Kenny Keller’s ground-school lesson explains Loss of Tail Rotor Effectiveness (LTE): an uncommanded rapid yaw toward the advancing blade caused by aerodynamic interactions between main and tail rotors (not mechanical failure). The transcript enumerates three wind-regions that commonly produce LTE (main rotor disc interference 285–315°, weathercock stability 120–240°, tail-rotor vortex ring state 210–330°), lists contributing factors (low-and-slow, tailwinds, high density altitude, etc.), prevention recommendations (maintain rotor RPM, avoid tailwinds below 30 kt, avoid OGE in high power demand), and step-by-step recovery actions (forward cyclic to gain speed, reduce collective if altitude permits, maintain full left pedal until rotation stops).
Chapters
Loss of tail Rotor Effectiveness LTE what is it the Helicopter Flying Handbook tells us? Loss of tail rotor effectiveness LTE, or an unanticipated yaw, is defined as an uncommanded rapid yaw towards the advancing blade which does not subside of its own accord. It can result in the loss of the aircraft if left unchecked. It is very important for pilots to understand the LTE is caused by an aerodynamic interaction between the main rotor and tail rotor and not caused by from a mechanical failure. Some helicopter types are more likely to encounter LTE due to the normal certification thrust produced by having a tail rotor that, although meeting certification standards, is not always able to produce the additional thrust demanded by the pilot. So first let's cover the three winds that you have to be familiar with so that you don't get yourself in an LTE situation. First, we'll talk about main rotor disc interference. If you have a wind off the quartering left front degrees 285 to 315 degrees and winds at velocities of 10 to 30 knots from the left front cause the main rotor vortex to be blown into the tail rotor by the relative wind. This main rotor disc vortex causes the tail rotor to operate in an extremely turbulent environment. So try to visualize a great big huge donut of air around the outside of that rotor system. Turbulent air being recirculated and that wind coming on that angle is pushing all that dirty turbulent air into that tail rotor. The second is weathercock stability. Winds from 120 degrees to 240 in this region, the helicopter attempts to weather vane or weathercock 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. If the pilot allows a right yaw 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. The third one you need to be aware of is tail rotor vortex ring state. These are winds from 210 to 330 degrees. Winds within this region cause the tail rotor vortex ring state to develop. The result is a non uniform unsteady flow into the tail rotor. The vortex ring state causes tail rotor thrust variations which will result in yaw deviations. 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 over controlling can lead to LTE. So again those three are main rotor disc interference 285 to 315, weathercock stability 120 to 240 degrees and tail rotor vortex ring state 210 to 330. So there are a number of contributing factors to this LTE that you need to be aware of. Number 1 Low and slow flight outside of ground effect. Number 2 Winds from plus or minus 15 degrees of the 10 o' clock position and possibly around the 5 o' clock position. Number 3 Tailwinds that may alter the onset of translational lift and transitional thrust that induce high power demands and demand more anti torque left pedal than the tail rotor can produce. Number four, Low speed downwind turns. Number five, large changes of power at low air speeds. Number six low speed flight in the proximity of physical obstructions that may also alter a smooth airflow to both the main rotor and tail rotor. You need to be aware of LTE at altitude. And we know anytime we have high gross weight, high ST altitude, that always is a contributing factor to anything bad that goes on in the helicopter. So of course at higher density altitudes, higher altitudes you're going to be more susceptible to lte. So there are some steps for reducing the onset of LTE and these steps are one Maintain maximum power on rotor rpm. Even if the main rotor RPM is allowed to decrease, the anatoric thrust available is decreased proportionally. Number two avoid tailwinds below airspeeds of 30 knots. If loss of transition lift occurs, it results in an increased power demand and additional anti torque pressures. Avoid OGE operations in 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 to 12 knots. A loss of transitional lift results in unexpected high power demand and an increased anti torque requirement. 5. Be alert to changing wind conditions which may be experienced when flying along ridge lines and around buildings. 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 and unanticipated right yaw. And number seven execute slow turns to the right which would limit the effects of rotating inertia and the loading on the tail rotor to control yawing would be decreased. So the best thing to do is avoid these situations and not even get yourself into it, but let's say you did. For whatever reason you got yourself into a LTE situation, then you need to know the recovery technique. If a sudden, unanticipated right yaw occurs, you need to 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 execute immediately if an uncommitted right yaw is evident. Collective pitch reduction aids in arresting the yaw rate, but may cause an excessive ray 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 so you can dig into the helicopter flying book a little deeper if you want to go even deeper than that. And for more information on LTE, you can go to Advisory Circular AC 90 95. Unanticipated right yaw and Helicopters. So my thing, avoid those winds. Know how to reduce the onset. Use good pilot technique and make sure you understand the recovery technique in the event you got into it.
