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22 contaminants in the field environment. One challenge is that which it returns to its normal state. This characteristic allows aircrafts are moving objects exposed to various PDPs and other most ADAFs to be applied easily and then resist being blown contaminants, climatic conditions, and maintenance practices off the aircraft surface during flight.). Rates of in-flight in- at different airports. For instance, Cd-plated steel was found to cidents connected to thickener deposits from these deicers be affected by paint vapors (Brough 1987), by microorganisms appeared to rise during the mid-1990s, shortly after the intro- in hydraulic fluids (Weyandt and Schweisfurth 1989), and by duction of alkali-metal-salt-based PDPs (Ross 2006). A greater aircraft fuel additives (Hinton and Trathen 1992). proportion of these reports appear to have come from Euro- pean operators than from North America or Asia (Hille 2007). The geographic imbalance of these reports is believed to be INTERACTION WITH AIRCRAFT DEICING connected to the general method of aircraft deicer application AND ANTI-ICING FLUIDS favored at European airports. In Europe, aircraft are treated in a single application with a solution of Type II fluid in hot The SAE Aircraft Deicing/Anti-icing Fluid (ADAF) Specifi- water. The two-step method favored elsewhere consists of an cations provide guidelines for the holdover time of Types I, initial application of heated Type I (nonthickened) deicing II, III, and IV fluids ("FAA-Approved . . ." 2007). To meet fluid, followed by application of a heated Type IV solution these criteria, Types II, III, and IV fluids currently used for (Hille 2007). User experience has shown that application of aircraft anti-icing contain thickeners to keep these fluids on the pure or diluted Type I fluid removes thickener residue surfaces after application. These thickeners are gel polymer from previous deicer applications. additives known to gradually precipitate out of solution and form dry residues that can remain in aerodynamically quiet The synergistic generation of residue when an ADAF on areas of the aircraft for long periods. If not discovered, these aircraft is splattered with modern PDPs (e.g., KAc and NaF) residues can accumulate over time, rehydrate and expand in presents serious concerns about residue gel rehydration and rain or aircraft washes, and freeze during cold weather or refreezing in flight and has produced potentially dangerous high altitude flight. This can negatively affect in-flight han- rough residues on leading edge surfaces on aircraft. Air- dling of the aircraft if deposits occur on or near control sur- craft and runway deicing fluids tend to mix in two different faces or linkages. Initial research has shown that thickener locations--the aircraft and the runway (Hille 2007). Aircraft separation is accelerated by contact between aircraft deicing deicer may run off the aircraft, mix with runway deicer, and fluids and runway deicing fluids (Ross 2006; Hille 2007). A then be splashed back onto another aircraft by landing gear typical aircraft deicing operation is shown in Figure 12. spray or blown on by thrust reversers. Runway deicer may reach freshly applied aircraft deicer by the same means or from The rest of this section synthesizes the information on the overspray during runway application. After mixing, the now validity and nature of the interaction between modern PDPs less viscous aircraft deicer may remain in place or it may and ADAFs, describes the related standards and test proto- migrate to aerodynamically quiet areas through control surface cols, discusses ways to prevent and mitigate such interaction, gaps or vent holes, where thickeners can precipitate unnoticed. and identifies pertinent knowledge gaps. Nature of the Interaction Between Modern Pavement Deicing Products and Aircraft Validity of Interaction Between Modern Pavement Deicing/Anti-Icing Fluids Deicing Products and Aircraft Deicing/ Anti-Icing Fluids Thickeners used in aircraft deicers increase viscosity through The first glycol-based, non-Newtonian ADAFs were intro- chargecharge interaction; organic salts such as KAc and KF duced in the early 1960s. ("Non-Newtonian" describes a are known to disrupt this interaction (Ross 2006). In theory, fluid that, when subjected to an external force, experiences contamination with KAc or KF should cause a measurable increased viscosity until the external force is removed, upon reduction in the viscosity of the aircraft deicer. Preliminary evidence shared with the SAE G-12 committee by Kilfrost, Ltd. appears to support this theory. Samples of Type II and IV aircraft deicer fluids contaminated with small amounts of KF- or KAc-based PDPs experienced immediate reductions in vis- cosity, followed quickly by precipitation of thickener addi- tives. These laboratory data appear to corroborate anecdotal reports of increased rates of thickener residues in environ- ments where alkali-metal-salt-based PDPs have been used. Nonetheless, this issue is being addressed by Kilfrost. Kilfrost also gathered data on the effect of runway deicers FIGURE 12 Typical aircraft deicing operation [adapted from on dried thickener residue. The thickener was observed to Ambrose (2007)]. rehydrate only slightly in a 5% KF solution when compared