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.

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.

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