SEA04LA090

The pilot reported that while in cruise flight at about 900 to 1,000 feet above ground level, 95 knots, and 80% torque, without warning, the helicopter suddenly lost altitude and yawed violently to the left and rapidly rolled (pilot unsure as to which direction). The pilot initiated an autorotation to a field, maneuvering to avoid trees. The pilot stated that during the flare, he pulled in collective, and the helicopter yawed, so he "rolled off the throttle." The helicopter touched down with almost no forward speed, the right landing skid collapsed and rolled over onto its right side. The pilot stated that he did not recall hearing the engine out beeper or the low rotor RPM horn and he did not note the engine or rotor RPM indications during the descent. Post accident inspection of the airframe and subsequent inspection and teardown of the engine and associated components did not find evidence of a mechanical failure or malfunction.

On May 23, 2004, about 1415 Pacific daylight time, a Bell 206B, N206AF, registered to Global Helicopter LLC, and flown by an airline transport pilot as a 14 CFR Part 91 personal flight, experienced a sudden loss of altitude and yawing while in cruise flight near Auburn, Washington. The pilot initiated an autorotation to an open field where after touch down, the helicopter rolled over onto its right side. Visual meteorological conditions prevailed at the time and no flight plan was filed. The helicopter was substantially damaged and the airline transport pilot and his four passengers were not injured. The flight departed from Bow, Washington, about one hour prior to the accident.

During a telephone interview and subsequent written statement, the pilot reported that while in cruise flight at about 900 to 1,000 feet above ground level, 95 knots, and 80% torque, without warning, the helicopter suddenly lost altitude and yawed violently to the left and rapidly rolled (pilot unsure as to which direction). The pilot initiated an autorotation to a field, maneuvering to avoid trees. The pilot stated that during the flare, he pulled in collective, and the helicopter yawed, so he "rolled off the throttle." The helicopter touched down with almost no forward speed. The right side landing skid was forced upward into the structure. The helicopter subsequently rolled over onto its right side.

The pilot stated that he did not recall hearing the engine out beeper or the low rotor RPM horn and he did not note the engine or rotor RPM indications during the descent.

After the helicopter came to rest, the pilot reported that the engine was still running so he shutoff the fuel valve and rolled the throttle to cutoff and shutdown the engine.

During an inspection of the airframe and engine by investigators from the National Transportation Safety Board, Federal Aviation Administration and Rolls Royce Engines, on June 9, 2004, it was found that the tailboom had separated as a result of main rotor blade contact to the structure. Impact damage was also noted to the horizontal stabilizer and the tailrotor gearbox and tail rotor blades. The tailrotor drive shaft was broken in three locations, however, the pieces were recovered and continuity was established from the cockpit pedals after to the tailboom section separation point.

The tail rotor blades displayed impact damage. Blade "A" was bent aft approximately 135 degrees at a point about nine inches outboard from the hub. The blade material was split. The pitch change link was attached at one end and partially broken at the other. Blade "B" separated about 14 inches from the hub. The pitch change link was broken on the arm.

The tail rotor drive and gearbox were found to rotate freely.

Cyclic and collective continuity was established from the cockpit controls to the transmission. The transmission was displaced aft and downward. All control tubing actuators to this area were damaged and bent and bound at the transmission, however, all control tubes remained connected.

Electrical power was applied to the system and all caution lights illuminated on the panel. The engine out/low rotor horn, transmission/oil pressure lights and low rotor rpm lights were functional. The engine out and low rotor rpm horns were functional.

The main rotor mast separated and the main rotor blades remained attached to the hub, however, both main rotor blades displayed wrinkles and tearing to the blade material throughout the length of the blades. The white blade was missing about two feet of its tip. The end weight of the red blade remained in place.

Inspection of the engine found that the N1 and N2 drive systems rotated freely. The compressor blades displayed evidence of foreign object debris (FOD) damage and ingestion. The transmission was displaced aft and loose material was present in the area forward of the compressor. Both magnetic indicating plugs were free of debris. The fuel nozzle inlet line contained approximately one teaspoon of fuel and the fuel nozzle inlet screen was intact and clear of contaminants. The airframe filter was full of fuel. No particulates or water were present. The rotor transmission to engine couple displayed evidence of rotational fractures on both ends and was dislodged from its mating points. Continuity was established from the cockpit controls.

The engine was removed from the airframe and transported to Rolls Royce Engines, Indianapolis, Indiana, for further inspection. On July 8, 2004, investigators from the Federal Aviation Administration and Rolls Royce Engines examined and tore down the engine. The FAA inspector present reported that metal pieces were found inside the fire shield. Metal splatter was found throughout the entire turbine section on the nozzles and on the turbine wheels. The compressor noted damage from the first stage through the sixth stage on both the rotors and stators. Evidence of rub was noted on the impeller shroud and metal was found in the impeller. The compressor forward housing support and anti-ice vanes displayed heavy damage. The FAA inspector reported that there was evidence of rotation throughout the engine.

The main fuel control unit and the power turbine governor were removed for bench testing. The main fuel control unit passed all functional testing with only minor discrepancies noted.

The power turbine governor found test points 3.010 and 4.090 (see attached report) were recorded out of limits. Disassembly of the unit found no anomalies to explain the out of limits recordings.