On January 2, 1998, at 0830 central standard time, a Hughes 269C helicopter, N1091Y, owned and operated by Smith Helicopters, Inc., under Title 14 CFR Part 91, sustained substantial damage during an autorotation following a loss of engine power while maneuvering near Riviera, Texas. The commercial pilot and the passenger were not injured. Visual meteorological conditions prevailed for the local cattle herding flight and a flight plan was not filed. The flight originated from the King Ranch at 0745. During telephone interviews, conducted by the investigator-in-charge (IIC), and on the Pilot/Operator Aircraft Accident Report (NTSB Form 6120.1/2), the pilot and the operator reported that the pilot was maneuvering the helicopter at 75 feet agl herding cattle on the King Ranch when the engine "started missing briefly then quit." The pilot executed an autorotation to a dirt road confined by mesquite trees 15 to 20 feet in height. During the landing, the main rotor struck the tailboom. The tailboom, vertical and horizontal stabilizer, main rotor blades, and the tail rotor driveshaft received structural damage. A review of the engine maintenance records, by the FAA inspector and the IIC, revealed that the Lycoming HIO-360-D1A engine, S/N L-25772-51A, was manufactured in 1991. The engine was overhauled in January 1995. During the engine overhaul, the crankshaft received a magnaflux inspection and re-nitriding. The crankshaft was certified and approved for return to service by Aircraft Engine & Accessory Co., Inc., Dallas, Texas. The engine was reassembled by a mechanic and installed in N1091Y. The engine was returned to service by company maintenance personnel on February 21, 1995. At the time of the accident, the engine time since overhaul was 1,150.6 hours. The last 100 hour inspection was performed on October 7, 1997, and the airplane had accumulated 69 hours since the inspection. On March 10, 1998, at Laredo, Texas, the FAA inspector examined the helicopter and the engine. Structural damage to the airframe was confirmed. The inspector reported that the engine crankshaft would not rotate. When cylinders #1 and #3 were removed from the engine, the FAA inspector found that a portion of the crankshaft appeared to be separated in the area of the #3 main bearing. A piece of metal was found in the oil screen sump. The engine was shipped to Textron Lycoming, Williamsport, Pennsylvania, for further examination. On September 16, 1998, in Williamsport, Pennsylvania, under the surveillance of an FAA inspector, a teardown of the engine revealed a broken crankshaft and camshaft. The camshaft exhibited deformation consistent with overload. All four cylinder mounting flanges were fretted and all piston rings showed "excessive" wear. The #1 and the #4 cylinder top compression rings were broken. The engine and aircraft logs provided no indications as to what service bulletins or FAA Airworthiness Directives had been complied with from overhaul to the time of the accident. The #1, #2, and the #3 center main bearing saddle faces of the crankcase showed heavy fretting. Fresh fretting was seen on the #4 main saddle surface. The oil holes and galleys were normal. See the enclosed reports for additional details. The crankshaft was examined by the Textron Lycoming Materials Laboratory. The mode of fracture of the crankshaft was fatigue. The fatigue originated from the oil hole (between #3 main and #3 crankpin journals) in the cheek and near the rear fillet radius of the #3 main bearing journal. The cause of the fatigue was not determined. The helicopter was released to the owner's representative.