Accident Overview
On August 12, 2009, about 1000 eastern daylight time, a McDonnell-Douglas 369E, registration N527BB, experienced a loss of engine power during a descent and subsequently made a hard autorotation landing in Acworth, Georgia. The commercial 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 14 Code of Federal Regulations Part 91, and no flight plan was filed. Visual meteorological conditions prevailed at the time.
Pilot Narrative
The pilot stated that he and an observer were conducting an aerial observation of power lines. After climbing to 1,400 feet mean sea level, he was awaiting clearance from McCollum Field, Georgia (RYY) to enter its airspace. Upon receiving clearance, he began a descent. Approximately 5 to 10 seconds into the descent, he heard a "'bang and braking" noise. Subsequently, an "ENG OUT" audio alert and caution lights activated. He then performed an emergency autorotation into a field.
Examination of Airframe and Controls
A Federal Aviation Administration inspector examined the helicopter and found that it collided with a runoff ditch behind residential homes. Examination of the airframe and flight control system components revealed no evidence of preimpact mechanical malfunction.
Engine Examination
The engine was recovered and examined. No evidence of impact damage or fire was found, and all engine mounts were intact. The gas producer fuel control lever and power turbine governor control lever traveled freely, and after engine removal both control levers operated fully. All fuel, lube, and pneumatic lines, coupling nuts, and B-nuts were tight and marked with torque paint. Inspection of the inlet plenum chamber and compressor inlet showed no foreign material, missing hardware, erosion, or foreign object damage. The compressor rotated freely by hand. The accessory gearbox was undamaged, and both gear trains turned freely and smoothly. Drive train continuity was confirmed.
Examination of the turbine section revealed rub marks consistent with an out-of-balance condition caused by a third stage turbine airfoil release. The third stage wheel airfoil had fractured in fatigue near the root, progressing approximately 0.55 inch from the trailing edge before final separation in overload. No anomalies were found at the fatigue origin. Damage to 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 engineering drawing requirements. The fracture surface showed high cycle fatigue failure over an extended period, with crack propagation estimated at greater than 2,000 cycles. No additional cracks were found on other third stage wheel airfoils.
Maintenance History
Review of logbook pages indicated that the airframe total time was 4,257 hours, and the engine total time was 3,820 hours. The airframe and engine had undergone a 100- and 300-hour inspection at 4,246.6 airframe hours and 3,809.6 engine hours. All airworthiness directives, applicable service bulletins, and 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 1,210.6 hours and 1,633.0 cycles, and was due for replacement at 4,710.6 hours. At the time of the accident, the number 3 turbine wheel had 2,609.7 hours and 2,412.0 cycles, with 1,940.0 hours and 3,588.0 cycles remaining before its replacement limit.