Accident Overview
On July 24, 2010, at 1400 central daylight time, a Bell 47G-4A helicopter, registration N124SH, sustained substantial damage when it impacted terrain after a loss of engine power at low altitude near Rochester, Minnesota. The commercial pilot was fatally injured. The helicopter was operated by Scotts Helicopter Services under 14 Code of Federal Regulations Part 137 as an agricultural flight. It had just refueled and reloaded its chemical load and was en route to a field. Visual meteorological conditions prevailed, and no flight plan was filed.
A witness, the spray truck driver, reported that the pilot had completed about 16 spray runs earlier in the day. He observed the helicopter depart toward a field about a mile away, with the engine sounding normal. He then saw a "brown puff of smoke" from the helicopter in flight, after which it almost immediately descended and impacted a small hill. The wreckage came to rest on the opposite side of the hill; no ground fire occurred.
Pilot Information
The 68-year-old pilot held a commercial pilot certificate with single-engine land airplane and helicopter ratings. A second-class medical certificate issued on July 13, 2010, required corrective lenses. The pilot reported approximately 16,000 total flight hours, with 150 in the last six months, including about 4,500 hours in Bell 47 helicopters and 1,000 in single-engine airplanes. The last biennial flight review was in April 2010.
Aircraft Information
The Bell 47G-4A, serial number 7599, was manufactured in 1967 and originally held a normal category airworthiness certificate. In July 2003, it was reconfigured for agricultural use and its certificate changed to restricted. The last annual maintenance inspection occurred on July 21, 2010, at which time the airframe had 15,020.2 total hours and a recording hour meter reading of 323.5 hours. The helicopter had been originally powered by a Lycoming VO-540-B1B3 engine, but was retrofitted with an Allison 250-C20B in June 2003 per Supplemental Type Certificate SH657NW. On July 21, 2010, the operator installed a 317 horsepower Allison T63-A-700 engine (military equivalent of the Allison 250-C10D), serial number AE-403309 BCEF, during the annual inspection. This engine had been removed from another aircraft in March 2009 and had not been in service since. At installation, the engine had 3,782.8 total hours and 2,026.8 hours since overhaul. The recording hour meter at the accident site read 328.7, indicating 5.2 hours of operation since the annual inspection.
Meteorological Conditions
At 1353, the surface weather observation at Rochester International Airport (RST), about 15 miles southeast of the accident site, reported wind from 210 degrees at 11 knots gusting to 15 knots, visibility 10 miles, a broken ceiling at 25,000 feet, temperature 33°C, dew point 22°C, and altimeter 30.06 inches of mercury.
Wreckage and Impact
The accident site examination showed the helicopter was traveling on a northwesterly heading. The initial impact point on the front side of a small hill displayed a red paint transfer consistent with a tailboom strike. The tailboom separated from the fuselage and came to rest about 10 feet from the main wreckage, which was partially inverted on its right side facing southeast. Both main rotor blades were present; one was broken from the hub, but both had minimal leading-edge damage.
A Federal Aviation Administration (FAA) airworthiness inspector verified continuity of engine controls, drive train from engine to transmission and rotors, and flight controls (cyclic, collective, and tail rotor).
Engine Examination
A partial on-site engine teardown revealed no signs of uncontained compressor, turbine, or gearbox failure. Fuel and oil filters were clean. The compressor No. 1 bearing housing was torn from its struts, and the bearing could not be rotated due to impact damage. The compressor assembly was removed; most axial compressor blades were bent opposite the direction of rotation. The turbine-to-compressor coupling was engaged without damage. The combustion housing and burner can showed no cracks, hot spots, or abnormal wear. Metal fragments were found at the No. 1 turbine stator, and the No. 1 turbine rotor blades were burned and eroded, with about 50 percent of the blade span missing. The engine was shipped to Rolls-Royce in Indianapolis, Indiana, for further inspection.
On August 12, 2010, at Rolls-Royce with FAA oversight, examination of the compressor rotor assembly found a single first-stage blade fractured at approximately one-third span. All compressor blades exhibited extensive foreign object damage. A metallurgical examination on February 8, 2011, determined that the first-stage compressor wheel airfoil had cracked in high cycle fatigue, initiating subsurface from a cluster of delta ferrite islands on the suction side, about 0.36 inch outboard of the root and 0.25 inch from the leading edge. No corrosion was associated with the crack origin. The microstructure, hardness, and chemistry conformed to engineering drawing requirements.
Additional Findings
An AG-NAV GPS navigation unit for agricultural spraying was installed but its hard drive was damaged, yielding no useful information. The pilot's handheld Garmin 496 GPS was also found; data retrieved for the last recorded flight did not match the accident flight parameters.
Autopsy results, conducted on July 25, 2010, at the Mayo Clinic, listed the cause of death as "multiple blunt force injuries due to a helicopter collision." Forensic toxicology was negative for carbon monoxide, cyanide, and ethanol; ibuprofen was detected in urine.