque yaw control system, which uses a transmission-driven fan with variable pitch blades to supply air to circulation control slots and a directional jet thruster nozzle. Examination of the control system revealed a fracture and separation in the force limiting control rod.
When this rod assembly becomes disconnected, the NOTAR fan blades revert to a neutral pitch. This prevents the delivery of sufficient airflow to the tail boom circulation control slots to provide anti-torque control and maintain heading. The rod assembly had been replaced approximately 779 hours before the accident following previous mechanical issues.
Using a scanning electron microscope, investigators examined the fracture surface and determined that the rod separated due to a fatigue crack originating from the base of the thread roots, followed by a final tension overload. No evidence of foreign material inclusions or other initiating conditions was found.