Incident Overview
Two commercial helicopter pilots, both holding helicopter instructor certifications, were engaged in practice autorotations with power recovery in a turbine-powered helicopter. During a power recovery maneuver, the flying pilot inadvertently activated the flight stop augmented fuel flow switch. This action caused the engine and main rotor to exceed their normal operating speeds.
Sequence of Events
The other pilot joined the flying pilot on the controls and increased collective pitch to reduce rotor rpm. In response, the helicopter climbed abruptly to approximately 60 feet above the ground. At that point, the tail rotor drive shaft separated. Subsequently, the engine lost power, and the pilots executed an autorotation to the ground.
Investigation Findings
Examination after the event revealed that the engine and main rotor system had been subjected to significant overspeed conditions. This resulted in a catastrophic failure of the turbine engine and the tail rotor drive shaft coupling. The flight stop switch on the collective is not equipped with a protective guard, making it susceptible to inadvertent engagement. When engaged, the switch permits the engine to enter the augmented fuel flow regime, which, under certain conditions, can cause an engine overspeed. The switch has a documented history of inadvertent activation and resulting engine overspeed incidents.