An Explanation of Unanticipated Yaw Phenomenom on Helicopters

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). Thanks to official reports, the first part of this paper will be devoted to the genesis of this problem and recommendations provided at that time. An analysis of accident databases, available in different countries, is astonishing with 18 accidents occurring on average each year. Surprisingly, three out of four accidents took place close to the ground where the recovery actions recommended in AC 90-95 are not applicable. A part of the mystery remains and especially no clear explanation of the phenomenon was provided at that time. The benefit of this paper is to provide an explanation of the phenomenon using the pedal curve approach. During hovering flight with wind, the pedal curve is useful to understand how and under what conditions unanticipated yaw can develop. The lack of effectiveness of the tail rotor perceived sometimes by the pilot is a consequence of a too limited action on the pedal during the recovery maneuver. Finally, flight tests carried out on AH130 confirm the explanation given thanks to the pedal curve and provide a clear recommendation to recover from such situation. A quick and large action on the pedal (up to the stop if necessary) in the opposite direction of the yaw is the only possible response to unanticipated yaw, regardless of the yaw rate reached during the phenomena. Authorities, Manufacturers and Schools must convey the same message towards the pilot community in order to reduce the number of accidents and to restore confidence in the effectiveness of the tail rotor.