VII Understanding Touchdown and Rollout

Touchdown – Friction Forces
Upon touchdown of the main landing gear, the aircraft transitions from the “ laws of flight dynamics” to the “ laws of ground dynamics”.
The following are among the events that occur upon touchdown:
.  
Wheels rotation, unless hydroplaning is experienced. To minimize the risk of hydroplaning and ensure a positive spin up of wheels, it is recommended to perform a firm touchdown when landing on a contaminated runway. 

.  
Buildup of friction forces between the wheel tires and the runway surface (Figure 4), under the combined effect of:

. Wheels/tires-braking forces; and,
. Tire-cornering forces.

Flight Operations Briefing Notes
Crosswind Landings

Figure 4
Friction forces between the wheel and the runway

Wheels/tires-braking forces and tires-cornering forces depend on the tire and runway conditions but also on each other, the higher the braking force, the lower the cornering force, as illustrated by Figure 5.

Free rolling wheel Slip Ratio (%) Blocked wheel
Figure 5
Tire Braking and Cornering Forces

Transient effects such as the distortion of the tire thread (caused by any yawing movement of the wheel) or the activation of the anti-skid system affect the tire-

Flight Operations Briefing Notes 
Crosswind Landings
cornering and wheel-braking forces (in both magnitude and direction) and, thus, affect the overall balance of friction forces.
As a consequence, the ideal balance of forces illustrated by Figure 4 is rarely steadily maintained during the initial landing roll.

Effect of Touchdown on Alignment
When touching down with some crab angle on a dry runway, the aircraft automatically realigns with the direction of travel down the runway.
　
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