getting to grips with COLD WEATHER OPERATIONS 寒冷天气运行(46)

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Although the thickener content is less than 1%, it gives the fluid particular properties. The viscosity of the fluid and the wetting agents causes the fluid to disperse onto the sprayed aircraft surface, and acts like a protective cover.
The fundamental idea is a lowering of the freezing point. Due to precipitation such as snow, freezing rain or any other moisture, there is a dilution effect on the applied fluid. This leads to a gradual increase of the freezing point until the diluted fluid layer is frozen due to the low ambient temperature. By increasing the viscosity, a higher film thickness exists having a higher volume which can therefore absorb more water before freezing point is reached. In this way, the holdover time is increased.
The following summarizes the properties of particular constituents of Type II and IV fluids:
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 The glycol in the fluid reduces the freezing point to negative ambient temperatures.

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 The wetting agent allows the fluid to form a uniform film over the aircraft’s surfaces.

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 The thickening agent in Type II and IV fluids enables the film to remain on the aircraft’s surfaces for longer periods.

Type II and IV fluids can be diluted with water. Because of the lower glycol content, compared to the Type I fluids, the freezing point rises all the time as water is added. The viscosity of Type II and IV fluids is a function of the existing shear forces. Fluids showing decreasing viscosity at increasing shear forces have pseudo-plastic or non-Newtonian flow properties.
During aircraft take-off, shear forces emerge parallel to the airflow at the fluid and aircraft surface. With increasing speed the viscosity decreases drastically and the fluid flows off the wing. The protective effect of the Type II and IV fluids is much better when compared to the Type I fluids. Therefore they are most efficient when applied during snowfall, freezing rain and/or with long taxiways before take-off.
Type II/IV and Type I fluids can all be diluted with water. This may be done if due to
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weather conditions, no long conservation time is needed or higher freezing points are sufficient. All above types of fluid have to meet the specified anti-icing performance and aerodynamic performance requirements as established in the respective specifications (ISO, SAE, AEA). This has to be demonstrated by the fluid manufacturer.
● Anti-icing process

The anti-icing fluid which freezes at a very low temperature (e.g. -30°C), is applied on a clean surface. It forms a protective layer.

This fluid layer absorbs the frozen precipitation. It keeps the freezing temperature of the diluted fluid well below OAT or aircraft skin temperature, thus preventing frozen precipitation to accumulate.
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Then the layer becomes more and more diluted by the melted precipitation; its freezing temperature increases. When it reaches OAT or the aircraft skin temperature, anti-icing fluid fails and the frozen precipitation accumulates.
　
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