NTSB Factual Report

On September 19, 2012, about 1225 Pacific daylight time, a Bell 206-B helicopter, N222GR, was destroyed following a precautionary landing due to a reported partial loss of engine power about 18 nautical miles east of Mammoth Lakes, California. The helicopter was registered to Aerial Resources LLC, of Chesapeake, Virginia, and operated by Chesapeake Bay Helicopters, also of Chesapeake, Virginia. The commercial pilot and his sole passenger were not injured. The local aerial inspection flight was being operated in accordance with 14 Code of Federal Regulations Part 91, and no flight plan was filed. Visual meteorological conditions prevailed at the time of the accident. The flight departed the Mammoth Yosemite Airport (MMH), Mammoth Lakes, California, at 1205.

In a statement submitted to the National Transportation Safety Board investigator-in-charge (IIC), the pilot reported that while surveying power lines at an elevation of about 7,500 feet mean sea level (msl), he maneuvered the helicopter adjacent to the power lines and began a 40 knot pass at an altitude of 200 feet above ground level (agl). The pilot stated that he visually confirmed that all [engine] gauges were operating in the green arc, and that the power was about 70 percent torque. About 1 mile from the start point the pilot observed linemen on a pole ahead of him, at which time he attempted to increase altitude for better separation. The pilot reported that the helicopter appeared to climb 50 feet, when the engine experienced a reduction in power and an unusual engine sound. The pilot opined that as there was no response to his increase in collective and when he realized that he would not be able to climb to a suitable altitude to clear the ridge ahead, he abandoned further climb attempts. He then focused on preserving rotor revolutions per minute (rpm) and maintaining straight and level flight at about 30 knots. As the helicopter continued to descend, and with the torque nearing 100 percent, he elected to maneuver the helicopter to the left to clear the wires, avoid the terrain, and established a shallow approach to a nearby dirt road. The pilot stated that while crossing the wires he thought he heard an audible warning tone and visual indication, but could not recall which warning light/lights were illuminated. The pilot reported that when the helicopter was about 500 feet laterally and 50 to 75 feet agl from the landing site, and while descending in a shallow approach, he observed a decay in rotor rpm, as well as the inability to increase power. At this point the helicopter was traveling at 30 knots and descending about 150 to 250 feet per minute. The pilot added that when the helicopter was about 10 feet laterally and 10 to 15 feet agl from the landing site it appeared to lose altitude, followed by impact with terrain about 2 feet short of the road. The helicopter subsequently rolled over on its left side. A postcrash fire consumed the helicopter.

In a postaccident interview conducted by a Federal Aviation Administration (FAA) operations inspector, the pilot reported that he became aware of the lack of power when the climb performance deteriorated. When he realized that further climb was not possible, he proceeded to a possible landing site on a road that was near his position. The pilot reported that he was unable to arrest the descent rate and that he also heard a noise that he assumed was the low rotor rpm warning. The pilot further reported that he was attempting a run-on landing and did not remember the last 2 seconds prior to the accident. When asked by the inspector if he had considered density altitude (DA) in his preflight planning, the pilot said the DA was at or near 11,000 feet. When the inspector asked the pilot if he thought the engine was operating normally, he said that he thought that it was.

In a statement provided to the NTSB IIC by the passenger who was photographing and carrying out the data collection for the flight, the passenger reported that while climbing to a higher altitude the pilot said something to the effect of, "This isn't working out." The passenger stated that the pilot began to maneuver the helicopter around to the left, crossing over the wires below and thinking, "Boy, that's really close." That passenger also revealed that he thought the engine sounded different in a way that he wasn't used to hearing, like it was "spinning or whining with a higher pitch," and as if it was "being pushed to its limits." During the descent the passenger stated that the pilot mentioned the horn, which he didn't remember [hearing].

Statements from 4 individuals who were working on the power lines and support structures being photographed were submitted to the IIC. The witnesses reported that they observed the helicopter flying close to the power lines before it turned and descended prior to impact with terrain. None of the witnesses reported hearing any anomalies with the engine during the accident sequence.

The single-engine Bell helicopter, serial number 1638, was powered by a Rolls-Royce Allison 250-C20B, 425 horsepower engine. The helicopter's most recent annual inspection was completed on September 13, 2012, at a total airframe time of 21,650.6 hours. The operator reported that the engine had accumulated a total time of 21,237 hours, and 253 hours since its most recent overhaul.

An initial examination of the engine was performed at a salvage facility and overseen by an FAA inspector. The inspector reported that the engine had sustained extensive damage due to the postcrash fire. The compressor exhibited internal heat distress damage and would not rotate, and the N2 power turbine rotor would not rotate by hand. The accessory gearbox, compressor bleed valve, power turbine governor, and fuel pump assembly had been consumed. Additionally, the fuel control unit was partially consumed in the postcrash fire, and most pneumatic, fuel and oil lines were crushed, melted or broken. Any remaining lines were checked and observed to be at least hand tight. The compressor inlet was clear of debris and was not damaged. The power turbine 4th stage wheel was unremarkable. The engine was subsequently shipped to Rolls-Royce Corporation in Indianapolis, Indiana, for further examination.

At the request of the NTSB IIC, and under the supervision of FAA aviation safety inspectors assigned to the Indianapolis Flight Standard District Office, Indianapolis, Indiana, on November 26, 2012, an examination of the engine was performed at the facilities of Rolls-Royce, Indianapolis, Indiana. As a result of the examination, the inspectors reported that the lower half of the compressor case was removed, with no signs of compressor blade separation or distortion observed. Melted material was present in the compressor section, although the type and/or origin could not be determined through this inspection. The number 1 and number 2 bearing(s) were removed; the condition of both bearings indicated signs of normal wear, and both were able to be rotated by hand. The power turbine section was disassembled. The rotating assembly was intact with no visual signs of separation or distortion observed, and no signs of Foreign Object Damage (FOD) or ingestion. Further, the Rolls-Royce Engine Investigation Report revealed no evidence of a preimpact failure, fire, damage, or any other mechanical condition, which would have prevented the engine from operating as designed. Additionally, Standard Aero, who was also a party to the investigation, reported that as a result of the engine investigation, it was concluded that there were no findings in the engine that would have caused the reported loss of engine power. (Refer to both the Rolls-Royce and Standard Aero Engine Reports, which are appended to the docket for this report.)

At 1235, the weather reporting facility at the Mammoth Yosemite Airport (MMH), Mammoth Lakes, California, elevation 7,135 feet msl, located about 10.5 nautical miles west of the accident site, reported wind 120 degrees at 4 knots, gusts to 14 knots, visibility 10 miles, sky clear, temperature 24 degrees Celsius (C), dew point -05 degrees C, and an altimeter setting of 30.38 inches of mercury. The computed density altitude was about 9,742 feet.

NTSB Probable Cause

The pilot’s inability to maintain sufficient main rotor rpm while maneuvering in highdensity altitude conditions, which resulted in a precautionary landing and subsequent rollover.