Prof. Dr. Önder ALTUNTAŞ
Eskişehir Technical University
Faculty of Aeronautics and Astronautics
AVIATION LUBRICANTS
Aircraft are highly sophisticated machines that operate in demanding environments, requiring specialized lubricants to ensure safe and efficient performance. These lubricants serve critical functions, including reducing wear on engine components, minimizing friction, and effectively dissipating heat. Additionally, they play a crucial role in preventing corrosion and removing harmful contaminants. With 39 years of expertise, the Eskişehir Technical University Faculty Aeronautics and Astronautics has a deep understanding of aircraft oils and their significance. The faculty continues to provide invaluable international support to the industry, advancing knowledge and innovation in this field. Aviation lubricants can be classified into three main categories: engine oils, hydraulic fluids, and greases, each tailored to specific operational needs to enhance performance and reliability.
ENGINE OILS are essential for the smooth and reliable operation of an aircraft engine. They play a crucial role in reducing friction between moving components, thereby minimizing wear and tear. Additionally, they aid in cooling the engine, preventing corrosion, and maintaining cleanliness by effectively removing contaminants. Beyond these primary functions, engine oils serve as an invaluable diagnostic tool. Aircraft engine oils endure some of the most extreme operating conditions, including high temperatures, intense pressures, and rapid temperature fluctuations. As a result, they must possess outstanding thermal stability, oxidation resistance, and precise viscosity properties to ensure optimal performance. There are three main types of engine oil used in aviation: mineral-based oils, dispersant oils, and synthetic oils.
Mineral oils are commonly used during the break-in phase of new or reconditioned engines due to their lower lubricity, which allows for the proper seating of piston rings and controlled wear of cylinder walls. During this initial period, oil consumption may temporarily increase as engine components adapt and settle into their optimal operating conditions. However, it typically stabilizes with continued use. Additionally, mineral oils remain a suitable choice for older piston engines originally designed for non-dispersant oils. They also perform well in low-performance aircraft, where lubrication demands are less rigorous.
Ashless Dispersant (AD) oils are highly recommended for standard use in most aircraft piston engines following the break-in period, thanks to their exceptional ability to maintain engine cleanliness and prevent harmful deposits. Their superior detergency and dispersancy make them particularly beneficial for high-performance engines, which generate greater heat and operate under increased stress. Additionally, AD oils play a critical role in turbocharged engines, effectively preventing carbon buildup around the turbocharger shaft, a common issue at high temperatures and speeds.
Synthetic oils are the preferred choice for modern piston engines, particularly in high-performance aircraft, due to their superior viscosity control and oxidation resistance, which enhance both engine efficiency and longevity. Additionally, aircraft operating in extreme conditions, such as fluctuating temperatures or high altitudes, benefit significantly from the advanced thermal stability and protective properties of synthetic oils.
HYDRAULIC FLUIDS serve as the power transmission medium in aircraft hydraulic systems, playing a crucial role in controlling flight surfaces, landing gear, and brakes. To ensure smooth and reliable operation, these fluids must possess excellent lubrication, viscosity stability, and anti-wear properties while remaining compatible with the various seals and materials used in hydraulic systems. Aircraft utilize different types of hydraulic fluids, each with unique advantages and limitations: Mineral-based hydraulic fluids are the most commonly used, offering good performance and wide material compatibility. However, they have a limited operating temperature range and are prone to oxidation over time. Synthetic hydraulic fluids, such as phosphate esters, provide enhanced fire resistance and high-temperature stability, making them ideal for demanding conditions. However, they tend to be more expensive and require special handling procedures to maintain optimal performance.
GREASES are semi-solid lubricants designed for applications where oil cannot be effectively retained, such as bearings, gears, and actuators in aircraft. These specialized greases must endure high temperatures, intense pressures, and extreme environmental conditions while delivering long-lasting lubrication and protection against wear and corrosion. The selection of grease depends on specific operational requirements and environmental factors. Lithium-based greases are the most commonly used, offering versatility and reliable performance across a wide range of applications. For high-temperature conditions, synthetic greases formulated with complex soap thickeners or polyurea compounds are preferred.
STANDARDS
Aircraft oils must adhere to rigorous specifications and standards to ensure optimal quality, performance, and compatibility with aircraft systems. These standards are established by key organizations such as the Society of Automotive Engineers (SAE), the American Society for Testing and Materials (ASTM), and the United States Department of Defense (MIL-SPEC).
The SAE classification system categorizes engine oils based on viscosity, which is their resistance to flow. Viscosity is a critical factor influencing an oil’s ability to lubricate, cool, and seal engine components efficiently. The SAE viscosity rating system utilizes a numerical code to represent an oil’s performance at both low and high temperatures. For example, a 15W-50 oil has a viscosity rating of 15 at low temperatures and 50 at high temperatures, ensuring optimal flow characteristics across a broad temperature range.
