On March 1, 2011, about 0830 mountain standard time, a Bell 206B-III, N206BY, impacted the terrain near the north end of Tooele Valley Airport, Tooele, Utah. The commercial pilot, who was employed by the operator, and the Airline Transport Pilot (ATP), who had accompanied her on the flight, were not injured, but the helicopter, which was owned by N206BY LLC, and operated by Upper Limit Aviation, sustained substantial damage. The 14 Code of Federal Regulations Part 91 local area personal proficiency flight, which departed Salt Lake City International Airport about one hour prior to the accident, was being operated in visual meteorological conditions. A company visual flight rules flight plan had been filed and activated. According to the commercial pilot, who had accumulated about 85 hours of flight time in a Bell 206, the primary purpose of the flight was for her to gain additional experience and proficiency in helicopter long-line external load operations, in preparation for participation in the operator's upcoming FAR Part 133 contract. Upon arriving at Tooele Valley Airport, the ATP exited the helicopter, inspected the long-line (which was hooked to a net with a 50 pound tire in it), connected the long-line to the helicopter, removed both forward cockpit doors, and then re-entered the helicopter. The commercial pilot then picked the helicopter and the long-line load off the ground, and established a working pattern altitude about 500 to 600 feet above ground level (agl). She then flew three separate approaches to the airport's compass rose, where she brought the helicopter to a stationary hover, and placed the external load upon the surface. After her third approach, the commercial pilot offered the ATP the opportunity to fly an approach to place the long-line load in the same location. The ATP, who had accumulated a total of 1.5 hours of flight time in a Bell 206, none of which included long-line operations, then took control of the helicopter. He first flew a downwind leg, and then began the final approach to the compass rose. When the ATP started his final approach portion of the pattern, the commercial pilot put her head out the door in order to observe the load hanging below the helicopter. While he was on final approach, the ATP kept the helicopter about 140 to 150 feet agl, which placed the external load about 30 to 40 feet agl. As he continued to move toward the compass rose, the helicopter was flying into a headwind of about 3 knots, and was moving forward about 10 to 12 knots. Reportedly, as the load approached the compass rose, the helicopter began an uncommanded yaw to the right. The ATP put in left pedal, but he was unable to arrest the nose right yaw. At the same time that the helicopter started yawing to the right, its nose pitched up slightly, which the pilot countered with forward cyclic input, which successfully arrested the upward movement of the nose. Because the helicopter continued to yaw to the right, the ATP lowered the collective and added additional forward cyclic in an attempt to arrest the yaw and gain additional airspeed. At that point in time, without advising the ATP, the commercial pilot applied some additional downward pressure to the collective in an effort to lower it still further, and then reportedly took her hands off the collective (although the ATP reported feeling what he thought were both downward and upward collective inputs from the commercial pilot as the helicopter neared the terrain). Shortly thereafter the helicopter impacted the terrain in a slight nose low shallow right bank on a heading of about 300 degrees. Immediately after the initial ground contact, which was on the forward portion of the right skid, the helicopter rolled onto its right side, coming to rest on a heading of about 210 degrees. As the helicopter rolled onto its side, the main rotor blades came in contact with the terrain, and the main rotor mast fractured at the location where it comes in contact with the static stops. During the accident sequence, one of the main rotor blades also contacted and severed the helicopter's tail boom. After the helicopter came to rest, the engine was still running, so the ATP shut the engine down by rolling the throttle past the detent to the full off position, while at the same time the commercial pilot moved the fuel shut-off valve toggle switch to the off position. In a phone interview with the NTSB Investigator-In-charge (IIC) about 30 minutes after the accident, both pilots said that they were not sure what had initiated the accident sequence. Both pilots stated that the onset of the yaw was without warning, and that there was no audible or visual indication of a loss of engine power or main rotor rpm. The commercial pilot said that at the time of the initiation of the yaw, her vision was focused entirely outside, and therefore she did not know what the engine or rotor instruments were indicating at that time. The helicopter was recovered to the facilities of the operator, where it underwent an inspection by a team made up of the NTSB IIC, an FAA Inspector from the Salt Lake City Flight Standards District Office, and an accident investigator from Bell Helicopter Company. That inspection determined that the main rotor mast had fractured in a manner consistent with overload, as a result of the over-travel of the hub assembly, and forces generated when the mast made contact with the static stops. It could not be determined whether the static stop contact occurred as a result of a main rotor blade impacting the ground, or as a result of a main rotor blade hitting the tailboom. It was also determined that the freewheeling unit had fractured adjacent to the drive-spline, in a manner consistent with forces generated upon impact. Due to the direction of the impact and the helicopter's attitude at the time of impact, those forces would have resulted in the front of the engine being displaced downward, while the transmission was being displaced both down and aft as a result of the combination of the downward impact forces and the aft forces generated by a main rotor blade impacting the ground at the right front of the helicopter. The main driveshaft was inspected, and it revealed that there was rotational scoring displayed on the spike plate, and the flexures at the transmission end of the drive shaft were fractured in a manner consistent with the transmission being displaced to a degree that the main driveshaft made contact with the spike plate while still being driven. Preimpact control continuity was confirmed, with all control tube fractures displaying 45 degree shear-lips consistent with overload. All attachment hardware was either in place, or was confirmed to have been removed by the FAA prior to the beginning of the inspection process. Rotational scoring and torsional twisting of the components throughout the drive system was consistent with the presence of energy at the time of impact, and evidence of preimpact drive continuity was displayed by damage and scarring to the rotor blades, main driveshaft, spike plate, and the tail rotor driveshaft. At the conclusion of the inspection process no evidence had been found of any preimpact anomaly or malfunction associated with the helicopter's airframe or flight control system.