Few lubricants are asked to do so much – or endure so rigorous an approval process
This article is based on a webinar originally presented by STLE Education featuring Edward Barnes of ExxonMobil. Barnes is an active member of STLE and obtained his Certified Lubrication Specialist™ certification in 1999. This content has been edited for length. Read the full article in Tribology & Lubrication Technology’s July 2018 edition.
Lubricants used in the aviation world are subject to significantly different performance demands compared to other industries. There are several themes making this industry unique. Lubricant technology development is a function of engine and airframe design changes. As aircraft equipment evolves, so do the lubricants. Turbine oils are exposed to much wider temperature ranges, from -73°C to 200°C. Also, the restrictive and regimented regulatory environment for aviation is like nothing else.
Commercial aviation lubricant-approval process
The aviation industry is highly regulated. Aviation turbine lubricant specifications contain requirements that are quite regimented compared to the other industries lubricant businesses serve. There are many requirements in the specifications. They contain compositional limitations, indicating exactly which chemistries can be used, and specifying that all additives must be ashless for aviation turbine oils. There is a long set of minimum physical and chemical properties to meet, which stipulate minimum performance requirements and thermal stability, tribological and deposition properties.
They also provide specific instructions on how to qualify a product and how to make changes to approved materials if a substitute material is needed. For example, if an additive supplier goes out of business, it specifies how to use the same additive from another supplier and the need to obtain permission from regulatory authorities to do so. If a significant formulation change is made, it specifies the need to fully requalify the fluid, requiring many years of testing.
Specifications also provide instructions on how to supply products to the US government, requiring the authorized package types and labeling specifications. It provides instructions on how manufacturers are expected to certify the quality of their approved materials, vetting and investigating its own facilities and suppliers.
The commercial aviation lubrication industry is extremely conservative and very careful.
Qualifying a new product is now a decade-to-decades-long process. The starting point involves new product development, establishing product performance goals, deciding performance objectives to meet, and performing research to decide base oils and a formulation. Once the formula is decided, qualification occurs with laboratory testing in-house, bench testing in rigs and with regulatory agencies. Eventually, qualification is performed against specifications (e.g., MIL-PRF-23699F or SAE AS5780B).
Airframe and component manufacturers must then approve the oil for their hardware. Ground testing and/or on-wing service evaluations take place with the builders and manufacturers of engines, components and anything the jet oil will touch for what they require for approval. Their requirements vary. Some are happy with extensive ground testing in actual engines, but some require service evaluations on wings in actual aircraft. These could require 5,000 hours of experience in multiple engines, which could include the full removal and disassembly process, inspection of oil-wetted hardware as well as oil and filter analysis. Maintenance manuals also must be updated.
All data, documents and reports are submitted to regulatory authorities, either the Federal Aviation Administration or the European Aviation Safety Administration. They endorse the results and conclusions of the ground or on-wing testing, allowing the OEMs to issue final approval.
Final approvals often come in the form of a document known as a Service Bulletin. This is an official communication to commercial airlines that an oil is approved for an application. Getting through all the requirements for all of the engines and related hardware, done in parallel, takes 10-15 years. It is an enormous process to get a new oil approved in this industry.
New turbine oils must be backward compatible with existing lubricants since the preference in the commercial aviation industry is for broad application. Managing multiple turbine oils for multiple engines becomes a significant challenge for airlines, so the ability to use a single lubricant in as much of the hardware as possible is greatly desired, making universal approval highly desirable.
General trends in engine temperatures in terms of gas path continue to increase, which may translate into still higher lubricant temperatures. While alternative chemistries are being assessed, the high heat capacity of esters is one reason why they continue to be used over alternate chemistries.
The materials in engines themselves also are changing. There are new alloys used in bearings, gears and accessory gearboxes, so new additive platforms are used. There’s also an increased use of non-metallic components. Oils must have improved compatibility with seals, paint and various composite materials. Additives are expected to produce less volatile antioxidants and better antiwear along with higher loading of degradation products while protecting elastomer seals over long periods.
The military sector is performing research on enhanced esters – ester-based oils that provide higher load carrying. There also are several exotic synthetic chemistries under consideration that have promise for turbine engine applications.
New oil technologies in commercial aviation follow the military experience where they have more latitude to take risks. The commercial aviation lubrication industry is extremely conservative and careful, which is why it can take 15 years for new products to get approved. It’s all about safety.
