Accident Overview
On March 1, 2011, about 0830 mountain standard time, a Bell 206B-III helicopter, registration N206BY, impacted terrain near the north end of Tooele Valley Airport in Tooele, Utah. The helicopter, owned by N206BY LLC and operated by Upper Limit Aviation, was being flown under 14 Code of Federal Regulations Part 91 as a local area personal proficiency flight. The flight departed Salt Lake City International Airport about one hour prior to the accident. Visual meteorological conditions prevailed, and a company visual flight rules flight plan had been filed and activated.
Flight Purpose
The commercial pilot, who had about 85 hours of flight time in the Bell 206, was conducting the flight to gain experience in helicopter long-line external load operations in preparation for an upcoming FAR Part 133 contract. The airline transport pilot (ATP) accompanied her. Upon arrival at Tooele Valley Airport, the ATP exited the helicopter, inspected the long-line (attached to a net with a 50-pound tire), connected it to the helicopter, removed both forward doors, and re-entered. The commercial pilot then lifted off, established a pattern altitude of about 500 to 600 feet above ground level (agl), and flew three approaches to the airport's compass rose, placing the external load on the surface while in a stationary hover.
Accident Sequence
After the third approach, the commercial pilot offered the ATP an opportunity to fly an approach. The ATP, who had only 1.5 total hours in the Bell 206 with no long-line experience, took control. He flew a downwind leg and began the final approach to the compass rose. During the final approach, the commercial pilot observed the external load from the open door. The ATP maintained the helicopter at about 140 to 150 feet agl, placing the load at about 30 to 40 feet agl. The helicopter was flying into a headwind of about 3 knots at a forward speed of about 10 to 12 knots. As the load neared the compass rose, the helicopter experienced an uncommanded yaw to the right. The ATP applied left pedal but could not arrest the yaw. Simultaneously, the nose pitched up slightly; the ATP countered with forward cyclic, stopping the pitch-up. As the yaw continued, the ATP lowered the collective and added forward cyclic to gain airspeed. Without advising the ATP, the commercial pilot applied additional downward pressure on the collective, then reportedly released it, though the ATP felt both downward and upward collective inputs from the commercial pilot as the helicopter neared the ground. Shortly thereafter, the helicopter impacted terrain in a slight nose-low, shallow right bank on a heading of about 300 degrees. After initial ground contact 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. During the roll, main rotor blades struck the ground, and the main rotor mast fractured at the static stops. One blade also severed the tail boom. The engine continued running; the ATP shut it down by rolling the throttle off, while the commercial pilot moved the fuel shut-off valve to off.
Post-Accident Inspection
In a phone interview about 30 minutes after the accident, both pilots stated they were unsure what initiated the sequence. They noted that the yaw onset was without warning and that there was no audible or visual indication of loss of engine power or main rotor rpm. The commercial pilot said her vision was focused outside at the time, so she did not know the engine or rotor instrument indications. The helicopter was recovered and inspected by the NTSB, FAA, and Bell Helicopter. The inspection determined that the main rotor mast fractured from overload due to hub assembly over-travel and forces from static stop contact, but it could not be determined whether this contact occurred from a blade striking the ground or the tailboom. The freewheeling unit fractured at the drive-spline from impact forces. No evidence of any preimpact anomaly or malfunction was found in the airframe or flight control system. Preimpact control continuity was confirmed, and all drive system components showed damage consistent with energy at impact.