Wheel Slip: 0 kt or 0% Wheel Slip: 20 kt or 20% Wheel Slip: 60 kt or 60% Wheel Slip: 100 kt or 100%
Free-rolling Locked Wheel
Figure C2
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The friction force depends on the slip percentage. It is easily understood that a free-rolling wheel (in other words, zero % slip) does not resist aircraft motion, therefore does not create a friction force; so, in theory, there is no braking action.
It is a well-known fact that a locked-up wheel simply .skidding. over the runway has a bad braking performance. Hence, the advent of the so-called .anti-skid. systems on modern aircraft.
PRINCIPLE OF ANTI-SKID SYSTEM Extracted from A320 FCOM, 1.32.30
The system compares the speed of each main gear wheel (given by a tachometer) with the speed of the aircraft (reference speed). When the speed of a wheel drops below 0.87 times the reference speed, the system orders brake releasing in order to maintain the brake slip at that value.
Somewhere in between these two extremes, lies the best braking performance. The following figure shows that the maximum friction force, leading to the maximum braking performance, is obtained for a slip ratio around 12%.

SLIP-RATIO PERCENTAGE
Figure C3
Tests have demonstrated that the friction force on a dry runway varies with the aircraft speed as per the following graph:

Figure C4

C2.2 Ground spoilers
Ground spoiler extension increases the aerodynamic drag and leads to deceleration. Extending the ground spoiler also significantly degrades the lift, thereby increasing the load on the wheels and brake efficiency.

DRAG INCREASE
LOAD INCREASE
Figure C5
C2.3 Thrust reversers
Similar to ground spoiler extension, reversers create a force opposite to the aircraft’s motion, inducing a significant decelerating force which is independent of the runway contaminant.
Regulations do not allow crediting reversers’ effect on performance on a dry runway.
However, regulations presently allow crediting reversers’ effect on takeoff performance for wet and contaminated runways.
The situation is a bit different for landing performance where regulations allow crediting the effect of reversers only for contaminated runways, and not for dry and wet runways.
Remark: This may lead to a performance-limited weight on a wet or contaminated runway being greater than the performance-limited weight on a dry runway. It is compulsory to restrict the performance-limited weight on a wet/contaminated runway to that of a dry runway.
As illustrated by the following graphs, reversers proportionally have a more significant effect on contaminated runways than on dry runways, since only low deceleration rates can be achieved on contaminated or slippery runways.
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Figure C6

Figure C7

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