Accident Overview
A helicopter conducting a sling load operation impacted the ground after an interruption of power. The flight was the first of the day, intended to transport fuel cans from a landing strip on the rim of the Grand Canyon to the canyon floor to refuel tour boats on the Colorado River. The pilot used a 50-foot line and a net to move 15 fuel cans.
Sequence of Events
The pilot performed a power check after picking up the load at an elevation of 4,800 feet msl, noting that 75% torque was required for an out-of-ground-effect hover. After stabilizing the load, he began a descent to the canyon floor (elevation 1,300 feet msl) at a rate of about 50 feet per minute. Witnesses and the pilot reported calm or nearly calm wind conditions.
While watching the ground crew, the pilot heard a tone followed by a series of three low-frequency popping noises. He was unsure whether the sounds originated from the engine or the main rotor blades. The helicopter then began to settle and descended to a hard impact into desert scrub brush. The pilot stated he closed the throttle after hearing the popping noises and entered an autorotation.
Witness Observations
Witnesses reported that the approach was controlled, with no sway or unusual movement of the load. As a ground crewmember reached for the load approximately 5 feet above the ground, he observed the helicopter sink down and impact the scrub brush and ground. This witness did not perceive any unusual popping sounds before the descent and impact.
Post-Accident Examination
The Rolls-Royce 250-C30P engine was removed from the airframe and tested at a repair facility with a test cell. No significant irregularities were noted during the engine test cell runs. The power turbine governor and gas producer fuel control were tested at the manufacturer's facilities, and no conditions were identified that would have prevented normal operation of either unit.