Incident Overview
A helicopter pilot flew below and behind the flight path of an airliner, encountering wake turbulence. During the encounter, the pilot inadvertently rolled the throttles to manual and did not restabilize the engines or main rotor rpm. The pilot also did not understand the reset procedures for the engine controls (FADEC) and never returned the engines to FADEC control.
Flight Sequence
After approximately two minutes of flight with several power changes and a climb of 700 feet, rotor rpm decreased to 73 percent. The pilot declared an emergency, reported a double power loss, and ditched the helicopter in a river.
Post-Impact Observations
Video footage of the last several seconds of the flight showed periodic bursts of flames and bright objects emitted from the rear of the helicopter before water contact. The left engine had been overtempted and experienced turbine failure, while the right engine was capable of producing power at water impact.
Mechanical Findings
A failed hydraulic line was found in-line with a failed coupling on the tail rotor drive shaft, in an area where a fire had burned. The manufacturer reported that the tail rotor drive shaft could become unstable above 168 percent Nr, or lower if the mounts were loose or rubber grommets deteriorated. Sound analysis recorded the main rotor momentarily at 125 percent Nr during the autorotation. A fault code from the right engine FADEC indicated the power turbine had reached 127 percent Nr.
Maintenance Issues
The hanger bearings for the long tail rotor drive shaft had not been retorqued as required after being replaced. Additionally, non-mechanic rated pilots had signed off 100-hour inspections and required inspections from airworthiness directives.