More Than Just an Oil Change

There is an oil-change happening in the aviation industry, and it’s leading to better engine health and longer maintenance cycles. Air BP Lubricants’ Ron Yungk explains what maintenance professionals need to know as more fleets convert from standard to high performance capability (HPC) class lubricants.

Over the past 15 years, while the number of civil jet engines in operation has increased by 22 percent overall, the number using high performance capability (HPC) class oils has increased by a massive 150 percent.

That relative proportion seems set to increase significantly for the remainder of the decade, as next generation engines emerge with increasingly high temperatures. This trend is demonstrated in Figure 1 (on page 20), which shows GE’s new engine launches and their respective exhaust gas temperature (EGT) data.

This trend is even prompting some OEMs to require exclusive use of HPC oils in these engines. For example, both of Rolls-Royce’s latest engine designs, the Trent 1000 and Trent XWB, are only certified to use HPC oils. This is expected to extend to further engine types in the Trent family either requiring HPC class oils or strongly recommending HPC class oil use.

What this trend means for us in maintenance terms is that it is more important than ever to understand the differences between HPC and standard performance capability (SPC) class oils, the characteristics of both, and the economic and technical implications of converting a fleet to HPC lubricants.

What is an HPC oil?

The core requirement specification AS5780 has become the new industry standard, particularly for lubricants used in civil aircraft engines. Under the auspices of the SAE, it was developed jointly by engine manufacturers, military specification authorities and lubricant manufacturers and first issued in late 2000. This aerospace standard combines requirements from the MIL-PRF-23699 specification with specific requirements from the major aviation OEMs. The AS5780 standard specifies two classes of turbine engine oil – SPC and HPC.

How do HPC and SPC differ?

In principle, the chemical building blocks for these oils are inherently similar and, as such, the limits for physical and chemical properties such as viscosity, acid value (TAN), flash point and pour point are the same for both classes.

Differentiation begins with the improved oxidative stability for HPC oils, which translates to reliable oil life, allowing engines to comfortably reach shop visits while reducing the need for disruptive oil condition monitoring programs or oil changes.

Oil life is becoming an increasingly important consideration in civil aviation as new engine bulk oil temperatures are increasing at the same time oil consumption is decreasing. The AS5780 specification distinguishes between SPC and HPC oil oxidative capability through several oil stability test method requirements. The Def Stan 05-50 (part 61) Method 9 Resistance to Oxidation and Thermal Decomposition is a good example of how HPC oils can demonstrate greatly improved effective oil life. The 250oC requirements for HPC class oils for viscosity increase, acidity increase and volatilization loss are all roughly double that of the SPC requirements.

Another critical differentiator between classes is improved thermal stability, or the ability to resist “coking”. Coking is essentially severe degradation of the lubricant down to solid carbon deposits. The use of standard class (SPC) lubricants in modern large turbofan engines has led to many incidents of coking, with the buildup of these carbon deposits being exacerbated by the lubricant’s intolerance of very high temperature regimes.

Coke deposits can cause the blockage of oil feed pipes leading to starvation of mainline bearings (see Figure 2). Coke can also block scavenge tubes, which leads to flooding of bearing chambers and possible leaks into high temperature areas of the engine which can potentially lead to engine fires. Oil starvation can also lead to poor lubrication of shaft spline connections causing wear and possible disengagement of these important mechanical connections.

The formation of these potentially harmful carbon deposits requires airlines to regularly inspect, clean or replace critical lubrication system components, significantly adding to the line maintenance workload and risk to aircraft downtime.

To ensure HPC oils provide improved coking resistance, AS5780 requires HPC oils to be able to perform in two test rigs for twice the duration of SPC oils while producing half the amount of coke deposits. These rig tests are the U.S. Navy ERDCO Bearing Deposition test (FED STD 791 Method 3410) and the Hot Liquid Process Simulator (HLPS) SAE ARP5996 coking propensity test. Figure 3 shows the performance differentiation in the HLPS test.

To meet the HPC performance requirement, oils need to combine a thermally robust ester base stock with an optimized blend of performance additives. When designing lubricants, manufacturers will always look to exceed, rather than meet, the specification requirement to provide OEMs and airlines with confidence of extra thermal and oxidative protection.

