LAX03LA212 · Hughes 369D

The pilot made a hard landing in the single-engine helicopter after experiencing a loss of engine power at 40 knots and 150 feet agl. The pilot was conducting fire suppression operations and had maneuvered the helicopter over a pond to pickup water. He heard a loud bang and looked over his shoulder to observe 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. The engine accumulated approximately 1,043 hours since undergoing a modification to its turbine section (approximately 707 hours short of its recommended overhaul time). The modification was part of a commercial engine bulletin, which converted the engines to obtain better performance "from the improved power turbine air flow characteristics." The conversion included a replacement of the 3rd and 4th stage turbine nozzles and wheels. A post-accident examination of the engine revealed the airfoils on the 3rd 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 3rd 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; however, some were attributed to overload. The 3rd 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.

Verbatim NTSB analysis and probable cause from the NTSB dataset