To support the air transport industry's holistic effort to reduce maintenance costs and safety risks, research is needed to identify airport best practices that could reduce the influence of airfield pavement deicing on accelerated structural deterioration of aircraft carbon brakes and provide guidance to airports on selection and application of these best practices.
Since the 1990's, airports worldwide have adopted airfield pavement deicers which contain organic alkali metal (potassium, K and sodium, Na) based freezing point depressants. The use of alkali-metal deicers is wide spread because these deicers offer significantly improved deicing and anti-icing performance, operational advantages (e.g., reduced slipperiness, reduced odor, improved spraying characteristics) and significant environmental benefits compared to previously used glycol and urea-based products. However, unlike glycols and urea, the alkali deicers can promote accelerated deterioration of carbon-carbon aircraft brakes through catalytic oxidation.
To address the effects of CBCO, aircraft are subjected to more down time and more frequent inspections. Higher brake maintenance/replacement costs are incurred because brakes are replaced prematurely before they reach the normal wear limit. Using the ICAO Safety Management Manual terminology, the International Air Transport Association (IATA) CBCO project team rates severity of consequence of CBCO as "B" (Hazardous) and the probability of consequence as "2" (Improbable), the latter primarily because aircraft inspection and maintenance mitigation practices tend to detect and address deterioration early. The cost implications of CBCO are significant. A presentation given during the 2013 EASA Annual Safety Conference cited the cost of replacing all carbon heatsinks on a 777-300 aircraft at approximately $700,000. A representative of Honeywell Aerospace estimated that the in-service life of these components decreases by roughly 20 – 30 percent with routine exposure to alkali metal pavement deicers. A KLM representative indicated that they experience 15 - 20% early removals of brake disks, and 10-20 delays and 3–5 cancellations annually related to CBCO.
ACRP Synthesis 6: Impact of Airport Pavement Deicing Products on Aircraft and Airfield Infrastructure (2008) summarized the CBCO materials compatibility issues associated with airfield deicers containing K and Na freezing point depressants at the time of publication. Additional insights and understanding of the phenomenon have been gained since then through ongoing research efforts. For example, potassium has been found to have a greater impact on CBCO than sodium, and relationships have been described between the rate of oxidation and temperature and between the loss of carbon mass and reductions in structural strength of carbon brake disks. SAE International AIR5567A Test Method for Catalytic Carbon Brake Disk Oxidation has facilitated these insights into the effect of deicing chemicals on CBCO.
Aircraft, carbon brake and deicer manufacturers have active research and development programs involving the CBCO issue and are collaborating on strategies to reduce the risks and costs of CBCO. These efforts have resulted in better carbon anti-oxidant coatings on carbon brake disks, improved brake designs, improved aircraft maintenance mitigation practices, and better operational practices to reduce brake temperatures. Alternative pavement deicer formulations, typically blends of potassium acetate and another freezing point depressant, have been developed and found to reduce impacts on CBCO, but with higher Biochemical Oxygen Demand (BOD) content, presenting an unresolved environmental compliance dilemma.
IATA's CBCO Working Group is facilitating these collaborations in close coordination with the SAE G-12RDF Catalytic Oxidation of Carbon Brakes Working Group. Commercial airports participate in the industry discussions, providing perspective on environmental sensitivities, especially the higher BOD content associated with alternative deicer formulations. Outreach and discussions with environmental regulatory agencies in Europe and North America have led to better understanding of the specific environmental concerns regarding airfield pavement deicer formulations.
One of the conclusions of these efforts is that further significant progress in CBCO risk reduction strategies will require airport practices that reduce exposure of aircraft carbon brakes to deicing chemicals that promote CBCO. Some examples include adoption of more aircraft- and environmentally-friendly runway deicers, improved application of runway deicers and innovative approaches for handling and possibly treating runway runoff. It is recognized that the applicability of such practices will depend on facility-specific conditions, including environmental permit compliance requirements, and the benefits are likely to be incremental, but they have the potential to significantly contribute to the overall effectiveness of other industry measures. The IATA CBCO working group noted that the absence of information and guidance on such airport practices represents a significant gap in the holistic approach being taken by the industry. The recommended research will address this gap through development of an ACRP guidebook on relevant airport best practices to reduce exposure of aircraft brakes to deicing chemicals that promote CBCO.
Ongoing industry efforts to address carbon brake catalytic oxidation (CBCO) have focused on advances in aircraft brake design, operational mitigation practices, and innovative pavement deicer formulations. Industry working groups leading this effort have noted a lack of information and guidance on best practices that airports might consider to reduce the contributions of airfield deicing operations to CBCO. The objective of the recommended research is to address this gap in the body of knowledge regarding methods for reducing the potential liabilities/risks associated with CBCO.
The recommended research and guidance development should be conducted with explicit recognition of certain key principles/assumptions. First, the safety of wintertime aircraft operations and compliance with environmental regulations must both be maintained. Secondly, a wide range of potential best practices should be investigated individually and in combination. Practices to be investigated should include but not be limited to airfield deicer product selection, operational protocols, containment and treatment of pavement deicing runoff, and airfield drainage design.