By offering improved high temperature performance, today’s HPC lubricants are augmenting the on-wing life of the engine by reducing scheduled and, more significantly, unscheduled maintenance activity.

According to American Airlines repair/part development engineer, Dan Foust, commenting on the airline’s conversion to BP’s HPC lubricant 2197, the maintenance implications can be significant.

“Since introduction of BPTO 2197 oil into AA’s CF6-80C2 Boeing and Airbus fleets ... we have experience far less coking buildup in our turbofan engines,” Foust says.

“It has eliminated two field trips per year to replace excessively coked oil tubes. This has translated into better system reliability and reduced maintenance costs.”

Making the decision to convert

The decision to change to a higher performing oil involves both technical and economic considerations, and can sometimes instigate a divergence between engineering and procurement decision-makers. Often, the cheapest AS5780-approved oil is procured to secure immediate financial benefit, but at the risk of increasing scheduled and nonscheduled maintenance action and thus increased cost in the long-term.

The cost of fleet engine oils represents a comparatively small proportion of an airline’s operating costs, and can account for less than 0.01 percent of the annual operating cost for a twin-engine, single aisle, standard body commercial aircraft. In contrast, engine and aircraft maintenance represents a significant percentage of an airline’s annual operating costs. The implication of this is that, to obtain noteworthy cost savings, commercial airlines need to use oil that will support long-term engine performance and reduce maintenance costs both directly and indirectly.

Airline turbine oil procurement practices will differ depending on the airlines themselves as well as the financial climate that may exist at a particular time. In turn, the relevant drivers used by the key decision-makers may well change. Should turbine oil be purchased on a simple tender basis to derive the cheapest product or should it be viewed as a critical engine component requiring more strategic considerations? Air BP Lubricants has recognized that these two contrasting “engineering vs. procurement” viewpoints may exist within many commercial airlines and has developed a ‘value comparison tool’ to assist airline customers in their decision-making around oil procurement (see box on page 19 for more information).

Any decision to convert to a new turbine oil is generally made on the basis of technical benefits, and evaluated in terms of the financial implications during the later stages of the decision. The decision to convert to an HPC oil should be strategically influenced by both commercial and technical considerations during the planning phase. By determining the value of HPC oil to both the “operational health” of the fleet and to the business, decision-makers can ensure the long-term procurement goals of the airline whilst achieving improved performance for their fleet.

Making the conversion

Brazilian airline GOL Linhas Aereas Inteligentes SA recently converted from an SPC oil to an HPC oil and, according to maintenance director Alberto Correnti, has experienced improved operations as a result of the switch to BPTO 2197.

“The transition to a high performance capable oil was part of the airline’s strategy to maintain its low-cost position while improving operational performance and safety,” Correnti says.

The Air BP Lubricants Technical Service team provides a comprehensive transition program to make the conversion process as seamless as possible. This experienced team has guided many operators through the conversion process, which is consistent with OEM requirements. A disciplined change management process is required including the preparation of appropriate Engineering Orders and provision of placards identifying the HPC oil to be used. As part of ABPL service, we also offer customers post-conversion support to ensure engineers maximize the benefits of a HPC oil and have access to the BP’s on-line Turbine Oil Academy – an exhaustive knowledge database and modular learning tool.

The future is here

The future for HPC class oils is looking certain as increased thermal regimes within new generation engines continue escalating, while incentives for developing new SPC class oils all but fade away. In evidence of this increasing usage, BP recently announced that our 2197 turbo oil had accumulated more than 250 million hours of on-wing experience and was chosen by Airbus for the first test and public flights of the A350 XWB.

So, more than ever before, it is critical for maintenance teams to consider the oil needs of their airlines with HPC oils top-of-mind, as they become the standard in meeting the demanding lubrication needs of modern engines, now and into the future.

 

Ron Yungk is a chemist with 34 years’ experience in aviation fuels and lubricants. He started his career managing Pratt & Whitney’s Chemistry department. Yungk also chaired the SAE committee on Aviation Propulsion Lubricants.

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