SEA04TA163 · Bell 206B

The helicopter was pulling sock line (rope) that was to be used to install a static wire at the top of 220-foot-tall towers supporting a 500-kV power line. (The stringing of sock line is a Class C external load operation, meaning an operation in which the external load is jettisonable and remains in contact with land or water during the rotorcraft operation.) The sock line was attached to the helicopter's remote cargo hook and played out of a truck mounted reel machine operating in the power payout mode. The reel machine operator stated that "suddenly the rope wrapped over another rope or pulled down in the drum, causing the rope to reverse on the drum." The reel machine operator immediately moved the machine's shift lever from "OUT" to "NEUTRAL," but by the time he had accomplished this, the rope between the reel and the helicopter went taut. Numerous witnesses, who were all members of the line crew installing the wires, reported that when the rope went taut, the helicopter pitched up and rolled right. The helicopter descended, impacted the ground and came to rest on its right side. Post-accident interviews revealed that the pilot, the line crew, and company management did not adequately recognize and mitigate the risks inherent in the procedures they were using to conduct the sock line pull. Specifically, the rigging used was a 25-foot long line with a 31-pound ballast weight, while other operators pulling sock line use longer long lines (50 to 100 feet) and heavier ballast (150 to 300 pounds) in order to provide pilots with an earlier warning of an impending shock load due to a snag so that the pilot will have more time to respond. Additionally, at the pilot's request, the reel machine was being operated in the power payout mode, instead of the free wheeling mode. This was due to a miscommunication between the pilot and the chief pilot, who stated that when he discussed the operation with the pilot, he assumed the pilot understood that he intended for the reel to be freewheeling. In the freewheel mode, when a snag occurs, at worst, the reel stops. In the power payout mode, when a snag occurs, the line can double back on the reel and begin to pull back in, as occurred in this accident. Also, while preparing for the sock line pull, the line crew was paying out sock line on the ground and experienced a snag, which resulted in the line being pulled in instead of paying out, just as occurred in the accident. However, there was no communication of this occurrence to the pilot. Finally, although the pilot had 21,803 hours rotorcraft flight time, 16,000 hours in the accident make and model helicopter, and 4,000 hours conducting external load operations, his most recent Class C external load experience was 5 years and 3 months prior to the accident.

Verbatim NTSB analysis and probable cause from the NTSB dataset