LTE (Loss of Tail Rotor Effectiveness)
If you are helicopter pilot you may have already gone through something very unusual in a flight, for example, the tendency of the helicopter to aproar an undesired direction during low airspeed. This can be an indication of a situation that could have caused an LTE. ( Loss of Tail Rotor Effectiveness )
In helicopters equipped with only one main rotor, it is necessary the presence of a tail rotor to control the tendency of the helicopter to rotate in the opposite direction of rotor blades caused by the effect of torque. If the anti-torque system (tail rotor) does not provide enough energy to keep the helicopter against this trend, the risk of a sharp and unexpected turn becomes imminent.
THE PHENOMENA OF LTE
LTE is a critical; low-speed aerodynamic flight characteristic which can result in an uncommanded rapid yaw rate which does not subside of its own accord and, if not corrected, can result in the loss of aircraft control.
LTE is not related to a maintenance malfunction and may occur in varying degrees in all single main rotor helicopters at airspeeds less than 30 knots. LTE is not necessarily the result of a control margin deficiency.
CONDITIONS UNDER WHICH LTE MAY OCCUR
Any maneuver which requires the pilot to operate in a high-power, low-airspeed environment with a left crosswind or tailwind creates an environment where unanticipated right yaw may occur.
Image: unknown source
There is greater susceptibility for LTE in right turns. This is especially true during flight at low airspeed since the pilot may not be able to stop rotation. The helicopter will attempt to yaw to the right. Correct and timely pilot response to an uncommanded right yaw is critical. The yaw is usually correctable if additional left pedal is applied immediately. If the response is incorrect or slow, the yaw rate may rapidly increase to a point where recovery is not possible.
Extensive flights and wind tunnel tests have been conducted by aircraft manufacturers. These tests have identified four relative wind azimuth regions and resultant aircraft characteristics that can, either singularly or in combination, create an LTE conducive environment capable of adversely affecting aircraft controllability.
For more information on how to avoid an LTE read the Slideshow below:
PS: This article is for main rotor that rotates counterclockwise as
viewed from above, for clockwise rotors invert the wind azimuth regions.
Source: FAA AC ( Unanticipated right yaw in helicopters ).