WPR13TA051 · Eurocopter AS350B3 2B1

About 2 hours into his mission, the pilot decided to take a lunch break. After landing the helicopter in a suitable area, although with potholes and cattle hoof prints in the dried mud, the pilot performed a stability check with the cyclic and then lowered the collective to the full-down position. With the engine at 100-percent power, the pilot added cyclic and collective friction to prevent the controls from inadvertently moving while he reached to retrieve his lunch from a bag located just aft and right of his seat; he did not engage the collective lock. The pilot let go of the cyclic with his left hand and, while guarding it with his legs, used both hands to seal the lunch bag and replace it behind the seat, which resulted in the pilot experiencing a momentary loss of situational awareness due to distraction. At that moment, the pilot felt the nose of the helicopter begin to get light on the skids with a slight upward pitch change. The pilot then placed both hands back on the controls and added forward cyclic to correct for the increase in pitch and to ensure positive skid contact with the ground; the cyclic and collective friction remained applied. The helicopter then began to exhibit vertical dynamic oscillations, which continued to resonate and worsen, consistent with the onset of ground resonance. It is likely that at least one, if not both, ground resonance straps were in a pothole depression and not in contact with the ground. When the condition worsened, the pilot made a positive collective application to lift the helicopter off the ground in an attempt to regain stability. As the helicopter began to ascend, it lurched forward and downward in an unusually nose-low attitude, which resulted in the lower wire strike blade contacting the ground and establishing a pivot point. The pilot then made an aggressive aft cyclic movement with a positive collective input to arrest the nose-down attitude. The tail skid subsequently impacted the ground, pushed the vertical fin and tail rotor gearbox upward, and severed the tail rotor drive shaft forward of the tail rotor gearbox. The tail of the helicopter then bounced upward from the impact followed by an uncommanded left yaw around its nose. The helicopter continued to spin left until the pilot reduced the collective to arrest the spin. The helicopter then touched down in a nose-up attitude and settled upright on its skids. A postaccident examination of the helicopter revealed that the pilot's inability to regain control was due to the loss of the tail rotor drive, which occurred as a result of the pilot's aggressive overapplication of the flight controls. The pilot's actions resulted in the tail skid impacting terrain, which damaged the tail rotor gear box and severed the tail rotor drive shaft.

Verbatim NTSB analysis and probable cause from the NTSB dataset