Tail Rotor Vortex Ring State (210–330°) Recovery Technique (Uncontrolled Right Yaw) LTE at Altitude Main Drive Shaft or Clutch Failure Reducing the Onset of LTE 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. power demand and additional antitorque pressures. Winds within this region cause a tail rotor vortex ring state to  develop.  [Figure 11-11]  The result is a nonuniform, 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 lead to LTE. If a sudden unanticipated right yaw occurs, the following  recovery technique should be performed. Apply full left  pedal. Simultaneously, apply forward cyclic control to  increase speed. If altitude permits, reduce power. As recovery  is affected, adjust controls for normal forward flight. A  recovery path must always be planned, especially when  terminating to an OGE hover and executed immediately if  an uncommanded yaw is evident. When the tail rotor thrust being generated is less than the  thrust required, the helicopter yaws to the right. When  hovering in left crosswinds, concentrate on smooth pedal  coordination and do not allow an uncommanded 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. Collective pitch reduction aids in arresting the yaw rate but  may cause an excessive rate of descent. Any large, rapid  increase in collective to prevent ground or obstacle contact  may further increase the yaw rate and decrease rotor rpm.  The decision to reduce collective must be based on the pilot’s  assessment of the altitude available for recovery. If the rotation cannot be stopped and ground contact is  imminent, an autorotation may be the best course of action.  Maintain full left pedal until the rotation stops, then adjust to  maintain heading. For more information on LTE, see Advisory  Circular (AC) 90-95, Unanticipated Right Yaw in Helicopters. At higher altitudes where the air is thinner, tail rotor thrust  and efficiency are reduced. Because of the high-density  altitude, powerplants may be much slower to respond to  power changes. 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.  This may not be noted as criteria on the performance charts. To help reduce the onset of LTE, follow these steps: The main drive shaft, located between the engine and the main  rotor transmission, provides engine power to the main rotor  transmission. In some helicopters, particularly those with  piston engines, a drive belt is used instead of a drive shaft.  A failure of the drive shaft clutch or belt has the same effect  as an engine failure because power is no longer provided to  the main rotor and an autorotation must be initiated. There  are a few differences, however, that need to be taken into  11-21 1.  Maintain maximum power-on rotor rpm. If the main  rotor rpm is allowed to decrease, the antitorque thrust  available is decreased proportionally. 2.  Avoid tailwinds below airspeeds of 30 knots. If loss  of translational lift occurs, it results in an increased  3.  Avoid OGE operations and high-power demand  situations below airspeeds of 30 knots at low altitudes. 4.  Be especially aware of wind direction and velocity  when hovering in winds of about 8–12 knots. A loss  of translational lift results in an unexpected high power  demand and an increased antitorque requirement. 5.  Be aware that if a considerable amount of left pedal  is being maintained, a sufficient amount of left pedal  may not be available to counteract an unanticipated  right yaw. 6.  Be alert to changing wind conditions, which may be  experienced when flying along ridge lines and around  buildings. 7. Execute right turns slowly. This limits the effects of  rotating inertia, and decreases loading on the tailrotor  to control yawing.