The pilot's failure to adequately secure the external load rigging of an airplane that was suspended from the helicopter, which resulted in a load shift during cruise flight, producing an abrupt nose-down attitude, and the severing of the tailrotor drive shaft by the main rotor blades.

The pilot's encounter with a ground resonance condition during the landing touchdown, which resulted in the shearing of a main rotor spacing cable bolt, and buckling of the tail boom.