The ASTM (American Society for Testing and Materials) establishes comprehensive standards for various aircraft oils, including engine oils, hydraulic fluids, and greases. These standards define critical properties such as viscosity, flash point, pour point, oxidation stability, and anti-wear performance, ensuring that aircraft oils meet essential performance criteria for their designated applications.
Meanwhile, MIL-SPEC (Military Specifications) are developed by the United States Department of Defense to meet the rigorous demands of military aircraft and equipment. These standards are typically more stringent than SAE and ASTM specifications, reflecting the extreme operating conditions military aircraft encounter. MIL-SPEC oils are identified by a MIL-PRF number, which outlines the precise performance requirements they must fulfill to ensure reliability under intense operational stresses.
MAINTENANCE AND SERVICE LIFE
Proper maintenance and monitoring are essential to ensure the optimum performance and longevity of aircraft oils. Regular oil changes, filter replacements, and oil analysis help maintain the health of aircraft engines and hydraulic systems. Oil changes are conducted at scheduled intervals based on operating hours or calendar time, with the frequency depending on factors such as the type of oil used, operating conditions, and manufacturer recommendations. During an oil change, the old oil is drained and replaced with fresh oil, and the oil filter is also replaced to ensure contaminants do not damage engine components. Oil filters should be changed at every oil replacement or more frequently if oil analysis indicates high contamination levels.
Oil analysis plays a crucial role in monitoring the condition of aircraft oils and detecting potential issues before they result in costly repairs or failures. Samples are periodically sent to a laboratory for testing, where key properties such as viscosity, wear metal content, and contamination levels are measured. This analysis helps identify wear patterns, detect contamination, and estimate remaining oil life, allowing for more informed maintenance decisions. While replacing oils at the planned maintenance intervals is vital for flight safety and reliability, an equally important consideration is what happens to these oils afterward.
WASTE AVIATION LUBRICANT MANAGEMENT: DISPOSAL METHODS, REGULATIONS, AND ENVIRONMENTAL CONSIDERATIONS
The management of waste aviation lubricants involves its collection and disposal as a hazardous waste due to potential contaminants, but there is a growing shift toward recycling and resource recovery through various methods. This process is strictly regulated by national and local environmental laws, with the U.S. Environmental Protection Agency (EPA) establishing specific guidelines for handling used oil, including storage, transportation, processing, and disposal to mitigate environmental pollution. Aircraft operators and maintenance facilities must ensure compliance with state and local regulations, which can be even more stringent than federal guidelines. Additionally, international regulations, such as those set by the International Civil Aviation Organization (ICAO), address hazardous material management on aircraft.
Improper disposal of waste aviation lubricant poses serious environmental risks, including soil and water contamination, which can impact drinking water sources and aquatic ecosystems. Furthermore, burning used oil releases harmful pollutants into the air, posing significant health hazards. To minimize environmental impact, it is essential to implement sustainable waste management practices that align with circular economy principles, emphasizing resource recovery and waste reduction while ensuring safe disposal and recycling methods.
HARNESSING CIRCULAR ECONOMY STRATEGIES FOR WASTE AVIATION LUBRICANTS
The user-side cycle of circular economy encompasses Reuse, Redistribution, Regeneration, Remanufacturing, Recycling, and, importantly, Biological Transformation. The key principle is to assess and repurpose waste before it reaches the recycling stage. Biotransformation, positioned within this cycle, is particularly comprehensive as it integrates more advanced and innovative applications.
Reuse. Reusing used aviation lubricant, even after basic filtration, poses significant challenges due to the stringent performance standards required for aircraft engine lubricants. These oils must maintain precise viscosity and lubrication properties under extreme conditions to ensure engine safety and efficiency. However, used oil is often degraded by thermal and oxidative processes and contains contaminants that hinder its ability to meet these high standards. While filtered used aircraft oil can be repurposed for less critical aviation applications—such as lubricating non-essential components or ground support equipment—extensive testing is necessary to ensure it meets the required specifications. Additionally, residual contaminants can compromise sensitive systems, making direct reuse in primary aircraft engine applications impractical without further processing.
Redistribution. Establishing an effective system for redistributing used aviation oil is crucial for the successful implementation of a broader circular economy strategy. This requires a well-developed infrastructure and streamlined logistics to facilitate the collection of used oil from various sources, including airports, maintenance facilities, and individual aircraft owners. Efficient collection involves the use of appropriate containers, clearly labeled to indicate “used oil,” and stored safely away from heat sources to prevent degradation. Depending on the assessment method employed, sorting processes may be necessary to classify the oil by type or contamination level. To ensure compliance and safety, dedicated carriers with EPA identification numbers should handle transportation, with meticulous record-keeping of quantities, transfer dates, and carrier details. Developing robust collection networks and, where feasible, centralized collection points is essential to maintaining a stable and reliable supply of raw materials for remanufacturing, refinement, or recycling efforts.
