Crosswind After Landing Roll
 
 Particularly during the after landing roll, special attention must be given to maintaining directional control by use of rudder, or nosewheel/tailwheel steering, while keeping the upwind wing from rising by use of aileron.

   When an airplane is airborne it moves with the air mass in which it is flying regardless of the airplane's heading and speed. However, when an airplane is on the ground it is unable to move with the air mass (crosswind) because of the resistance created by ground friction on the wheels.

Characteristically, an airplane has a greater profile or side area, behind the main landing gear than forward of it (Fig. 9-13). With the main wheels acting as a pivot point and the greater surface area exposed to the crosswind behind that pivot point, the airplane will tend to turn or "weathervane" into the wind.

   Though it is characteristic of most airplanes, this weathervaning tendency is more prevalent in the tailwheel type because the airplane's surface area behind the main landing gear is greater than in nosewheel type airplanes.

Wind acting on an airplane during crosswind landings is the result of two factors - one is the natural wind which acts in the direction the air mass is traveling, while the other is induced by the movement of the airplane and acts parallel to the direction of movement. Consequently, a crosswind has a headwind component acting along the airplane's ground track and a 
crosswind component acting 90 degrees to its track. The resultant or relative wind, then, is somewhere between the two components. As the airplane's forward speed decreases during the after landing roll, the headwind component decreases and the relative wind has more of a crosswind component. The greater the crosswind component the more difficult it is to prevent weathervaning.
 
The headwind component and the crosswind component can be determined by reference to Figure 9-14. For example:

A relative wind at 20 knots at an angle of 60 degrees to the runway has a headwind component of 10 knots and a 90 degree crosswind component of 18 knots. Federal Aviation Regulations require that all airplanes, type certificated since 1962, have safe ground handling characteristics with a 90 degree crosswind component equal to 0.2 Vs0. 

Thus, an airplane that stalls at 55 knots in the landing configuration, must have no uncontrollable ground looping (weathervaning) tendencies with a 90 degree crosswind component of 11 knots (0.2 x 55). It is imperative that pilots determine the maximum crosswind component of each airplane they fly, and avoid operations in wind conditions that exceed the capability of the airplane.

 While the airplane is decelerating during the after landing roll, more and more aileron must be applied to keep the upwind wing from rising. Since the airplane is slowing down there is less airflow around the ailerons and they become less effective. At the same time the relative wind is becoming more of a crosswind and exerting a greater lifting force on the upwind wing. Consequently, when the airplane is coming to a stop the aileron control must be held fully toward the wind.

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