Why Oil Analysis Is Important

Much is heard these days about the use of oil analysis as a tool for helping to determine engine condition. However, the vast majority of the general aviation public do not understand how this tool is to be used. We will attempt here to set forth a brief summary of the subject.

Oil analysis is not new, but it came late to general aviation as a maintenance tool. The object is to examine oil samples from an engine, and break down the sample in parts per million in order to determine the internal health of the engine. This is based on the fact that all lubricated engine parts wear and deposit a certain amount of metallic particles in the oil. The number of particles per million of each metal determines the wear pattern for the particular engine being analyzed. It is of the utmost importance to understand that the result of the analysis is only pertinent to the engine being analyzed, although accumulation of data on any specific engine series is a basis for establishing standards for that series of engine.

The fact that is important is a sharp rise above normal of the amount of a particular metal in the oil. It is imperative then to build a case history of each engine, wherein a sharp rise in any one metal will indicate abnormal engine wear. The analysis can also tell you whether the oil contains other liquid contaminants such as gasoline or water. Gasoline contamination of the oil can result from blow-by from the combustion chamber caused by poor combustion, bad timing, improper fuel mixture, worn rings and the like. Water contamination is usually restricted to condensed vapor, but this vapor combines with the fuel combustion products to form harmful metal-attacking acids. Basedon this contamination in the oil, the analysis will be able to pinpoint improper mixture, poor maintenance, etc.

Lycoming Service Letter No. L171, entitled “General Aspects of Spectrometric Oil Analysis,” provides a guide for the use of oil analysis in measuring engine health. The information is in general terms since the health of each engine must be determined on its own merits.

Differences in manufacturing processes may cause a variation in analysis results for different engine models. The amount of tin plating, copper plating, nitriding, etc., performed during manufacture has a definite relationship to the oil analysis reports. It is not uncommon, for example, to see what seems to be high copper content early in the life of an engine, only to have this content continually decrease as the engine accumulates time, and then disappear altogether. Poor air filter maintenance, running the aircraft on the ground with carburetor/ alternate air on, and holes in the air intake system are all factors which will allow an engine to ingest dirt and foreign matter. The result of this will show up as high iron (cylinder barrels) and chrome (piston rings) content at the next oil analysis. Neither time nor space permits us here to list all of the variables involved (indeed we do not profess to know them all) but it should be obvious to everyone that a continuing history of each engine is the only criteria by which its health can be determined.

Remember that several samples taken at the regular oil change intervals must be analyzed to determine the normal characteristics of an engine, and also remember that the first few samples on factory fresh engines will read high as new parts are wearing in and conforming to each other.

Excessively heavy wear of internal engine parts will show up as traces in parts per million during analysis long before detrimental flaking or scoring takes place, and almost always before any outward indication of trouble. This initial departure from normal is not usually any reason to tear the engine down. An investigation and timely and appropriate corrective action (replacing the air filter, perhaps) by the operator will usually result in trace elements returning to normal at the next oil change. If long TBOs are to be achieved, it is most important that clean air be provided to the engines.

Basically and briefly, that is the oil analysis story. It is a good tool if properly used. Like any other tool, it is only one of many things that must be used to determine engine health.