Incident Overview
A helicopter on a camping reconnaissance flight experienced a loss of engine rpm while hovering over a lake, resulting in a controlled descent into the water. The aircraft came to rest upright on its skids in approximately six to eight feet of water, with main rotor blades damaged upon contact with the surface. No injuries were reported.
Sequence of Events
Upon reaching the lake area, the pilot conducted two high reconnaissance passes to survey the terrain before descending to a lower altitude for closer inspection of potential camping sites along the shoreline. After completing a systems flow check, the pilot initiated a hover taxi over the lake. Once the hover was established, the oil pressure and oil temperature were rechecked. The pilot then proceeded to hover down the middle of the lake, scanning for suitable landing spots. When he reached the far end, he concluded that the best site was back at the other end where he had first passed over the shoreline.
As he prepared to turn around, he noticed a gradual decrease in engine rpm, and the helicopter began a slow descent toward the water. Despite increasing the throttle, he was unable to increase or stabilize the rpm. The engine rpm, initially at 3,200, dropped to 2,400 by the time the aircraft contacted the water surface. The helicopter settled into the lake in a level and upright position.
Post-Accident Examination
As part of the investigative process, the left magneto, which was older, was removed from the helicopter and tested at various rpm levels. (The right magneto had been newly installed and was not removed for testing.) During the testing, the left magneto performed erratically, failing to produce a consistent continuous spark at high rpm. The testing was subsequently terminated, and the magneto was disassembled and inspected. However, the specific reason for its inconsistent performance could not be determined.