What happened
Two commercial helicopter pilots, both certified as helicopter instructors, were conducting training maneuvers in a turbine-powered helicopter. The crew was practicing autorotations with a power recovery prior to touchdown.
During the power recovery phase, the flying pilot accidentally engaged the flight stop augmented fuel flow switch. This action caused an overspeed of both the engine and the main rotor. In an attempt to reduce the rotor RPM, the second pilot joined the flying pilot on the controls and increased the collective. This resulted in an abrupt climb to approximately 60 feet above the ground. At this altitude, the tail rotor drive shaft separated from the aircraft. Following the separation, the engine lost power, and the crew successfully completed an autorotation.
The investigation
The investigation determined that the engine and main rotor system had been subjected to significant overspeed conditions. These conditions led to a catastrophic failure of the turbine engine and the tail rotor drive shaft coupling.