The myth of losing tail rotor effectiveness

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Dequin, A.-M.
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The helicopter community has been plagued during the latest forty years by accidents due to unanticipated yaw, also called Loss of Tail rotor Effectiveness (LTE). How the problem was identified and what answers were given are first reconstructed from period documents and Airbus experience. A part of the mystery still remained and especially no clear explanation of the phenomenon was given. An analysis of accident databases existing in different countries is then presented. The figures are somewhat amazing and a yearly average exceeding 18 accidents was identified. Surprisingly three out of four accidents take place in the close vicinity of the ground where the recovery actions recommended in AC 90-95, the authoritative document about the topic, are not applicable. An explanation of the phenomenon is proposed, using the pedal curve as a tool. It allows understanding how and in what conditions unanticipated yaw occurs in the simplest case, hovering with wind. It also shows that recovery is affected by the modification of the pedal position at trim induced by the change in heading coming from the yaw rate, which makes the pilot feel the tail rotor to be ineffective. Accidents most probably occur because pilots do not use pedal corrections of sufficient amplitude during recovery. A more complex case is also analyzed, low speed turns to the right in ground reference with wind, as used during photographing or filming flights where unanticipated yaw often takes place. Such events are shown to occur when entering tailwind conditions, where the airspeed is reduced while a side wind component exists. A few myths grown on our poor understanding of the issue are then corrected, highlighting the unsafe way of flying helicopters with left wind, when performance is limited. The pedal curve provides a clear understanding of unanticipated yaw and gives an opportunity to solve that problem. This asks to build a unique, clear and consistent message toward pilots, appropriate to the low height conditions where the problem occurs, that shall be propagated by Authorities, Industry and Flight Schools.