Background
On February 23, 2011, at approximately 1620 central standard time, a Hughes 369D helicopter, registration N112AS, operated by Aerial Solutions, Inc., was substantially damaged when it struck the ground and rolled over about 12 miles southwest of Henryetta, Oklahoma. Visual meteorological conditions prevailed at the time. The flight was conducted under 14 Code of Federal Regulations Part 133 as an external load operation without a flight plan. The pilot, the sole occupant, was not injured. The local flight had originated at 1400 from the same landing zone where the accident occurred.
The external load was a patented Aerial Saw, used for cutting vegetation along power line right-of-ways. It consisted of ten 24-inch diameter circular blades mounted on a saw bar channel, driven by belts connected to a gas-powered engine. The engine and saw assembly were suspended below the helicopter by aluminum tubing sections, with hinges allowing fore-and-aft swing but not sideways movement. The saw weighed 822 pounds and was suspended on an 80-foot beam. At the time of the accident, the saw was idling.
Accident Sequence
According to the operator's accident report, the helicopter was facing into the wind and hovering with the external load over an open field adjacent to the work area, approximately 120 feet above the ground. The pilot initiated a 180-degree right pedal turn to enter the work area. While trimming for a left quartering tailwind (reported as 200 degrees at 11 knots), he heard a loud "pop" and felt a "thump" in his seat. He was unable to move the cyclic control to the left, and the helicopter began a right banking turn. The pilot lowered the collective and applied aft cyclic control, which lowered the external load to the ground and stabilized the helicopter by pulling aft on the load. Despite all efforts, the cyclic control could not be moved left. At about 5 feet above the ground, as the pilot leveled the helicopter, it rolled sharply to the right and impacted the ground, coming to rest on its left side.
Post-Accident Examination
Federal Aviation Administration (FAA) inspectors examined and documented the wreckage at the accident site. The helicopter was then transported to Air Salvage of Dallas, Texas, where on April 12, 2011, a more detailed examination was conducted by FAA inspectors and representatives from Boeing Helicopters and Rolls Royce. Flight control continuity was established from the cockpit to the control deck, and all controls were moved without obstructions or binding. The trim actuators were tested; the lateral trim actuator did not operate due to severed wires from the accident sequence, but full left lateral trim input was noted. When the wires were reconnected, the lateral trim motor functioned normally. A support on the lower non-rotating portion of the swashplate was fractured in overload during the accident. Damage was found on the mating surfaces between the non-rotating and rotating swashplate parts. The cowling around the swashplate showed evidence of fiberglass debonding and was missing a fastener. Damage to the inlet fairing was attributed to the accident sequence from the helicopter rolling over and contacting rotating and broken upper flight control parts. Boeing representatives found no anomalies that would cause control binding. Damage to the rotor system was consistent with power being developed. No powerplant discrepancies were noted. No post-accident examination of the Aerial Saw was performed.