LTE Loss Of Tail Rotor Effectiveness Lesson

Aug 27, 2018

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LTE Loss Of Tail Rotor Effectiveness Lesson

How well do you understand LTE? Loss of tail rotor effectiveness. 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 it's own accord. It can result in a 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 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, all though meeting certification standards, is not always able to produce the additional thrust demanded by the pilot.

LTE Loss Of Tail Rotor Effectiveness Lesson

First, let's cover the three winds that you have to be familiar with so that you don't get yourself in a LTE situation. First, we'll talk about main rotor disk interference. If you have a wind, off the quartering left front, degrees two hundred and eighty-five to three hundred and fifteen degrees, and the wind velocity is from ten to thirty knots from the left front cause the main rotor vortex to be blown into the tail rotor by the relative wind. This main rotor disk vortex causes the tail rotor operate in an extremely turbulent environment. Try to visualize a great big huge doughnut 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 a hundred and twenty degrees to two hundred and forty, in this region the helicopter tends to weather vane or weathercock it's nose into the relative wind. Unless the 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 Loss Of Tail Rotor Effectiveness 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 two hundred and ten to three hundred and thirty degrees. Winds within this region cause a 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 at a left crosswind. Maintaining a precise heading in this region is difficult. This presents no significant problems unless corrective action is delayed. However, high pedal workload, lack of concentration and over controlling can lead to LTE.

 LTE Loss Of Tail Rotor Effectiveness Lesson

Again, those three are main rotor disk interference, two hundred and eighty-five to three fifteen, weathercock stability, one twenty to two hundred forty degrees, and tail rotor vortex ring state, two hundred and ten to three hundred and thirty.

There are a number of contributing factors to this LTE that you need to be aware of. Number one, low and slow flight outside of ground effect. Number two, winds from plus or minus fifteen degrees of the ten o'clock position and possibly around the five o'clock position. Number three, tail winds that may alter the onset of translational lift and translational 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 airspeeds. Number six, low speed flight in the proximity of physical obstructions that may alter a smooth airflow to both the main rotor and tail rotor.

You need to be aware of LTE at altitude. We know anytime we have a high gross weight, high [inaudible 00:03:50] altitude, that always is a contributing factor to anything bad that goes on in the helicopter. Of course, at higher density altitudes, higher altitudes, you're going to be more susceptible to LTE.

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 anti-torque thrust available is decreased proportionately. Number two, avoid tail winds below airspeeds of thirty knots. If loss of translational lift occurs it result in an increase power demand and additional anti-torque pressures. Avoid OGE operations in high power demand situations below airspeeds of thirty knots at low altitudes. Number four, be especially aware of wind direction and velocity when hovering in winds of about eight to twelve knots. A loss of translational lift results in a unexpected high power demand in an increased anti-torque requirement. Number five, be alert to changing wind conditions which may be experienced when flying along ridge lines and around buildings. Be aware that if a consideral amount of left pedal is being maintained, a sufficient amount of left pedal may not be available to counteract an unanticipated right yaw. 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.

LTE Loss Of Tail Rotor Effectiveness Lesson

The best thing to do is avoid these situations and not even get yourself into it. 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 from 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 resting 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 auto-rotation may be the best course of action. Maintain full left pedal until the rotation stops then adjust to maintain heading.

You can dig into the helicopter flying book a little deeper if you want to go even deeper than that. For more information on LTE, you can go to advisory circular, AC ninety dash ninety-five, unanticipated right yaw in helicopters. 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.

 LTE Loss Of Tail Rotor Effectiveness Lesson

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