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Page 35
Suggested Citation:"Chapter Five - Conclusions." National Academies of Sciences, Engineering, and Medicine. 2008. Impact of Airport Pavement Deicing Products on Aircraft and Airfield Infrastructure. Washington, DC: The National Academies Press. doi: 10.17226/13913.
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Page 35
Page 36
Suggested Citation:"Chapter Five - Conclusions." National Academies of Sciences, Engineering, and Medicine. 2008. Impact of Airport Pavement Deicing Products on Aircraft and Airfield Infrastructure. Washington, DC: The National Academies Press. doi: 10.17226/13913.
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Page 36
Page 37
Suggested Citation:"Chapter Five - Conclusions." National Academies of Sciences, Engineering, and Medicine. 2008. Impact of Airport Pavement Deicing Products on Aircraft and Airfield Infrastructure. Washington, DC: The National Academies Press. doi: 10.17226/13913.
×
Page 37
Page 38
Suggested Citation:"Chapter Five - Conclusions." National Academies of Sciences, Engineering, and Medicine. 2008. Impact of Airport Pavement Deicing Products on Aircraft and Airfield Infrastructure. Washington, DC: The National Academies Press. doi: 10.17226/13913.
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Page 38

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35 Responses representing approximately 100 airports were gath- ered from the recent (2006) EPA questionnaire, which indi- cated that potassium acetate (KAc) and sand are most widely used at U.S. airports for snow and ice control of airfield pave- ments, followed by airside urea, sodium acetate, sodium for- mate, propylene glycol-based fluids, ethylene glycol-based flu- ids, and other. Responses representing 12 U.S. airports and 3 non-U.S. airports were gathered from the ACRP synthesis sur- vey distributed in this project to the 50 busiest U.S. airports among others. From the ACRP survey results, the selection of pavement deicing products (PDPs) by airport staff was based on many factors, including cost, effectiveness, environmental impact, risk of corrosion, and electrical conductivity. “Effec- tiveness” was ranked as the most important criterion and “elec- trical conductivity” as the least. The effectiveness criterion also exhibited the lowest standard deviation, with “corrosion risk” being the highest. Interestingly, the challenges and dilemmas faced by the airports pertinent to snow and ice control were highlighted because no airport selected “unimportant” or “not very important” for any of the criteria options in the survey. Alkali-metal-salt-based PDPs such as KAc and potassium formate (KF) entered the European market to a significant extent in the mid- to late-1990s. A few years later, these mod- ern PDPs entered the U.S. market. In both cases, these salts were introduced as alternatives to urea and glycols used in traditional PDPs for freezing point depression, to mitigate the environmental concerns related to airfield deicing and anti- icing operations. It became apparent soon after their intro- duction that these new deicers presented new challenges, to both the aircraft and airfield infrastructure. • Catalytic Oxidation of Carbon–Carbon Composite Brakes Thermal oxidation is the primary design specification govern- ing durability of aircraft carbon–carbon (C/C) composite brakes. Catalytic oxidation of C/C composite brakes resulting from airfield PDPs has become a growing concern that needs to be monitored in the ever-changing operation environment. In recent years, as non-traditional chemical contaminants, modern PDPs may be responsible for the more rapid structural failure of C/C composite brakes. To avoid potential safety implications, this concern has to be mitigated through more frequent proactive maintenance and inspection activities incur- ring high direct and indirect costs. A growing body of field evi- dence from airline operators suggests that the use of KAc and KF on airfield pavements leads to premature oxidation of C/C composite brake components. As the brake frictional char- acteristics are changed by the use of alkali-metal-salt-based deicers on airfield pavements, airline operators are concerned about the adverse effect of these PDPs on the braking perfor- mance and safety of aircraft. There are potential opportunities for all stakeholder groups to collaborate to address the catalytic oxidation issue of C/C aircraft brakes, with respect to aircraft and component design, brake testing, aircraft operations, airfield maintenance, etc. In the domain of brake technologies, the combination of