Loss of helicopter tail rotor effectiveness (LTE) has been flagged by the UK Civil Aviation Authority as an accident-causing condition needing emphasis among the pilot community. In a flight operations department communication, the CAA has described three LTE helicopter accidents in which the aircraft went into an uncontrollable spin or turn, and describes the flight conditions in which the risk is high.
LTE is not caused by a mechanical fault but is purely an aerodynamic effect, the CAA emphasises. It is caused by disturbance of the airflow entering the tail rotor by main rotor downwash or blade tip vortices, and is most likely to occur during slow speed or transitional flight when the main rotor is at high power. This can be worsened by gusting or yaw putting the airflow relative to the helicopter in roughly the 10 o'clock position for US helicopters or 2 o'clock for European; the difference due to the opposite directions of main rotor rotation.
Source: Flight International
