Incident Description
A pilot with 10,300 hours of flight experience reported an in-flight emergency while cruising at 500 feet and 110 knots. He heard a loud bang, which was immediately followed by severe and constant vibration. The pilot noted that pedal inputs seemed ineffective in correcting the aircraft's yawing. Concluding that continued flight was impossible, he initiated a forced landing.
Emergency Maneuver and Landing
The pilot began a 180-degree autorotation to the left. During the turn, he broadcast a mayday call and reduced the throttle, which had no effect. After closing the throttle, the pounding and vibration diminished. When the aircraft rolled out and was level at 100 to 150 feet, the pilot realized they would strike a pond levee. He applied collective to clear the obstacle, executed a cyclic turn to the right, and cushioned the landing. Upon landing, the main rotor blade contacted the tailboom.
Post-Accident Examination
Examination of the wreckage by the operator and a Federal Aviation Administration inspector revealed structural damage to the tailboom. Strike marks on the tailboom, corresponding to the main rotor blade top path plane, were found adjacent to the #7 tail rotor drive shaft segment. Additionally, the left horizontal stabilizer winglet was missing. Further inspection uncovered a ten-inch crack in the fixture securing the left winglet to the horizontal stabilizer. The right winglet, which remained attached, also had a pre-existing crack in the same area. According to the operator, these cracks were not visible during visual inspection because the crack area lies under a line of structural adhesive used during manufacture, becoming visible only when the winglet is removed.
Operator's Assessment
The operator stated that the ten-inch crack on the left winglet appeared to have existed for some time. They suggested that the winglet could have separated from the helicopter in flight, struck the tail rotor, and caused the loud bang heard by the pilot, leading to a loss of directional control. Other than the winglet issues, examination of the helicopter and its drive and flight control components revealed no pre-existing anomalies that could have contributed to the accident.