Incident Summary
During a hovering maneuver, the pilot heard a whine from the engine that increased in intensity. Torque fluctuations were observed, followed by a loud explosion. The pilot initiated an autorotation and landed on the ramp.
Investigation Findings
Post-incident examination revealed that the power turbine inner shaft had failed in torsion. The failure was attributed to corrosion on the inner surface of the shaft. This failure allowed the third stage turbine wheel to overspeed and subsequently fail.
Maintenance History
The helicopter was 11 years old with a total time of 2,146 hours on both the airframe and engine. Allison had issued three service bulletins addressing the power turbine inner shaft; however, none of these bulletins specified compliance based on calendar time—only engine total time was used. The first scheduled maintenance on the power turbine was set at 3,550 hours of operation.