getting to grips with COLD WEATHER OPERATIONS 寒冷天气运行(52)

C3 BRAKING PERFORMANCE

The presence of contaminants on the runway surface affects the braking performance in
various ways.

The first obvious consequence of the presence of contaminants between the tire and the
runway surface is a loss of friction force, hence a reduced  μ. If this phenomenon is quite
natural to understand, it is difficult to convert it to useable figures. That is why the
mathematical model is still evolving and is monitored by regulations.

The presence of a fluid contaminant like water or slush can also lead to a phenomenon
known as aquaplaning or hydroplaning. In such a configuration, there is a loss of contact,
therefore of friction, between the tire and the runway surface.
Fluid contaminants produce a lot of precipitation on the airframe and landing gears,
causing additional drag.

Hard contaminants:
Compacted snow and ice
Decrease of friction forces
Figure C8

Fluidcontaminants:
Water, slush and loose snow
Decrease of friction forces
+
 precipitation drag

+
 aquaplaning

Figure C9
C3.1 Reduction of the friction coefficient μ
Friction force reduction is due to the interaction of the contaminant with the tire and the runway surface. One can easily understand that this reduction depends directly on the contaminant. Let us review the μ reduction by contaminant.

C3.1.1 Wet runway
The following text is extracted from the ICAO Airport Services Manual, Part 2. Quote
.Normal. wet friction is the condition where, due to the presence of water on a runway, the available friction coefficient is reduced below that available on the runway when it is dry. This is because water cannot be completely squeezed out from between the tire and the runway and, as a result, there is only partial contact with the runway by the tire. There is consequently a marked reduction in the force opposing the relative motion of the tire and runway because the remainder of the contacts is between tire and water. To obtain a high coefficient of friction on a wet or water-covered runway, it is, therefore, necessary for the intervening water film to be displaced or broken through during the time each element of tire and runway are in contact. As the speed rises, the time of contact is reduced and there is less time for the process to be completed; thus, friction coefficient on wet surfaces tend to fall as the speed is raised, i.e. the conditions in effect become more slippery.
Unquote
In other words, we expect μWet to be less than μDry, and to diminish as speed increases.
Until recently, regulations stated that a good representation of the surface of the wet
runway condition is obtained when considering μDry divided by two.
For example, for the A300/A310/A320 and A321 aircraft μWet= μDry/2.

As of today, a new method has been developed accounting for:
　
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