The recommended research process consists of the following series of tasks.
Assess the potential extent of the problem
Identify, evaluate and summarize information to describe the nature and extent of the CBCO problem in terms that are relevant to aircraft operators and airports. This should include the current airfield pavement deicer market, costs of CBCO to aircraft operators and airports, and the extent to which airports must use alkali metal pavement deicers to achieve environmental compliance.
Identify regulatory and legal issues that must be considered.
Define the constraining regulatory and legal issues that must be considered in assessing the feasibility of best practices at a given airport.
Identify contributing factors
Investigate airfield operational and infrastructure factors that can influence or contribute to CBCO. The following are examples that should be considered:
• Airfield pavement maintenance practices such as deicer product selection and application that drive the exposure of aircraft to K and Na based deicers.
• Airfield drainage design features that may constrain options for pavement deicer selection or feasibility of airfield deicing runoff treatment.
• ATC operational practices and local aircraft operating rules.
Develop best practices
Develop specific best practices to reduce exposure of aircraft carbon brakes to alkali metal-based pavement deicers at an airport while maintaining safety and environmental compliance. Development of these best practices should consider every aspect of airfield deicing at an airport from product selection to environmental compliance, including the following topics:
• Airfield deicing operational best practices, including product selection and optimized application to reduce the sources of chemicals associated with CBCO.
• Snow and ice removal and deicer application technology that reduces amounts of applied deicers that are available to be deposited on aircraft landing gear.
• Treatment of runoff from deiced airfield pavement to reduce discharges of BOD and other deicer constituents to the environment.
• Airfield pavement design best practices to minimize delivery of deicers in runoff to stormwater outfalls and potentially allow a wider range of options in selecting pavement deicer products.
The research findings will be incorporated into a Guidebook of airport best practices to minimize impacts of airfield deicing on CBCO of aircraft brake systems. The guidebook should include a primer on CBCO and its impacts on aircraft safety and operating costs, discussion of the relationship between airfield pavement deicing and environmental regulatory compliance, fact sheets of potentially applicable airport best practices and guidance on their selection by an individual airport.
Estimated cost for the recommended research is $450,000. This estimate is based on the experience of the author in conducting similar research for ACRP and other aviation organizations.
Several ACRP research projects are related to the recommended research. For example, ACRP Report 99: Guidance for Treatment of Airport Stormwater Containing Deicers describes 11 currently used treatment technologies for treating deicing runoff and ACRP Report 14: Deicing Planning Guidelines for Airport Stormwater Management Systems describes five best management practices that reduce the amounts of pollutants generated by airfield pavement deicing. The ongoing 02-87 project is describing the fate and transport of airfield pavement deicers as they move across an airport, which will inform the exploration of innovative airfield drainage design for treatment. Research conducted by Dr. Bill Hunt (North Carolina State University) on removal of stormwater pollutants as a function of airfield drainage features is also likely to provide useful information. The experience of airports that have implemented innovative solutions to airfield deicing runoff management will also be a valuable source of information. Ongoing related research by manufacturers, aircraft operators and aviation industry organizations provide a context and foundation for the recommended research.
Examples of related research include:
• ACRP Synthesis 6: Impact of Airport Pavement Deicing Products on Aircraft and Airfield Infrastructure (2008)
• ACRP Report 14: Second Edition: Deicing Planning Guidelines for Airport Stormwater Management Systems (in press)
• ACRP Report 99: Guidance for Treatment of Airport Stormwater Containing Deicers (2014)
• ACRP Report 174: Green Stormwater Infrastructure (2017)
• ACRP Report 169: Clean Water Act Requirements for Airports
• ACRP 02-87 (ongoing) Determining Airfield Pavement Deicer and Anti-icer Contributions to Airport Stormwater
• Assessing the Scope for Implementing Onsite Treatment Technologies such as Aerated Wetlands to treat De-Icer Contaminated Storm Runoff at Airports. A. Freeman, et al., R&D Technical Report NE/N012593, Natural Environment Research Council, 2016
• International Air Transport Association (IATA) Engineering and Maintenance Group Carbon Brake Catalytic Oxidation Working Group
• SAE G-12RDP Catalytic Oxidation of Carbon Brakes Working Group
• International Civil Aviation Organization (ICAO) Safety Management Manual
• Ongoing unpublished research by aircraft, brake and airfield deicer manufacturers
• Unpublished experience by airports that have implemented airfield deicing runoff treatment projects (e.g., IAD airfield biological treatment units, GRR stormwater and deicing natural treatment system, and aerated wetlands systems being used at European airports)
None of the related research provides specific practical guidance to airports on the full range of best practices that an airport might consider in reducing the exposure of aircraft to pavement deicers that contribute to CBCO. The recommended research would address this deficiency.