Incident Details
On September 19, 2012, at approximately 1225 Pacific daylight time, a Bell 206-B helicopter, registration N222GR, was destroyed during a precautionary landing about 18 nautical miles east of Mammoth Lakes, California. The pilot reported a partial loss of engine power, prompting the landing. The helicopter was operated by Chesapeake Bay Helicopters and registered to Aerial Resources LLC. The commercial pilot and sole passenger were not injured. The flight was an aerial inspection operating under 14 CFR Part 91 with no flight plan filed. Visual meteorological conditions prevailed. The helicopter had departed Mammoth Yosemite Airport (MMH) at 1205.
Pilot Report
According to the pilot's statement, while surveying power lines at approximately 7,500 feet mean sea level (msl), the helicopter was maneuvered adjacent to the lines at 40 knots and 200 feet above ground level (agl). Engine gauges indicated normal operation with about 70 percent torque. Approximately 1 mile from the start point, the pilot observed linemen ahead and attempted to increase altitude. The helicopter climbed about 50 feet before the engine experienced a reduction in power and an unusual sound. The pilot noted no response from collective input and abandoned climb attempts to preserve rotor rpm and maintain flight at about 30 knots. As the helicopter descended with torque nearing 100 percent, the pilot maneuvered left to clear wires and established a shallow approach to a dirt road. The pilot recalled an audible warning tone and visual indication but could not specify which lights illuminated. At about 500 feet laterally and 50–75 feet agl from the landing site, rotor rpm decayed and power could not be increased. The helicopter was traveling at 30 knots with a descent rate of 150–250 feet per minute. Approximately 10 feet laterally and 10–15 feet agl from the site, the helicopter lost altitude and impacted terrain about 2 feet short of the road, rolling onto its left side. A postcrash fire consumed the helicopter.
In a subsequent interview, the pilot stated he became aware of power loss when climb performance deteriorated. He proceeded to a road landing site but was unable to arrest the descent rate and heard a noise he assumed was the low rotor rpm warning. The pilot was attempting a run-on landing and did not recall the final seconds. When asked about density altitude, the pilot noted it was at or near 11,000 feet. He believed the engine was operating normally.
Passenger and Witness Accounts
The passenger, who was photographing and collecting data, reported that during the climb the pilot said, "This isn't working out." The pilot then maneuvered left, crossing wires closely. The passenger noted the engine sounded different—higher pitched, as if "being pushed to its limits." During descent, the pilot mentioned a horn, but the passenger did not hear it.
Four witnesses working on power lines observed the helicopter flying close to the lines before turning and descending. None reported engine anomalies.
Aircraft and Engine Information
The single-engine Bell 206-B, serial number 1638, was powered by a Rolls-Royce Allison 250-C20B engine producing 425 horsepower. The most recent annual inspection was completed on September 13, 2012, at 21,650.6 airframe hours. The engine had accumulated 21,237 total hours and 253 hours since its last overhaul.
Engine Examination
An initial examination at a salvage facility, overseen by an FAA inspector, revealed extensive postcrash fire damage. The compressor exhibited heat distress and would not rotate; the N2 power turbine rotor was also locked. The accessory gearbox, compressor bleed valve, power turbine governor, and fuel pump assembly were consumed. The fuel control unit was partially consumed, and most lines were crushed, melted, or broken. Remaining lines were hand-tight. The compressor inlet was clear and undamaged; the power turbine 4th stage wheel was unremarkable. The engine was shipped to Rolls-Royce Corporation in Indianapolis for further examination.
On November 26, 2012, under FAA supervision, a detailed examination was performed at Rolls-Royce. The compressor case lower half showed no blade separation or distortion, though melted material was present. Bearings 1 and 2 showed normal wear and rotated by hand. The power turbine section was intact with no separation, distortion, or foreign object damage. The Rolls-Royce Engine Investigation Report found no evidence of preimpact failure, fire, damage, or any condition that would have prevented normal engine operation. Standard Aero, also a party to the investigation, concluded there were no findings in the engine that would have caused the reported loss of power.
Weather Information
At 1235, the weather reporting facility at Mammoth Yosemite Airport (elevation 7,135 feet msl, about 10.5 nautical miles west) reported wind 120 degrees at 4 knots with gusts to 14 knots, visibility 10 miles, clear sky, temperature 24°C, dew point -5°C, and altimeter 30.38 inHg. Computed density altitude was about 9,742 feet.