chem- ical modification of C/C with structural changes or defect elimination seems to offer promising solutions to mitigating catalytic oxidation. Catalytic oxidation of C/C brakes may also be mitigated by utilizing more carbon-friendly PDPs on airfield pavements. • Cadmium Corrosion Cadmium (Cd) plating is the most popular surface treatment technology for corrosion protection of aircraft steel parts (e.g., airframe components and fasteners). Field reports in- creasingly suggest that the contact with modern PDPs pro- motes damage to aircraft components, including Cd-plated components. Until recently, the principal evidence connect- ing alkali-metal-salt-based PDPs with Cd-plating corrosion has been the increasing number of reports of the latter occur- ring concurrently with to the introduction of the former. There are potential opportunities for all stakeholder groups to collaborate to prevent and mitigate the effects of PDPs on aircraft components, from aspects of aircraft and component design, aircraft operations, and airfield maintenance. In the domain of corrosion-inhibiting compounds, there is still great potential for improvement when it comes to mitigating the effect of PDPs on aircraft frames and components. Little aca- demic research on interactions between alkali metal salts and Cd-plating is available, and still less is available on inhibition of these interactions. In lieu of a comprehensive prevention solution to Cd-plating corrosion or a satisfactory Cd-plating replacement, shop-level mitigation practices such as addi- tional and enhanced maintenance and inspection should help reduce the effects of PDPs on corrosion-prone, Cd-plated steel aircraft components. Such best practices would also min- imize the impact of PDPs on other aircraft components. In addi- tion, the corrosion of aircraft components (e.g., Cd-plating and CHAPTER FIVE CONCLUSIONS

36 aluminum parts) can be mitigated by using less corrosive PDPs on airfield pavements. • Interaction with Aircraft Deicing and Anti-Icing Fluids Thickeners used in modern aircraft deicing and anti-icing flu- ids increase viscosity through charge–charge interaction; organic salts such as KAc and KF are known to disrupt this interaction and cause a measurable reduction in their viscos- ity. Not only do alkali-metal-salt-based PDPs accelerate the precipitation and buildup of thickener residues, but under the right conditions, they may also encourage greater moisture uptake by the thickeners. Although interaction between runway and aircraft deicers is inevitable, there are opportunities to control the effects of the interaction by means of enhanced operational practices. Nonetheless, challenges such as financial and environmental constraints remain for such operational practices in commer- cial aviation. In addition, spray from PDP pools is unpredict- able during aircraft take-off and landing. Interaction with Type IV aircraft deicing and anti-icing fluids has been seen to rapidly promote rough, persistent residue on wing leading edges with unfavorable aerodynamic properties. • Impact of Pavement Deicing Products on Concrete Pavement The last decade has seen an increase in the premature deteri- oration of airfield portland cement concrete (PCC) pavements with the use of alkali-metal-salt-based PDPs. Such PDPs have been used more extensively and for more years in European countries for winter maintenance than in the United States. The degree of distress in the PCC pavements of European facilities ranged from mild to severe in terms of surface crack- ing, repair, and rehabilitation efforts needed. Limited exist- ing laboratory studies indicated that alkali-metal-salt-based deicers could cause or accelerate alkali-silica reaction (ASR) distress in the surface of PCC pavement by increasing the pH of concrete pore solution. To prevent or mitigate the effects of PDPs on concrete pavement, the first and most important countermeasure is to follow best possible practices in concrete mix design and con- struction. ASR has been conventionally controlled by limit- ing alkali content in cement and selecting aggregates of good quality. Furthermore, efforts have been made to mitigate ASR by adding various supplementary cementitious materials or chemical admixtures such as lithium compounds. • Impact of Pavement Deicing Products on Asphalt Pavement In addition to the effects of PDPs on PCC pavement, their effects on asphalt pavement are also of increasing concern. A laboratory study found that the use of PDPs (sodium chloride, KAc, and sodium formate, as well as urea) was damaging to both aggregates and asphalt mixes. Concurrent to the use of acetate