Accident Sequence
While the helicopter was accelerating at about 60 knots and approximately 150 feet above ground level, the pilot heard a loud bang, accompanied by activation of low rotor rpm and engine out audio warnings. The pilot initiated an autorotation to a grassy area, flared the aircraft, and landed in six to eight inches of water.
Passenger Account
A passenger reported that the pilot "pulled pitch right before the landing" and described the touchdown as "one medium bump and then followed very quickly with another medium bump with a loud sound with it." The passenger stated that the landing attitude was "relatively normal," but added that "it was possibly a little more pitched up than normal."
Damage
During the landing, a main rotor blade contacted the tailboom, partially separating it aft of the horizontal stabilizer.
Engine Examination
Post-accident examination of the engine revealed that the Spur Adapter Gearshaft was fractured at the forward splines. According to the engine manufacturer's metallurgist, "severe spline wear led to [the] spiral fatigue failure of the Spur Adapter Gearshaft and Compressor Adapter Coupling." The reason for the spline wear was not determined.