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22.3.3 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.
22.3.4 The following summarizes the properties of particular constituents of Type II and IV fluids:
.  
The glycol in the fluid reduces the freezing point to negative ambient temperatures.

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

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

22.3.5 Type II and IV fluids can be diluted with water. Because of the lower glycol content compared to the Type I fluids, their freezing points rise 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.
22.3.6 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.
22.3.7 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.
22.3.8 Type II/IV and Type I fluids can all be diluted with water. This may be done, if due to weather conditions, long conservation time is not needed or higher freezing points are sufficient.

22.4  Fluid Specification
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.
Issue II 02.07.2009  Rev. 0

 

22.5 Manufacturer Certificate
The users are, however, obtaining a certificate from the manufacturers that the fluids meet the SAE specifications. The SAE specifications are:
a.  
SAE Aerospace Material Specification (AMS) 1424 “Aircraft Deicing/Anti-icing Fluid SAE Type I”; and

b.  
SAE AMS 1428: “Deicing/Anti-icing Fluid SAE Type II, III and IV”.

 

22.6  Freezing point
The freezing points are determined by the American Society for Testing Materials (ASTM) D 1177 method, which measures the temperature of the first ice crystal formation in the fluid.
　
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