On August 12, 2009, about 1000 eastern daylight time, a McDonnell-Douglas 369E, N527BB, lost engine power during a descent and landed hard during an autorotation landing in Acworth, Georgia. The commercial rated pilot received minor injuries, and the passenger was seriously injured. The helicopter was substantially damaged by impact forces. The flight was operated as an aerial observation flight under the provisions of 14 Code of Federal Regulations Part 91, and no flight plan was filed. Visual meteorological conditions prevailed at the time of the accident. The pilot stated that he and an observer were conducting an aerial observation of power lines. He climbed to 1,400 feet mean sea level, and was awaiting clearance from McCollum Field, Georgia (RYY) to enter its airspace. After receiving the clearance to enter RYY, he began a descent. Approximately 5 to 10 seconds into the descent he heard a "'bang and braking" noise. Subsequently, he received an "ENG OUT" audio and caution lights. He then made an emergency autorotation into a field. Examination of the helicopter by a Federal Aviation Administration inspector revealed that the helicopter collided with a runoff ditch behind residential homes. Examination of the airframe and flight control system components revealed no evidence of preimpact mechanical malfunction. The helicopter was recovered from the ditch and an examination was conducted on the engine. Examination of the engine revealed no evidence of impact damage or fire, and all engine mounts were intact. The gas producer fuel control lever operated, and the power turbine governor control lever traveled freely, and after the engine was removed from the airframe both control levers operated fully. All fuel, lube and pneumatic lines, coupling nuts and B-nuts were checked for security and were found tight and marked with torque paint. Inspection of the inlet plenum chamber did not reveal any evidence of foreign material or missing hardware. Inspection of the compressor inlet showed no signs of erosion or foreign object damage. The compressor rotated freely by hand. The accessory gearbox was undamaged, and both gear trains turned freely and smoothly by hand. Drive train continuity was confirmed throughout. Examination of the turbine section revealed rub marks throughout the turbine section consistent with an out-of-balance condition caused by a third stage turbine airfoil release. The third stage wheel airfoil fractured in fatigue near the root of the airfoil, progressing approximately 0.55 inch from the trailing edge prior to the final separation in overload. No anomalies were found at the fatigue origin. Damage observed on the other turbine components was consistent with secondary damage after the third stage airfoil separation. The microstructure, hardness, and chemistry of the third stage turbine wheel met the engineering drawing requirements. The fracture surface of the failed third stage blade revealed high cycle fatigue failure of an extended period of time. The crack propagation is estimated to be greater than 2000 cycles. No additional cracks were found on the other third stage wheel airfoils. Review of the logbook pages revealed that the airframe total time was 4,257 hours, and the engine total time was 3,820 hours. The airframe and the engine had gone through a 100 and 300 hour inspection at 4,246.6 airframe hours and 3,809.6 engine hours. All airworthiness directives and applicable service bulletins as well as all instructions for continued airworthiness were complied with. The hot section and turbine inspection was completed on February 21, 2008 at 2,958 hours. The number 3 turbine wheel power turbine was overhauled on February 3, 2004 at 1210.6 hours and 1633.0 cycles, and was due to be replaced at 4,710.6 hours. The number 3 turbine wheel had 2609.7 hours and 2412.0 cycles at the time of the accident. The number 3 turbine wheel had 1940.0 hours remaining and 3588.0 cycles remaining at the time of the accident.