Aviation hydraulic fluids
Aviation hydraulic fluids have different sets of requirements. For mineral oil-based aviation hydraulic fluids, the military specification MIL-PRF-5606 is in effect. Products known as “red oil” that meet this specification are used in the landing gear of modern commercial aircraft as “strut fluids” and in older military aircraft.
Specifications for newer polyalphaolefin (PAO)-based fluids are given in MIL-PRF-83282 and are typical products found in most military aircraft. Another specification, MIL-PRF-87257, exists for lower viscosity fluids that are more flammable than 83282 fluids. Both are less flammable and more stable than MIL-PRF-5606.
Flammability is the most important reason totally different flame-resistant hydraulic fluids are used in modern commercial aircraft. Over the past 60 years, as large commercial jets became more common, the phosphate ester chemistry evolved and is now what is used in all large commercial jets. Its use was prompted by higher aircraft speeds during landing with higher brake temperatures and more concerns about fires.
Hydraulic fluid used to actuate brakes and landing gear is pressurized from 3,000-5,000 psi. If it were to leak near an ignition source, like a hot brake, the resulting fog of fluid could ignite, hence the need for fire resistance.
Phosphate ester fluids use trialkyl phosphates that have excellent low-temperature performance and will function at temperatures as low as -60°C, with pour points around -80°C. They offer many benefits over other types of hydraulic fluids, but the main reason they are used is for their fire-resistance properties. They are self-extinguishing fluids with high auto-ignition temperatures and zero flame propagation. Flames will not spread with these fluids (see Figure 1).
Figure 1 Comparison of commercial aviation hydraulic fluids.
These products are qualified by commercial airline manufacturers since no military specification exists. They must meet airframe OEM testing against various specifications and specialized OEM tests such as hot manifold and high-pressure spray. Boeing BMS 3-11 is an example of an OEM specification.
Phosphate ester hydraulic fluids evolved similarly to turbine oils over time. Types I, II and III are no longer used. Type V fluids used today typically have longer fluid lives, and some have lower density and less weight, which is important in aviation. They also can manage the stresses of the more demanding 5,000 psi hydraulic systems on the Boeing 787, Airbus A380 and A350 aircraft (see Figure 2).
Figure 2. Hydraulic fluid evolution.
Grease is used in many airframe applications. It lubricates drive screws in hydraulically powered actuators that move flight controls and landing gear, track-ways for flaps, ailerons, leading-edge slats, rudder and elevator linkages.
Low-temperature performance is critical for aviation. At 30,000-40,000 feet in altitude, the air temperatures can go down -40°C to -70°C. Airframe grease has to function in application temperatures from -73°C to 121°C.
Airframe grease products must qualify against both military and OEM specifications to be sold. Military specs are MIL-PRF-23827 and MIL-PRF-81322, and an example of an OEM spec is the Boeing BMS 3-33.
Typically, PAO and ester-based oils with a lithium complex thickener are used in most aviation greases. There are some clay or synthetic clay thickeners used as well. They all have additives, and some specialized aviation greases use solid additives such as molybdenum disulfide.
Most aviation greases have consistency ratings between NGLI 1 and 2, with most around 1.5 due to low temperatures. Generally low-base-oil viscosities are needed for extremely low temperatures in airframe greases.
Wheel bearing greases are used in sealed, tapered rolling element bearings on aircraft. There are two sets of bearings to handle axial loads in both directions. Since they’re sealed, there is no regreasing while in service, but typically wheels are removed and overhauled on an accelerated schedule in commercial aircraft. Because of tire wear, the wheels are removed every 100-200 landings. While the tires are off being inspected, refinished, retreaded or reused, the bearings also are removed, cleaned and inspected. Those that are deemed airworthy are repacked with grease and put back into service. The grease is completely removed and replaced each time there is maintenance. This can be fairly often, every two to six months depending on the usage of the aircraft. Short haul carriers may make numerous landings per day, decreasing the service life.
Wheel bearing grease is exposed to conditions different from airframe grease. Wheel bearings experience higher temperatures, ranging from -54°C to 200°C, and are subject to higher loads compared to airframe applications. Base oil viscosity is higher in wheel bearing grease than airframe grease.
Wheel bearing grease products also are qualified by both military (MIL-PRF-81322) and wheel bearing and airframe OEM testing and specifications. A new specification (SAE AMS 3058) is currently in development.
Reprinted with permission from the July 2018 issue of TLT, the official monthly magazine of the Society of Tribologists and Lubrication Engineers, an international not-for-profit technical society headquartered in Park Ridge, Illinois, www.stle.org.