In a conventional tail rotor system, a complete loss of tail rotor thrust can happen from an internal drive system failure or if an object contacts the tail rotor and damages the blades or gearbox.
A complete loss of thrust from a drive failure is the easier of the two emergencies for the pilot to handle. In flight (airspeed at least about 60 knots) the pilot will experience a yaw to the left or right (it depends of which direction the rotor turns) that is not correctable with pedal input. The airflow passing over the vertical fin will prevent the helicopter from spinning and in this situation the helicopter can most likely be flown to a suitable landing area. Landing without a tail rotor thrust requires an autorotation. When the throttle is closed and the engine stops applying torque, the need for tail rotor thrust goes away. It’s important to keep the helicopter into the wind to prevent sideways movement during touchdown. Collective pitch should be added carefully because friction in the transmission can have a tendency to turn the fuselage. If the helicopter starts sliding sideways it could easily roll over.
In a hover or with low airspeed a tail rotor drive failure requires quick action. The helicopter will immediately begin spinning and the pilot will need to close the throttle and perform a hovering autorotation. A failure low to the ground is normally recoverable; however, for pilots performing high hovers (utility helicopters doing lift work for example) it is much more dangerous. In cases where this has happened some pilots have survived some have not.
Loss of tail rotor thrust resulting from an object striking the tail rotor is very serious. Many times the damage causes such an imbalance that the tail rotor assembly and gearbox will break free from the tail boom. The loss of weight at that long of a moment arm will cause the CG to shift too far forward. In addition to issues resulting from the loss of tail rotor thrust, the helicopter will pitch down and the pilot most likely will not have sufficient aft cyclic movement to recover. When this happens in cruise flight or a high hover the results are normally severe aircraft damage with a high potential for serious or fatal injuries. When pilots in a low hover (EMS accident scenes for example) hit something with the tail rotor the damage to the helicopter can be severe as well, but the potential for human injury is low.
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