and formate-based deicers in the 1990s, asphalt pave- ment in Europe saw an increase in pavement durability prob- lems. At some Nordic airports, these problems emerged as degradation and disintegration of asphalt pavement, softening of asphalt binders, and stripping of asphalt mixes occurring together with loose aggregates on the runways. Such problems were not identified before the airports changed from urea to KAc- and KF-based deicers. According to laboratory and field investigations conducted under a joint research program—the JÄPÄ Finnish De-icing Project—the damaging mechanism of asphalt pavement by modern PDPs appeared to be a combi- nation of chemical reactions, emulsification, and distillation, as well as the generation of additional stress inside the asphalt mix. To prevent or mitigate the effects of PDPs on asphalt pave- ment, the first and most important countermeasure is to fol- low best possible practices in asphalt mix design and con- struction. Responses to the ACRP survey for this project pointed toward adoption of some of these preventive mea- sures: one European airport reduced asphalt pavement air void to 3.0%; another European airport indicated using polymer-modified binder; and one U.S. airport changed the asphalt binder to PG 76-32, citing current FAA specifications. Nonetheless, the JÄPÄ Project research showed that the resis- tance of asphalt pavement to deicers can be improved only partially by mix design. According to the laboratory results, binders with high viscosity or polymer-modified binders were recommended when formate/acetate-based deicers were to be used. High-quality (sound) aggregates could also improve the durability of asphalt pavements in the presence of such deicers, and so did the aggregates with higher pH. The void contents of the asphalt mixes were recommended to be kept low enough to limit deicer solution in pores. • Impact of Pavement Deicing Products on Other Airfield Infrastructure Other airfield infrastructure that comes into contact with PDPs includes ground support equipment, signage, and lighting and other electrical systems. Empirical evidence exists indicating that PDPs are responsible for damaging such infrastructure. However, no academic-peer-reviewed scientific information could be found to corroborate these empirical observations. • Looking to the Future When it comes to airfield pavement deicing and anti-icing there are no simple solutions to the competing, and sometimes conflicting, objectives of aircraft safety, environmental reg- ulatory compliance, materials compatibility, and operational implementation viability. The ACRP survey distributed for this project provided a forum to describe knowledge gaps and research needs, as well

37 PDP Impact What Is Known What Is Unknown Ongoing Research 2. Existing research in the laboratory has demonstrated the catalytic effects of potassium, sodium, and calcium on carbon oxidation. 2. More research is needed to better understand relationships between brake design, AO treatment, and PDP contamination as factors in catalytic oxidation. The SAE A-5A Brake Manufacturers Working Group is in the process of developing an oxidation test method for AO-treated coupons. 2. More research is needed to better understand the interactions among the aircraft component design, the CICs used, and the contamination of PDPs in the processes of metallic corrosion. 3. There is still a lack of academic research data from controlled field investigation regarding the aircraft metallic corrosion by PDPs. 2. Further research is needed to better understand the interactions between ADAFs and PDPs, as new ADAFs and PDPs are continually introduced to the market. 2. Limited existing laboratory studies indicated that alkali-metal-salt-based deicers could cause or accelerate ASR distress in the surface of PCC pavement, by increasing the pH of concrete pore solution. 2. There is a need to unravel the specific mechanism by which alkali metal salts cause or promote ASR. 2. Significantly accelerated deterioration of asphalt pavements was found in laboratories when exposed to acetate/formate-based deicers. 