Oil Analysis

With a single glance, even the most inexperienced eye can distinguish between fresh and used engine oil.


With a single glance, even the most inexperienced eye can distinguish between fresh and used engine oil. But examine a sample of used oil carefully in the laboratory, and it will reveal a detailed snapshot of the engine's overall health. Is a bearing wearing down and shedding alloy fragments? Has corrosion penetrated the engine; creating ferrous oxide particles? Has somebody dropped a wire into the crankcase; an intruder which has since been shredded into surface-destroying shards? With the right analysis at facilities such as Aviation Laboratories or Jet-Care International, used lubricating oil will bear witness to these and other signs of engine problems.

"Given that it costs about $40 to test an engine oil sample, oil analysis is a very inexpensive way to find problems before they become serious and costly," says Josh Wagner, Jet-Care International's sales manager for North and South America. "After all, a bearing failure can completely scrap an engine if not detected in time."

Beyond providing immediate engine health snapshots, periodic oil analysis can help mechanics track a given engine's wear and tear over time. Such information can be especially helpful in spotting friction-caused wear soon after it begins, and in identifying potentially catastrophic failures long before they occur. It can also be useful in deciding whether an engine can be maintained based on its TBO schedule, or sent in for an early overhaul to keep it healthy.

One caution: To round out the picture, aircraft mechanics also need to periodically open and inspect used oil filters; and inspect the suction screen for debris as well. By doing so, they'll get a detailed look at what's happening inside the engine, and early warnings that could result in equipment-saving preventative maintenance.

What ends up in the oil

When we speak of dirty engine oil, what we are really saying is that there are suspended particles in this fluid. These particles have accumulated since the last oil change; swept up by the constant flow of oil past the engine's moving parts.

"These particles come from the normal wear-and-tear that an engine undergoes while in use, and from abnormal events that result in fragments being generated inside the engine," says Brian Costello; a field service engineer with Lycoming Engines. Included in these particles are everything from alloys being shed by bearings in normal use, to shavings coming off parts that shouldn't be grinding against each other, but are.

"Then there's silicon particles, a.k.a. dirt," Costello tells AMT. "These are elements that come from outside the engine through improperly sealed air intakes, since silicon is never used in the manufacture of engine parts. Since it is abrasive and accelerates wear, dirt is never a good thing to find."

In general, piston engines are the ones that benefit most from oil analysis, says Rory Hammond, president of Aviation Laboratories. This is because piston engines create significant amounts of extremely fine wear particles in the 0-20 micron size range that are detectable by oil analysis, because of the constant rubbing of cylinders, sleeves, bearings, and other mechanical parts against each other. This extremely fine wear circulates in the oil and is not trapped by filtration. The larger piston engine wear particles trapped in the oil filter must be monitored by oil filter analysis.

Turbine engines, on the other hand, have 10 micron oil filters or smaller, and generate predominantly larger wear particles greater than 10 microns, which are trapped by the oil filter. In regard to turbine engines, says Hammond, it is critical to perform both oil analysis and oil filter analysis, because most of the diagnostic evidence of abnormal turbine engine wear modes can be contained in the oil filter. Larger wear particles are analyzed by Scanning Electron Microscopy Energy Dispersive Spectroscopy, SEM EDS.

The process of oil analysis

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