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23 with the control. Immediately following this treatment, the of improved thickener formulae for ADAFs. To this end, the same residue sample exceeded the control in weight gain SAE G-12 Fluid Residues Working Group is leading re- when rehydrated with dematerialized water (Ross 2006). This search efforts in this field. Currently available thickeners suggests that not only do alkali-metal-salt-based PDPs accel- were designed to enhance the holdover properties of ADAFs erate the precipitation and buildup of thickener residues, but and did not take the potential interaction with PDPs into under the right conditions they may also encourage greater account. From a residue mitigation standpoint, air carriers and moisture uptake by the thickeners. airports would be well-served by a controlled comparison of the single- and double-step processes currently favored for application of ADAFs to identify the most effective method Standards and Test Protocols for controlling thickener residue buildup. There are two independent standards used to designate Further research is needed to better understand the inter- ADAFs for military and commercial use. The military specifi- actions between ADAFs and PDPs, as new ADAFs and PDPs cation--MIL-A-8243D--is entitled Anti-icing and Deicing- are continually introduced to the market. For instance, envi- Defrosting Fluids and approves only propylene glycol-based fluids as Type I and ethylene glycol-based fluids as Type II to ronmentally benign alternatives to glycol-based ADAFs such be used by the U.S. Air Force. The commercial specifica- as formulations based on glucose-lactate have been tested tions--SAE AMS 1424 (Type I) and AMS 1428 (Types II, III, with promising results ("FAA-Approved . . ." 2007). In addi- and IV)--classify ADAFs based on their viscosity and hold- tion to the freezing point depressant and additives used, the over properties, other than their chemical composition (Pro- interactions between ADAFs and PDPs may be affected by Act Fact Sheet . . . 1998). the aircraft type, maintenance and inspection practices, and weather (Ross 2006). AMS 1428F is the accepted standard for SAE Types II, III, and IV thickened, non-Newtonian aircraft deicers, and CONCLUDING REMARKS there is no provision for testing compatibility with PDPs con- tained in the July 2007 revision of this standard. Likewise, The U.S. aviation industry as a whole has enjoyed greater AMS 1435, the SAE standard for liquid runway deicers, con- pavement frictional characteristics (safety) and longer oper- tains no provision for testing compatibility with ADAFs. ating hours for aircraft (nonclosure of runways) because of the effectiveness of modern PDPs. Prevention and Mitigation In spite of their environmental advantages over older for- Although interaction between runway and aircraft deicers is mulae such as urea and glycols, alkali-metal-salt-based PDPs inevitable, there are opportunities to control the effects of the present new challenges to the aircraft operating and manu- interaction. When applying runway deicer, extra care can be facturing industries. Table 11 summarizes the effects of mod- taken to avoid overspray around parked airplanes. Thickener ern PDPs on aircraft components, including the key findings residue can be reduced to a minimum through frequent in- and knowledge gaps. spection and cleaning of areas prone to buildup, such as spar areas and leading edge cavities. Dried residue can be re- It should be noted that the effects of modern PDPs on air- hydrated with warm water spray and then flushed or wiped craft components lead to substantial financial consequences away. Nonetheless, challenges remain for such operational such as increased maintenance, inspection, and replacement practices in commercial aviation, in light of the financial and costs and flight delay costs. Continental Airlines forecasted environmental constraints. In addition, spray from PDP pools out-of-service and flight delay losses owing to catalytic is unpredictable during aircraft take-off and landing. Inter- oxidation of C/C aircraft brakes starting at $200,000 and action with Type IV ADAFs has been seen to rapidly pro- $500,000 annually (Duncan 2006). Advances in anti-oxidant mote rough, persistent residue on wing leading edges with technology and C/C composite substrates are helping to con- unfavorable aerodynamic properties. trol this figure. Corrosion of Cd-plating and aluminum parts by runway deicers requires modifications and repairs to com- Knowledge Gaps ponents on, in, and around landing gear and wheel wells, with an the annual cost estimated at approximately $1.3 mil- The contamination effects of ADAFs by runway deicing flu- lion per national carrier for the foreseeable future (Duncan ids have been well-observed, but not yet thoroughly quanti- 2006). Increasing costs like these are making alternatives to fied. Acquisition of hard data will assist in the generation of Cd-plated steel such as anodized aluminum and stainless inspection schedules (Hille 2007) and may spur development steel more attractive to manufacturers (Duncan 2006).

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24 TABLE 11 SUMMARY OF EFFECTS OF MODERN PDPS ON AIRCRAFT COMPONENTS PDP Impact Information Sources What Is Known What Is Unknown 1. A growing body of field evidence from airline operators suggests that 1. There is still a need to the use of KAc and KF on airfield establish a comprehensive PDP 1. Academic-peer-reviewed pavements leads to catalytic oxidation catalytic oxidation test protocol. literature of C/C composite brake components. Catalytic oxidation of 2. Industry-peer-reviewed carboncarbon publications and reports 2. More research is needed to composite brakes 3. Survey of stakeholder 2. Existing research in the laboratory better understand relationships groups has demonstrated the catalytic effects between brake design, AO of potassium, sodium, and calcium on treatment, and PDP carbon oxidation. contamination as factors in catalytic oxidation. 1. There is still a need to establish a comprehensive 1. Until recently, the principal metallic corrosion test protocol evidence connecting alkali-metal-salt- for PDPs. based PDPs with Cd-plating corrosion has been a trend of increased 2. More research is needed to reports of the latter occurring better understand the simultaneously with the introduction interactions among the aircraft 1. Industry-peer-reviewed Corrosion of aircraft of the former. component design, the CICs publications and reports alloys (with a focus on 2. Very little research has been used, and the contamination of 2. Survey of stakeholder cadmium plating) conducted to investigate the PDPs in the processes of groups mechanism of Cd corrosion or Cd- metallic corrosion. steel corrosion in the presence of alkali-metal-salts (e.g., KF and KAc), 3. There is still a lack of partly owing to the high toxicity academic research data from associated with Cd and its controlled field investigation compounds. regarding the aircraft metallic corrosion by PDPs. 1. The contamination effects of ADAFs by runway deicing fluids have been well-observed, 1. Recent laboratory data appear to but not yet thoroughly 1. Industry-peer-reviewed quantified. Interaction with corroborate anecdotal reports of publications and reports aircraft deicing and increased rates of thickener residues 2. Survey of stakeholder 2. Further research is needed to anti-icing products in environments where alkali-metal- groups better understand the salt- based PDPs have been used. interactions between ADAFs and PDPs, as new ADAFs and PDPs are continually introduced to the market.