getting to grips with COLD WEATHER OPERATIONS 寒冷天气运行(53)

-Tire wear state
-Type of runway
-Tire inflation pressure
- Anti-skid effect demonstrated through flight tests on wet runways.
In any cases, the braking friction coefficient diminishes (non-linearly) with the aircraft ground speed.
C3.1.2 Fluid contaminated runway: Water, slush and loose snow
The reason for friction force reduction on a runway contaminated by water or slush is similar to the one on a wet runway. The loss in friction is due to the presence of a contaminant film between the runway and the tire resulting in a reduced area of tire/runway dry contact.
As for the μwet, μcont is often derived from μdry. Again, until recently, regulations stated that μcont = μdry /4. This is applicable to A300/A310/A320/A321.
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As for the wet condition, a new model has been developed to take into account tire wear
state, type of runway, tire inflation pressure and anti-skid effect.
C3.1.3 Hard contaminated runway: Compacted snow and ice
These two types of contaminants differ from water and slush, as they are hard. The wheels just roll over it, as they do on a dry runway surface but with reduced friction forces. As no rolling resistance or precipitant drag is involved, the amount of contaminant on the runway surface is of no consequence. Assuming an extreme and non-operational situation, it would be possible to takeoff from a runway covered with a high layer of hard compacted snow, while it would not be possible to takeoff from a runway covered with 10 inch of slush. One can easily imagine that the rolling resistance and precipitation drag would be way too important.
The model of the friction forces on a runway covered by compacted snow and icy runway as defined in the FCOM, leads to the following μ:
Compacted snow : μ = 0.2 Icy runway : μ = 0.05
C3.2 Precipitation drag
Regulation requires, e.g. AMJ 25x1591:
.During take-off acceleration, account should be taken of precipitation drag. During
accelerate-stop deceleration and at landing, credit may be taken for precipitation
drag..

Precipitation drag is composed of the two following drags:
1/ Displacement drag
Drag produced by the displacement of the contaminant fluid
from the path of the tire.
Increases with speed up to a value close to aquaplaning speed.

Figure C10
It is proportional to the density of the contaminant, to the frontal area of tire in the contaminant and to the geometry of the landing gear.
2/ Spray impingement drag Additional drag produced by the spray thrown up by the wheels (mainly those of the nose gear) onto the fuselage.
C3.3 Aquaplaning
As previously explained, the presence of water on the runway creates an intervening water film between the tire and the runway leading to a reduction of the dry area. This phenomenon gets more critical at higher speeds, where the water cannot be squeezed out from between the tire and the runway. The aquaplaning is a situation where .the tires of the aircraft are, to a large extent, separated from the runway surface by a thin fluid film. Under these conditions, tire traction drops to almost negligible values and aircraft wheels braking as well as wheel steering for directional control is, therefore, virtually ineffective.. ICAO Airport Services Manual, Part 2.
　
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