Loss of tail–rotor effectiveness (LTE) occurs when the tail rotor of a helicopter is exposed to wind forces that prevent it from carrying out its function—that of cancelling the torque of the engine and transmission. Any low-airspeed high-power environment provides an opportunity for it to occur.
Why is this bad? Loss of Tail-Rotor Effectiveness simply put means that you don’t have full pedal authority of the helicopter anymore. Given enough deterioration, you would be hard pressed to control the helicopter. LTE is created by certain wind scenarios. Although they can’t be avoided all together, you should be aware of them and recognize avoid them when able.
There are three scenarios that are particularly prone to causing LTE.
Main Rotor Disc Vortex Interference (285 – 315 degrees) Winds within this region can cause the main rotor vortex to be directed onto the tail rotor. The left quartering headwind forces the downwash from the main rotor system into the tail rotor, interfering with the normal efficiency of the tail rotor.
WEATHERCOCK STABILITY (120-240°) In this region, the helicopter attempts to weathervane its nose into the relative wind. Unless a resisting pedal input is made, the helicopter starts a slow, uncommanded turn either to the right or left depending upon the wind direction.
TAIL ROTOR VORTEX RING STATE (210-330°) Winds within this region will cause the tip vortices generated by the tail rotor blades to be recirculated through the rotor, in the same way that main rotors re-ingest wake vortices in an improperly executed descent.
LTE RECOVERY is initiated by increasing airspeed, using the vertical stabilizer to reduce yaw or, if uncorrectable by application of speed or tail-rotor thrust, entry into autorotation. Note that a full autorotative landing isn’t necessary—the mere entry into autorotation will eliminate the torque, and then the spin or yaw will reduce through friction, particularly with the buildup of forward speed. At that point power recovery (while maintaining airspeed) can be successfully accomplished.
The FAA has published an Advisory Circular about Loss Tail Rotor Effectiveness. AC90-95.
Video by Helicopter Online Ground School