Incident
During a training flight in a Bell 206B helicopter, a certified flight instructor was demonstrating a rapid deceleration maneuver to a grass area at the airport. The helicopter touched down hard, resulting in the tail boom being buckled downward. The subsequent misalignment caused the tail rotor drive shaft to fracture into several sections, and the tail boom was deflected to the right.
Examination
Examination of the fractured tail boom and tail rotor segments revealed overload fractures consistent with a hard, vertical landing at approximately 70 percent main rotor rpm. Although the pilots were not performing an autorotation, the conditions at touchdown were similar to procedures outlined in the Bell 206 Rotorcraft Flight Manual (RFM). The RFM instructed pilots to adjust collective pitch during an autorotation maneuver as required to maintain 90 to 107 percent rpm. It also recommended that a level touchdown be made prior to passing through 70 percent rotor rpm.
Manufacturer Warnings
The manufacturer had previously warned operators that the tail boom and aft fuselage could be damaged if during an autorotation landing the main rotor rpm was allowed to decay below 70 percent rpm.