Regeneration. Regenerating waste aircraft lubricant involves various technologies designed to remove contaminants and restore some of its original properties, enabling its continued use in less critical applications. Common techniques such as filtration, sedimentation, and demineralization help eliminate different types of impurities. In-situ regeneration can prolong the oil’s functional life, though it may not fully restore its original quality. More advanced methods, including centrifugal separation and vacuum dehydration, further remove water and fine particles, enhancing the oil’s suitability for specific industrial uses and low-demand lubrication tasks. However, achieving the level of purity and performance necessary for reuse in aircraft engines typically requires more intensive treatment, such as re-refining processes.
Remanufacturing (Rerefining). Rerefining is a well-established industrial process designed to remove contaminants and depleted additives from used oil, restoring it into high-quality base oils suitable for producing new lubricants. This method involves multiple stages, starting with pretreatment to eliminate water and solids, followed by vacuum distillation to separate oil fractions. Further refining techniques such as hydrogenation and solvent extraction help remove impurities like sulfur, nitrogen, and heavy metals, ensuring superior purity. Rerefined base oils can match or even exceed the quality of virgin oils derived from crude oil. While traditionally applied to used motor oil, this technology holds potential for waste aircraft oil—provided it meets strict industry standards—offering valuable base stocks for aviation lubricants. Beyond its technical benefits, rerefining significantly conserves natural resources, consumes less energy than conventional crude oil refining, and contributes to environmental sustainability.
Recycling. Even if waste aviation lubricants are not re-refined into high-quality lubricants, it can still be repurposed through recycling for various industrial and practical applications. Filtered or minimally processed used oil can serve as industrial burner fuel, providing heat for manufacturing processes. It can also be incorporated into bitumen-based products, such as asphalt for road construction, or utilized as a feedstock in oil refineries to produce gasoline and coke. Additionally, waste aviation lubricants can be repurposed for less critical lubrication needs, such as maintaining industrial machinery or household equipment. These recycling methods reduce waste disposal, maximize energy recovery, and support resource efficiency, contributing to a more sustainable and circular approach to oil management.
BioConversion. Bioconversion is an innovative approach to waste oil management that leverages biological processes to break down contaminants and promote sustainability. This method primarily relies on microorganisms such as bacteria and fungi to biodegrade harmful hydrocarbon molecules, transforming them into less toxic substances and aiding in environmental remediation. Beyond biodegradation, there is growing interest in converting waste oils—including those from aircraft—into Sustainable Aviation Fuels (SAF) through advanced biorefining techniques. Hydrogenated Esters and Fatty Acids (HEFA) processing enables the transformation of oils, fats, and greases into jet fuel that meets aviation standards. Additionally, researchers are exploring the potential of wet waste conversion, which could incorporate elements from degraded aircraft oil into SAF production. In general, bioconversion presents a promising solution for the aviation industry, offering both effective waste management and a pathway toward more sustainable fuel alternatives.
FUTURE TRENDS
The valorization of waste aviation lubricant through circular economy approaches involves a range of technological and industrial processes designed to enhance its usability. Advanced refining techniques, such as filtration and separation, remove contaminants, while vacuum distillation enables fraction separation. Further improvements in oil quality are achieved through hydrogenation, which eliminates impurities, and solvent extraction, which removes unwanted compounds. To convert waste oils into Sustainable Aviation Fuels (SAF), emerging technologies such as Alcohol-to-Jet (ATJ) and Catalytic Hydrothermolysis Jet (CHJ) are employed, alongside established methods like Hydrotreated Esters and Fatty Acids (HEFA). Additionally, microbial degradation plays a role in breaking down hydrocarbons within waste oil, offering a biological route for processing. Although re-refining is a well-established practice, adapting it for aviation oil remains necessary to ensure compliance with stringent industry standards. While the HEFA process is commercially viable for SAF production, other biocycle methods are still under development. The selection of an optimal technology depends on factors such as the desired end product, the quality and volume of available waste oil, and economic considerations that impact feasibility and scalability.
Implementing circular economy strategies for waste aviation lubricant yields substantial economic and environmental advantages. These approaches help lower disposal costs for airlines and maintenance facilities while generating potential revenue through re-refining and Sustainable Aviation Fuel (SAF) production. Additionally, they offer cost savings compared to conventional petroleum-based products. From an environmental perspective, these strategies significantly reduce energy consumption and greenhouse gas emissions associated with oil refining. By minimizing the lifecycle carbon footprint of jet fuel, they contribute to the decarbonization of the aviation industry. Furthermore, they aid in conserving natural resources and mitigating pollution caused by improper oil disposal. Despite their benefits, certain challenges remain. High initial investments in infrastructure and technology, along with the costs of collecting and processing waste oil, can be significant barriers. While these methods are more sustainable, their environmental impact still requires careful management. To accelerate adoption, robust regulatory frameworks and financial incentives are crucial for setting industry standards and encouraging investment in recycled products.
Within this framework, Eskişehir Technical University’s Sustainable Aviation and Fuel Research Group remains dedicated to advancing the sector and shaping the future through a robust infrastructure that supports cutting-edge research on innovative SAF fuels and comprehensive engine performance testing.
