The commercial certificated helicopter pilot was dispatched to a remote research site, about 40 miles northeast of the base camp to retrieve an external sling load. The helicopter was being operated under contract to the U.S. government as a public use flight under Title 14, CFR Part 91. The helicopter departed the base camp in the early evening, and did not return. There were no communications with the helicopter pilot, and weather conditions precluded a search until late the next morning. The helicopter's wreckage was located near the shore of a shallow lake, in several feet of water, about 1 mile south of the remote research site. There were no ground scars around the lake's perimeter. The external sling load was at the accident site. The helicopter was equipped for satellite flight following, and the track showed no unusual characteristics from the base camp to the remote research site, nor from the research site to the accident site. Prior to departing the base camp for the research site, the helicopter had recently returned from the research site with a passenger. Both the pilot and passenger told the camp manager that weather conditions at the research site were marginal and deteriorating. The passenger reported that typically from early evening to late morning, fog and low clouds blanketed the area. The pilot was asked to retrieve the load if possible, but not to take any chances. The helicopter was examined at the accident site, and re-examined after recovery. At the accident site, all the helicopter components were accounted for, however, the tail boom had separated from the fuselage, and lay farther out in the lake. The helicopter's cabin had the appearance of having tumbled, but the tail boom did not. The external load was located in the lake, between the tail boom and the fuselage. Examinations of the helicopter and its components established control continuity, and did not reveal any pre-accident mechanical anomalies. The helicopter's external load lifting gear revealed damage to the airframe attaching brackets, and supporting cable attachments. The sling load had damage to both the cargo net and the net's cinch ropes. The cargo net was made of white rope containing a black trace, and had several broken ropes, which were not there prior to the accident flight. The net's cinch rope, which was yellow with a blue trace, had been pulled apart at a spliced thimble. During an examination of the tail rotor tracking blade, blue rope fibers consistent with the blue trace fibers of the net's cinch rope, were found trapped under the blade's end cap bolt. Both the tail boom and the tail rotor blades had rope marks in their paint.
The National Transportation Safety Board determines the probable cause(s) of this accident as follows:
The pilot's failure to maintain clearance from the sling load during cruise flight, which resulted in the load becoming entangled in the tail rotor, and an in-flight loss of control. |
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