Hughes 369D N920JP accident near Marion, South Carolina

Probable cause

A momentary jam in the tail rotor control system from an undetermined source, resulting in a torsional fracture of the tail rotor control torque tube and a loss of helicopter control.

NTSB analysis

The commercial pilot of the helicopter was trimming trees on a power line right-of-way when the externally-mounted saw blades jammed. He climbed the helicopter out of the area and elected to return to the landing zone (LZ) to have the saw blades cleared. As he began a forward transition directly to the LZ, the helicopter yawed to the right. He initially corrected the situation with left pedal inputs. While maintaining a heading into the wind, he felt a "thump" and heard a "pop" sound, and the helicopter began to spin to the right out of control. The engine continued to run throughout the event. The helicopter settled into trees as the pilot attempted to cushion the landing with collective control inputs. The helicopter subsequently impacted the ground.

An examination of the wreckage revealed a spiral fracture in the tail rotor control torque tube that connected the left and right seat pedals. Metallurgical examination of the torque tube revealed that it failed in overload due to torsional stresses. A design review by the helicopter manufacturer's engineering department revealed that the torque tube met all airworthiness standards and design criteria.

It was apparent that, based on the pilot's comments and the fracture characteristics of the torque tube, it fractured in flight, immediately before the loss of helicopter control. Although no airframe or foreign obstructions were found in the tail rotor control system, it is possible that a momentary jam existed, though the source could not be determined despite a thorough examination of the wreckage. Although the tail rotor pitch control was replaced about 25 hours of time in service before the accident, and a tail rotor control rigging check was required at that time, aircraft damage prevented an evaluation of the tail rotor control rigging condition at the time of the accident.

Text from the NTSB. Open the NTSB docket for ERA16LA030 for the full record.