Accident Details
The helicopter was engaged in pulling sock line, a rope used to install a static wire at the top of 220-foot-tall towers supporting a 500-kV power line. This operation is classified as a Class C external load operation, where the load is jettisonable and remains in contact with land or water. The sock line was attached to the helicopter's remote cargo hook and played out from a truck-mounted reel machine operating in power payout mode.
The reel machine operator reported that suddenly the rope wrapped over another rope or pulled down on the drum, causing the rope to reverse on the drum. The operator immediately moved the shift lever from "OUT" to "NEUTRAL," but by the time this was accomplished, the rope between the reel and the helicopter became taut.
Sequence of Events
Multiple witnesses, all members of the line crew installing the wires, stated that when the rope went taut, the helicopter pitched up and rolled to the right. The helicopter then descended, impacted the ground, and came to rest on its right side.
Findings from Post-Accident Interviews
Post-accident interviews indicated that the pilot, line crew, and company management did not adequately recognize and mitigate the risks inherent in the procedures used for the sock line pull. Specifically, the rigging consisted of a 25-foot long line with a 31-pound ballast weight. In contrast, other operators pulling sock line use longer long lines (50 to 100 feet) and heavier ballast (150 to 300 pounds) to provide pilots with an earlier warning of an impending shock load from a snag, allowing more time to respond.
Additionally, at the pilot's request, the reel machine was operated in power payout mode instead of freewheeling mode. This resulted from a miscommunication between the pilot and the chief pilot, who stated that when discussing the operation, he assumed the pilot understood his intention for the reel to be freewheeling. In freewheel mode, a snag would at worst cause the reel to stop. In power payout mode, a snag can cause the line to double back on the reel and begin pulling in, as occurred in this accident.
Furthermore, during preparation for the sock line pull, the line crew experienced a snag while paying out sock line on the ground, causing the line to be pulled in instead of paying out—similar to the accident scenario. This occurrence was not communicated to the pilot.
Pilot Experience
The pilot had accumulated 21,803 hours of rotorcraft flight time, including 16,000 hours in the accident make and model helicopter and 4,000 hours conducting external load operations. However, his most recent Class C external load experience was 5 years and 3 months prior to the accident.