NTSB Factual Report

HISTORY OF FLIGHT

On July 24, 2010, at 1400 central daylight time, a Bell 47G-4A, N124SH, sustained substantial damage when it impacted the terrain after a loss of engine power at low altitude near Rochester, Minnesota. The commercial pilot was fatally injured. The helicopter was registered to and operated by Scotts Helicopter Services under the provisions of the 14 Code of Federal Regulations Part 137 as an agricultural flight. The helicopter was en route to a field after it had refueled and reloaded its chemical load when the accident occurred. Visual meteorological conditions prevailed at the time of the accident. No flight plan was filed.

A witness, who was the spray truck driver, reported that the pilot returned from a spray run and had the helicopter refueled and the hopper filled with chemical. The witness watched it depart toward a field about a mile away. He stated that the engine sounded normal, and that there was nothing unusual about its departure or flight path. Then he saw a "brown puff of smoke" from the helicopter in-flight, and the helicopter almost immediately descended and impacted a small hill. The helicopter wreckage came to rest on the other side of the hill. There was no ground fire.

The witness reported that the pilot had performed about 16 spray runs during the day, and that the pilot looked healthy and normal before the flight.

PERSONNEL INFORMATION

The 68-year-old pilot held a commercial pilot certificate with single-engine land airplane and helicopter ratings. The pilot's second class medical certificate issued on July 13, 2010, required that the pilot "must wear corrective lenses." The pilot reported that his total flight time was about 16,000 hours with about 150 hours flown in the last six months. The operator reported that the pilot had about 4,500 hours in a Bell 47 helicopter and about 1,000 hours in single-engine airplanes. He flew his last biennial flight review in April 2010.

AIRCRAFT INFORMATION

The helicopter was a Bell 47G-4A, serial number 7599, manufactured in 1967 with a normal category airworthiness certificate. The helicopter seated three and had a maximum gross weight of 2,950 pounds. In July 2003, the helicopter was configured for agriculture and pest control, and the airworthiness certificate was changed to a restricted category. The last annual maintenance inspection was conducted on July 21, 2010. At the time of the inspection, the helicopter had a total time of 15,020.2 hours and a recording hour meter reading of 323.5 hours.

The helicopter was originally equipped with a Lycoming VO-540-B1B3 reciprocating engine. On June 24, 2003, the helicopter was equipped with an Allison 250-C20B engine, in accordance with Supplemental Type Certificate (STC) SH657NW. Subsequently, Scott's Helicopter Sevices acquired the helicopter. On July 21, 2010, the operator installed a 317 horsepower Allison T63-A-700 engine, serial number AE-403309 BCEF, on the helicopter during the annual maintenance inspection. (The T63-A-700 is the military version of the commercial Allison 250-C10D engine) The engine had been removed from N3094G on March 24, 2009, and had not been in service until its installation in the accident helicopter. The engine total time was 3,782.8 hours with a time since overhaul of 2,026.8 hours at the time of the annual maintenance inspection.

At the accident site, the recording hour meter reading was 328.7 hours, which indicated that the helicopter was operated for 5.2 hours since the last annual maintenance inspection.

METEOROLOGICAL CONDITIONS

At 1353, the surface weather observation at the Rochester International Airport (RST), in Rochester, Minnesota, located about 15 miles southeast of the accident site was: wind 210 degrees at 11 knots, gusting to 15 knots; visibility 10 miles; ceiling broken at 25,000 feet; temperature 33 degrees Celsius; dew point 22 degrees Celsius; altimeter 30.06 inches of mercury.

WRECKAGE AND IMPACT INFORMATION

The examination of the accident site indicated that the helicopter was traveling on a northwesterly heading. The initial impact point was a found on the front side of a small hill, and it had a red paint transfer mark which was consistent with a tailboom strike. The tailboom was separated from the fuselage and was lying about 10 feet from the main wreckage located on the back side of the small hill. The main wreckage was found partially inverted on its right side, facing to the southeast. The main rotor blades were found with the main wreckage. Except for one blade was broken from the blade hub, both blades were intact and exhibited minimal impact damage to their leading edges.

A Federal Aviation Administration (FAA) airworthiness inspector examined the helicopter at the accident site. Engine control continuity from the cockpit to the engine was verified. Drive train continuity from the engine to the transmission, the main rotor, the tail rotor output coupling, and the tail rotor gear box were verified. Flight control continuity of the cyclic and collective from the cockpit to the main rotor blade grips, and the tail rotor control continuity from the cockpit to the tail rotor blades were verified.

A partial engine teardown was conducted during the on-site examination. The examination of the engine revealed that there were no signs of uncontained compressor, turbine, or gearbox failure. Fuel and oil filters were free of contamination. The compressor No. 1 bearing housing was torn from the No. 1 bearing struts, and the No. 1 bearing could not be rotated due to impact damage. The compressor assembly was removed from the engine. The majority of the axial compressor blades were bent opposite the direction of rotation. The turbine to compressor coupling was engaged with the no signs of damage, and no damage was noted on the compressor shaft. The compressor could be rotated by hand, but with some resistance due to the damage to the compressor blades and vanes. The combustion housing and burner can exhibited no signs of cracks, hot spots, or abnormal wear. Metal fragments were noted 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 the Rolls-Royce facility in Indianapolis, Indiana, for further inspection.

An AG-NAV GPS navigation unit used for agricultural spraying applications was installed on the helicopter. The unit was inspected by the manufacturer. It was determined that the hard drive was damaged and no useful information was able to be obtained from the unit.

The pilot's handheld Garmin 496 GPS was found at the accident site. The unit was downloaded, but the data retrieved from the unit for the last recorded flight did not provide information that was consistent with the accident flight.

MEDICAL AND PATHOLOGICAL INFORMATION

An autopsy of the pilot was conducted on July 25, 2010, at the Mayo Clinic in Rochester, Minnesota. The "Cause of Death" was noted as "multiple blunt force injuries due to a helicopter collision." A Forensic Toxicology Fatal Accident Report was prepared by the FAA Civil Aerospace Medical Institute. The results were negative for carbon monoxide, cyanide, and ethanol. Ibuprofen was detected in the urine.

TESTS AND RESEARCH

On August 12, 2010, the engine was examined at the Rolls-Royce facility with FAA oversight. The examination of the compressor rotor assembly revealed that a single first-stage blade had fractured at approximately 1/3 span. All compressor blades exhibited extensive foreign object damage.

On February 8, 2011, a metallurgical examination of the compressor and engine components was conducted at Rolls-Royce with FAA oversight. The examination revealed the following:

1. The first stage compressor wheel airfoil cracked in high cycle fatigue, initiating subsurface and emanating from a cluster of delta ferrite islands on the suction side airfoil approximately 0.36 inch outboard of the root and 0.25 inch from the leading edge. No corrosion damage was found associated with the crack origin.

2. The microstructure, hardness, and chemistry of the first stage compressor wheel conformed to the requirements of the engineering drawing.

ADDITIONAL INFORMATION

The pilot's Garmin 496 and the attaching mount were found at the accident site. The operator reported that the pilot used it for navigation, and that the pilot mounted it on the cyclic.

NTSB Probable Cause

The total loss of engine power due to a high-cycle fatigue fracture of a first-stage compressor blade. The reason for the fracture could not be determined during postaccident examination.