What happened
While conducting reconnaissance over a lake to identify potential camping sites, the pilot performed two high-altitude passes before descending to a lower altitude. After completing a systems flow check and establishing a hover taxi, the pilot rechecked the oil pressure and oil temperature. The pilot then proceeded to hover down the center of the lake to inspect various landing sites.
Upon reaching the far end of the lake, the pilot decided to return to the initial shoreline area. As the pilot prepared to turn back, a gradual drop in engine RPM was observed, causing the helicopter to descend toward the water. Despite an increase in throttle, the pilot could not stabilize or increase the RPM. The aircraft eventually settled into the water in a level and upright position. At the start of the event, the RPM was 3,200, but it had decreased to 2,400 by the time the aircraft touched the surface. The aircraft came to rest on its skids in approximately six to eight feet of water, though the main rotor blades sustained damage after contacting the water.
The investigation
Investigators removed and tested the older left magneto from the aircraft at various RPM levels; the right magneto had been newly installed and was not removed for testing. During this process, the left magneto performed erratically and failed to produce a consistent, continuous spark at high RPMs. Following the tests, the magneto was disassembled and inspected, but investigators could not determine the specific cause of its inconsistent performance.