19980618014279A

Internal failure of the reduction gear box and no continuity between N2 and the powre trubine. Awaiting on teardown. (.4)the pilot stated that he heard a loud metallic bang coming from the rear of the aircraft, followed immediately by a loss of power. He elected not to auto-rotate to a landing at the log-landing because of personnel in the vicinity, so he initiated an approach to a logging road adjacent to the log-landing. He landed with skids level, no power, and very low rotor RPM, but felt that he had the road made. After touching down, the helicopter rocked backward and rolled over on its left side. The engine was still running, and the pilot shut it down with the fuel valve. During the course of engine disassembly and inspection, output reduction gearbox damage was found to be consistent with a number one planet gearshaft separation caused by fatigue initiating at the outside diameter of the gear shaft within the shaft/gear web shoulder fillet radius. Laboratory analysis of the remnants of the number one planet gear noted that a cross section through the apparent fatigue origin did not exhibit evidence of material defects; however the fat igue initiation area was obscured as a result of secondary damage. The outer diameter of the planet gear shaft was found to have been plated with chromium or nickel and the case hardness of the carburized case of the planet gears was below that specified by the manufacturer. Investigation determined that none of the three planet gears in this engine were of oem maufacture. Plating of the planet gear is not an authorized repair, and no military repair was identified authorizing this type of repair. (-23)limited investigation revealed internal failure of the reduction gear box. There was no powertrain continuity between the N2 and the power turbine wheel. No corrective action planned pending the final investigation report from allied signal and NTSB.