Helicopter Hard Landing and Fire During Fire Suppression Operations (N633WA)

Incident Overview and Immediate Sequence of Events A pilot was conducting fire suppression operations in a single-engine helicopter when the aircraft experienced a loss of engine power at 40 knots and 150 feet above ground level. The pilot had maneuvered the helicopter over a pond to pick up water. During the operation, the pilot heard a loud bang and observed white smoke coming from the engine exhaust. The pilot entered an autorotation and began a flare at 50 feet AGL with a high rate of descent. The helicopter impacted the ground, and a fire ignited. ## Engine History and Modification The engine had accumulated approximately 1,043 hours since undergoing a modification to its turbine section, which was approximately 707 hours short of its recommended overhaul time. The modification was part of a commercial engine bulletin intended to improve performance through improved power turbine air flow characteristics. The conversion included replacement of the third and fourth stage turbine nozzles and wheels. ## Post-Accident Engine Examination A post-accident examination of the engine revealed that the airfoils on the third-stage wheel failed as a result of fatigue cracking that originated at the airfoil's trailing edge and progressed toward the leading edge of the blades. All of the third-stage airfoils displayed some form of cracking near the airfoil roots at their trailing edges. A number of the cracks were attributed to fatigue propagation, while some were attributed to overload. The third-stage turbine wheel and airfoils met the manufacturer's specifications, and no material flaws or defects were noted at the fatigue cracking origins. The reason for the fatigue cracking and subsequent failures could not be determined.