Narrative
While in straight and level flight at 1,000 feet, the pilot heard a "pop" in the engine area. Immediately, the turbine outlet temperature (TOT) began to rise into the high yellow range. The pilot reduced power to 50% torque, and the TOT returned to the green arc. The pilot then started to divert. However, after about 20 to 30 seconds, the TOT began rising again, so torque was further reduced to 30%. At that time, the helicopter was over an area of trees, but the pilot spotted a cul-de-sac. As the pilot was approaching to land, the TOT became uncontrollable and the engine was shut down. An autorotation was completed in a tail-low attitude to prevent a ground run. During touchdown, the tailboom was severed aft of the horizontal stabilizer.
Engine Examination
Disassembly of the engine revealed that the second stage turbine nozzle, part number 6893952, had separated from the vane portion of the nozzle.