Loss of helicopter tailrotor effectiveness has been identified by the UK Civil Aviation Authority as an accident-causing condition needing more emphasis among the pilot community. In a recent flight operations department communication, the CAA has described three accidents caused by loss of tailrotor effectiveness, in which the aircraft went into an uncontrollable spin or turn, and describes the flight conditions in which the risk is high.
Loss of tailrotor effectiveness is not caused by a mechanical fault but is an aerodynamic effect, the CAA emphasises, describing it as "a critical low-speed aerodynamic flight condition that can result in a rapid yaw rate that does not subside and can result in the loss of the aircraft if it is not checked".
Loss of tailrotor effectiveness occurs as a result of a "control margin deficiency", the marginal component being directional control. It is caused by disturbance of the airflow entering the tailrotor by main-rotor downwash or blade tip vortices, and loss of tailrotor effectiveness is most likely to occur during slow speed or transitional flight when the main rotor is at high power.
The CAA advises avoidance of the following combinations:
low and slow flight outside of ground effect; relative winds from plus or minus 15° of the 10 o'clock position (US helicopters) or of the 2 o'clock position (French). The difference is related to the opposite main-rotor turningdirections; tailwinds that may alter the onset of transitional lift, thus inducing high power demands; low speed downwind turns; large changes of power at low airspeeds; low speed flight close to obstacles that may disturb a smooth airflow (buildings, ridges).Source: Flight International
