**1. Introduction**

Lubrication is critical in any application where moving parts are involved. Aircraft involve large numbers of different types of moving parts, many of which are associated with the propul‐ sion system. The objectives of lubrication are primarily to improve energy efficiency by reducing friction and to improve component lifetime by reducing wear of the moving parts. An added benefit of an efficient lubrication system is a reduction in the engine operating temperature because of the heat dissipation through the lubricant system [1].

Turbine engine lubrication is an extremely demanding application for lubricants for several important reasons, as follows:

**•** Reliability due to the consequences of failure. Aircraft applications are the only applications where it is not possible to stop immediately and investigate a lubricant failure indication.

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The combination of these requirements eliminates the possibility of an ideal natural lubricant or synthetic lubricant meeting all of the requirements. As speeds and temperatures increased, automotive lubricants initially used in aircraft were replaced by specialty lubricants for aircraft engines. As the propulsion systems changed from conventional engines to turbine engines, lubricants based on mineral oils were inadequate and a new class of lubricants was needed [2].

In the years after World War II, turbine engines were developed and began to dominate both military and commercial aircraft propulsion. Turbine engines require the lubricant to be stable and fluid over a wide range of temperatures. Ground starting temperature may be as low as -54°C and temperature as high as 300°C may be observed at times in the operating engine. Over time, a number of different lubricants were developed with very different chemistries and compatibilities. Many different locations had lubricants that were developed to suit the local conditions and needs. With a number of incompatible fluids, the global nature of aircraft use, and the grave consequences of mistakes, a need for standardization became apparent. To address this problem, a system of lubricant specifications has been developed for turbine engine lubricants.
