0° 30° 60° 90° 120° 150° 180° 210° 240° 270° 300° 330° R e g i o n o f d i s k v o r t e x i n t e r f e n c e 1 0 k n o t s 360° 1 5 k n o t s 2 0 k n o t s 315° 285° Wind R e g i o n o f p o s i b l e y a $^{w}$  i n t r o d uc ti o n b y w e a t h e r c o c k $_{ }$s $_{t}$a b i l t y 0° 30° 60° 90° 120° 150° 180° 210° 240° 270° 300° 330° 1 0 k n o t s 360° 5 k n o t s 1 5 k n o t s 1 7 k n o t s Wind Figure 11-9.  Main rotor disk vortex interference. Figure 11-10.  Weathercock stability.  airflow in order to provide a steady and constant antitorque  reaction. The pitch and AOA of the individual blades will  determine the thrust. A change to either of these alters the  amount of thrust generated. A pilot’s yaw pedal input causes  a thrust reaction from the tail rotor. Altering the amount of  thrust delivered for the same yaw input creates an imbalance.  Taking this imbalance to the extreme will result in the loss  of effective control in the yawing plane, and LTE will occur. This alteration of tail rotor thrust can be affected by numerous  external factors. The main factors contributing to LTE are: 1.  Airflow and downdraft generated by the main rotor  blades interfering with the airflow entering the tail  rotor assembly. 2.  Main blade vortices developed at the main blade tips  entering the tail rotor disk. 3.  Turbulence and other natural phenomena affecting the  airflow surrounding the tail rotor.  4.  A high-power setting, hence large main rotor  pitch angle, induces considerable main rotor blade  downwash and hence more turbulence than when the  helicopter is in a low power condition. 5.  A slow forward airspeed, typically at speeds where  translational lift and translational thrust are in the  process of change and airflow around the tail rotor  will vary in direction and speed. 6.  The airflow relative to the helicopter; a. Worst case—relative wind within ±15° of the  10 o’clock position, generating vortices that  can blow directly into the tail rotor. This is  dictated by the characteristics of the helicopters  aerodynamics of tailboom position, tail rotor size  and position relative to the main rotor and vertical  stabilizer, size and shape.  [Figure 11-9]  b.  Weathercock stability—tailwinds from 120° to  240°  [Figure 11-10] , such as left crosswinds,  causing high pilot workload. c. Tail rotor vortex ring state (210° to  330°).  [Figure 11-11]  Winds within this region  will result in the development of the vortex ring  state of the tail rotor. 7.  Combinations (a, b, c) of these factors in a particular  situation can easily require more antitorque than the  helicopter can generate and in a particular environment  LTE can be the result. (EMS) environments can find themselves in low-and-slow  situations over geographical areas where the exact wind speed  and direction are hard to determine.  Certain flight activities lend themselves to being at higher  risk of LTE than others. For example, power line and pipeline  patrol sectors, low speed aerial filming/photography as well  as in the Police and Helicopter Emergency Medical Services  11-19