Accident Sequence
While on approach to the landing area, the pilot could not see a windsock and estimated the wind by watching leaves on trees. He approached at a "normal to steep angle," stabilized at approximately 40 knots indicated airspeed, about 100 feet above the ground. The helicopter then began a slow, un-commanded yaw to the right. The pilot responded by applying left anti-torque pedal, but the helicopter continued to yaw right. The pilot observed that engine and rotor rpm were "out of the green" but felt unable to enter an autorotation because a power line was below. He increased the throttle, and the right yaw continued. About 10 feet above the ground, after approximately 270 degrees of yaw, the helicopter descended rapidly and impacted the ground, seriously injuring the passenger.
Post-Accident Examination
Examination of the helicopter following the accident revealed that the tail rotor had sustained impact damage to both blades, and the tail rotor drive shaft was fractured at the coupling hub. Examination of the fractured portions of the coupling hub showed signatures consistent with a torsional event, such as a tail rotor strike, and no evidence that the fracture surfaces were rubbing under power.
Background on Loss of Tail Rotor Effectiveness
According to Federal Aviation Administration Advisory Circular 90-95, loss of tail rotor effectiveness (LTE) events typically occur in a low airspeed flight regime while maneuvering, such as on final approach to landing. Any maneuver that requires the pilot to operate at high power, low airspeed, and with a left crosswind or tailwind creates an environment where unanticipated right yaw may occur. Additional factors that can influence the severity of LTE onset include increases in gross weight and density altitude, low indicated airspeeds, and power droop. Recovery from an LTE event should include applying full left pedal, while simultaneously moving the cyclic forward to increase speed, and, altitude permitting, reducing power.