Sequence of Events
The pilot, who had 1,514 hours of flight experience, landed a helicopter on a platform in the Gulf of Mexico and remained parked for approximately 10 minutes with the main rotor RPM at ground idle. While parked, the pilot grabbed the flight manifest and placed it in his lap to make an entry. With his head down, the pilot felt a sensation that the helicopter was "rocking backwards" and grabbed both controls in an effort to "level" the helicopter. The pilot lifted up on the collective and pushed forward on the cyclic. The tail boom separated from the fuselage, and both sections of the helicopter fell off the platform and into the water.
Manufacturer's Safety Notice
Bell Helicopter Operations Safety Notice (OSN 206L-82-4) addresses tail boom skin compression wrinkles in all Bell model 206L/L-1/L-3 helicopter operations. The notice states: "Past 206 experience and flight tests...have revealed the tail boom and aft fuselage can be damaged if during an autorotation landing the main rotor RPM is allowed to decay below 70 percent RPM. Applying collective pitch in excess of that required will in some instances result in excessive flapping of the main rotor during or after touch down. This can cause a resonant response that can damage the tail boom and or aft of the fuselage. Touch down rotor RPM above 70 percent RPM is preferred. Upon ground contact collective pitch should be reduced smoothly without delay while maintaining cyclic pitch near the center position. Long ground runs with the collective up, or any tendency to float for a long distance prior to skid contact should be avoided."
Examination
Examination of the wreckage revealed the tail boom separated forward of the horizontal stabilizer, consistent with an autorotation/low RPM failure.
Operator's Comments
When asked how this accident could have been prevented, the operator stated: "Pilot should have remained more vigilant in monitoring control input and position. More aggressive use of control friction would have minimized the possibility of this occurrence."