Accident Details
A helicopter being used to support United States Forest Service (USFS) firefighting operations crashed while attempting to deliver supplies via external load. Prior to the approach to the intended landing zone (LZ), the pilot overflew the location and was informed that winds were from the west at approximately 7 knots. The pilot then performed a hover check about half a mile east of the LZ over a valley. He moved the helicopter west to the LZ at a speed just above effective translational lift and came to a hover with the load approximately 30 to 40 feet above the landing spot. The pilot checked the engine and operating instruments, which all indicated normal.
Loss of Control
As the pilot began looking down and outside the helicopter, he felt a yaw to the right. He noticed that he was "depressing the left [pedal] more as I pushed on the left to stop a right turn." The helicopter then made an uncommanded turn to the right and began spinning right. After two to three rotations, the pilot jettisoned the cargo portion of the long line and lowered the collective. He attempted to regain control by adding power and pitch to the main rotor, but the aircraft immediately started spinning again. The pilot then lowered the collective and began a glide into the trees. The helicopter subsequently impacted trees and terrain, coming to rest on its left side.
Examination and Findings
Examination of the aircraft revealed no anomalies that would have precluded normal operation. However, prior to the flight, the pilot incorrectly computed the performance and load calculations for the operation. These incorrect calculations were also not properly verified by the USFS helicopter manager. Additionally, USFS personnel miscalculated the weight of the external load, which was unknown to the pilot. A review of the rotorcraft flight manual indicated that the maximum gross weight limitation was exceeded given the environmental conditions at the time of the accident. The calculated density altitude was 11,465 feet.