2. There is a need to unravel the specific mechanisms by which alkali metal salts and other PDPs (e.g., bio-based deicers) deteriorate asphalt pavement. RITA Project: Mitigation of Moisture and Deicer Effects on Asphalt Thermal Cracking through Polymer Modification, conducted by Montana State University. A Cd-corrosion test protocol has been in development in the SAE G-12 Cd Corrosion Working Group since 2003 and is currently being refined for inclusion to AMS 1431 and 1435. The SAE G-12 Carbon Oxidation Working Group is in the process of refining a carbon compatibility test protocol. The SAE G-12 Fluid Residues Working Group is leading research efforts in this field. IPRF Project 05-7: Performance of Concrete in the Presence of Airfield Pavement Deicers and Identification of Induced Distress Mechanisms and IPRF Project 06-5: Role of Dirty Aggregates in the Performance of Concrete Exposed to Airfield Pavement Deicer, both conducted by Clemson University. I nteraction with aircraft deicing and anti-icing p roducts 1. Recent laboratory data appear to corroborate anecdotal reports of increased rates of thickener residues in environments where alkali-metal-salt-based PDPs have been used. 1. The contamination effects of ADAFs by runway deicing fluids have been well-observed but not yet thoroughly quantified. AAPTP Project 05-03: Effect of Deicing Chemicals on HMA Airfield Pavements, conducted by the Advanced Asphalt Technologies. Catalytic oxidation o f carbon–carbon composite brakes 1. A growing body of field evidence from airline operators suggests that the use of KAc and KF on airfield pavements leads to catalytic oxidation of C/C composite brake components. 1. There is still a need to establish a comprehensive PDP catalytic oxidation test protocol. Corrosion o f aircraft alloys (with a focus on cadmium p lating) 1.Until recently, the principal evidence connecting alkali-metal-salt-based PDPs with Cd-plating corrosion has been a trend of increased reports of the latter occurring simultaneously with the introduction of the former. 2. Very little research has been conducted to investigate the mechanism of Cd corrosion or Cd-steel corrosion in the presence of alkali metal salts (e.g., KF and KAc), partly owing to the high toxicity associated with Cd and its compounds. 1. There is still a need to establish a comprehensive metallic corrosion test protocol for PDPs. 1. The last decade has seen an increase in the premature deterioration of airfield PCC pavements with the use of alkali- metal-salt-based PDPs. 1. There is a need for research data from controlled field investigation regarding the effects of alkali-metal- Salt-based PDPs on concrete pavement.I mpact of P DPs on concrete p avemen t N/A I mpact of P DPs on asphalt p avemen t 1. Although it was observed in some Nordic airfields that exacerbated asphalt deterioration occurred with applications of alkali-metal-salt-based PDPs, there is thus far little observation reported in U.S. or Canadian airports. 1. No academic-peer-reviewed scientific information could be found to corroborate these empirical observations. 1. There is a need for research data from controlled field investigation regarding the effects of alkali-metal- salt-based PDPs on asphalt pavement. I mpact of P DPs on other airfield infrastructure 1. Empirical evidence exists indicating that PDPs are responsible for damaging other airfield infrastructure (GSE, signage, lighting and other electrical systems). AO = anti-oxidant; CICs = corrosion-inhibiting compounds; ADAFs = aircraft deicing/anti-icing fluids; ASR = alkali-silica reaction; N/A = not available. TABLE 13 SUMMARY OF PDP EFFECTS, KNOWLEDGE GAPS, AND ONGOING RESEARCH

as potential challenges for the future of airfield pavement deicing and anti-icing. Some of the key findings from the sur- vey and the literature review are summarized in Table 13. A dominant theme throughout all the responses provided by airports was the challenge of needing environmentally benign products that are simultaneously safe for aircraft, pavement, and electrical systems. Several respondents indi- cated the need for standards concerning the compatibility of 38 deicers with airfield infrastructure and airframe materials, as well as standards for environmental effects of deicers. There was a strong call for best practices and new test methods with pass/fail criteria, but also skepticism about the actual impacts of PDPs and whether scientific data are truly avail- able to confirm these impacts. One suggestion noted was to keep precise records of PDP use to develop the knowledge base needed to determine if PDPs are damaging aircraft and airfields.

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TRB’s Airport Cooperative Research Program (ACRP) Synthesis 6: Impact of Airport Pavement Deicing Products on Aircraft and Airfield Infrastructure explores how airports chemically treat their airport pavements to mitigate snow and ice, and the chemicals used. The report also examines the effects of pavement deicing products on aircraft and airfield infrastructure, and highlights knowledge gaps in the subject.

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