What happened
While conducting fire suppression operations, the pilot of a single-engine helicopter maneuvered over a pond to collect water. At an altitude of 150 feet above ground level and a speed of 40 knots, the pilot heard a loud bang and observed white smoke emitting from the engine exhaust. Following this loss of engine power, the pilot entered an autorotation. As the aircraft reached 50 feet above ground level, the pilot began a flare with a high rate of descent. The helicopter impacted the ground, which subsequently resulted in a fire. There were no injuries reported.
The investigation
The engine had accumulated approximately 1,043 hours since a modification to its turbine section, which was roughly 707 hours before its recommended overhaul interval. This modification, part of a commercial engine bulletin, was designed to improve performance through enhanced power turbine airflow characteristics and included the replacement of the 3rd and 4th stage turbine nozzles and wheels.
A post-accident examination of the engine revealed that the airfoils on the 3rd stage wheel had failed. This failure was caused by fatigue cracking that originated at the trailing edge of the airfoils and progressed toward the leading edge. All 3rd stage airfoils showed some form of cracking near the airfoil roots at their trailing edges. While many cracks were attributed to fatigue propagation, some were attributed to overload. The investigation noted that the 3rd stage turbine wheel and airfoils met all manufacturer specifications, with no material flaws or defects found at the origins of the fatigue cracking.