What happened
The pilot had requested clearance to transit the Seattle Terminal Control Area (TCA) for a landing at Boeing Field. During this request, air traffic control advised him of a DC-10 departing from Seattle-Tacoma International Airport and cautioned him about the possibility of wake turbulence.
The DC-10 passed the helicopter with approximately 1 1/2 miles of lateral separation and was positioned about 600 feet higher than the helicopter. Shortly after this pass, the helicopter encountered violent vortex turbulence. The aircraft exceeded a 90-degree bank angle and a 90-degree nose-down attitude. At this point, the engine flamed out.
The turbulence continued throughout the ensuing autorotation but diminished during the last 100 to 200 feet of descent. When the pilot attempted to flare for a landing, the main rotor blades flexed downwind and contacted the tail boom. Upon recovery, it was determined that less than 1 quart of fuel remained in the helicopter.
The investigation
Examination of the aircraft revealed that the main rotor blades had made contact with the tail boom due to flexing during the flare maneuver. The engine experienced a flameout likely induced by the extreme attitudes encountered during the turbulence event. Fuel analysis confirmed critically low fuel levels at the time of recovery.
Findings
The incident was caused by the helicopter encountering wake turbulence from a larger aircraft, which led to loss of control and subsequent structural damage. Contributing factors included the proximity of the departing DC-10 despite lateral separation and the pilot's attempt to flare with insufficient fuel remaining.