285°
11-12
 

WEATHERCOCK STABILITY (120-240°)
In this region, the helicopter attempts to weathervane its nose into the relative wind. [Figure 11-11] 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. If the pilot allows a right yaw rate to develop and the tail of the helicopter moves into this region, the yaw rate can accelerate rapidly. In order to avoid the onset of LTE in this downwind condition, it is imperative to maintain positive control of the yaw rate and devote full attention to flying the helicopter.
360°
 

TAIL ROTOR VORTEX RING STATE (210-330°)
Winds within this region cause a tail rotor vortex ring state to develop. [Figure 11-12] The result is a non-uniform, unsteady flow into the tail rotor. The vortex ring state causes tail rotor thrust variations, which result in yaw deviations. The net effect of the unsteady flow is an oscillation of tail rotor thrust. Rapid and continuous pedal movements are necessary to compensate for the rapid changes in tail rotor thrust when hovering in a left crosswind. Maintaining a precise heading in this region is difficult, but this characteristic presents no significant problem unless corrective action is delayed. However, high pedal workload, lack of concentration and overcontrolling can all lead to LTE.
When the tail rotor thrust being generated is less than the thrust required, the helicopter yaws to the right. When hovering in left crosswinds, you must concentrated on smooth pedal coordination and not allow an uncontrolled right yaw to develop. If a right yaw rate is allowed to build, the helicopter can rotate into the wind azimuth region where weathercock stability then accelerates the right turn rate. Pilot workload during a tail rotor vortex ring state is high. Do not allow a right yaw rate to increase.
90°
180° 360° 0° 17 Knots
 
Figure 11-12. Tail rotor vortex ring state.
LTE AT ALTITUDE
At higher altitudes, where the air is thinner, tail rotor thrust and efficiency is reduced. When operating at high altitudes and high gross weights, especially while hovering, the tail rotor thrust may not be sufficient to maintain directional control and LTE can occur. In this case, the hovering ceiling is limited by tail rotor thrust and not necessarily power available. In these conditions gross weights need to be reduced and/or operations need to be limited to lower density altitudes.
 
 

REDUCING THE ONSET OF LTE
To help reduce the onset of loss of tail rotor effectiveness, there are some steps you can follow.
1 Maintain maximum power-on rotor r.p.m. If the main rotor r.p.m. is allowed to decrease, the anti-torque thrust available is decreased proportionally.
2 Avoid tailwinds below an airspeed of 30 knots. If loss of translational lift occurs, it results in an increased power demand and additional anti-torque pressures.