The Savvy Aviator #21: Checking The Oil

The oil in your engine lubricates, cleans and cools. If you pay attention, it also provides some of the best tools available for monitoring the health of your powerplant.


Not long ago, I was chatting about piston aircraft engines with a group of aircraft owners when the longtime owner of a Cessna P210 asked me the following question:“What are the relative pros and cons of oil filter inspection vs. laboratory oil analysis? Do you recommend one over the other or do you recommend both?”The question struck me as surprising, particularly coming from an aircraft owner as experienced as I knew this one to be. After decades of inspecting oil filters and sending oil samples to the lab for my own airplanes, I guess I just assumed that most aircraft owners knew all about this stuff. Perhaps not.I explained to this owner that I considered both regular filter inspection and spectrographic oil analysis to be essential. Oil analysis reveals micron-scale wear metals (mostly 5 microns or less) but tells nothing about larger metal flakes since they are trapped in the filter and never make it into the oil sample. Filter inspection reveals macroscopic metal (mostly 20 microns or more) but tells nothing about micron-scale wear.It is not a question of which one is better, since the two look at things that are almost mutually exclusive. Both are essential in order to get a complete picture of what’s going on inside your engine.

Oil Filter Inspection

Oil filter inspection is probably the single most important tool for monitoring the health of a piston aircraft engine. If your engine isn’t equipped with a full-flow filter, it’s worth adding one simply for the diagnostic value. It’ll also pay for itself quickly by doubling your oil change interval from 25 to 50 hours. Oil screens are very coarse and unable to trap a lot of the macroscopic wear particles that we need to know about. (As an old mechanic’s joke puts it, oil screens are great for catching hunks of metal with serial numbers.)The full-flow filter should be changed at every oil change, and should always be cut open and inspected. Tossing an old filter in the trash without cutting it open for inspection is a capital offense. (Whenever I visit a new shop, I try to sneak a peek in the shop’s trashcan; if I see any uncut filters there, the shop is immediately crossed off of my good-guy list.)

Every shop — and every owner who does his own oil changes — needs a good oil-filter-can cutter with a sharp cutting wheel. Cheap cutters and hacksaws can leave shards of the can in the filter medium, which can really confuse the filter inspection. Champion makes a good cutter, albeit pricey. I personally use the oil-filter-can cutter and holder from Sacramento Sky Ranch.After cutting open the filter and cutting the filter medium from its spool with a sharp, serrated knife, take it outdoors and examine it in direct sunlight. If that’s not practical, inspect the medium under the brightest light you can find. Small metallic particles embedded in the filter medium are reflective and will generally glisten when viewed in direct sunlight, but may well be invisible under ordinary indoor lighting.Sometimes it’s difficult to determine whether flakes in a filter are metal or carbon. Here’s an easy way to tell them apart: Place some between your fingertips and rub your fingers while squeezing hard. Carbon flakes will break apart, while metal flakes won’t.A newly overhauled engine or one that has just had one or more cylinders replaced will often have a small amount of fine metal particles in the oil filter; but once the break-in has been completed and the break-in oil replaced, any appreciable amount of metal in the filter should be cause for concern.How much metal is considered “appreciable”? Lycoming has a service bulletin that addresses this question, and states that anything more than a quarter-teaspoon of metal particles, or any single metal chunk larger than a pencil point, warrants grounding the aircraft until the cause is found. (A quarter-teaspoon of metal particles is a lot of metal!) Lycoming goes on to suggest that if something between an eighth- and quarter-teaspoon of metal particles is found in the filter, change the oil and filter, fly for 10 hours, and recheck the filter.I take a different approach with my airplane. Since I’ve been changing the oil and cutting open the oil filters for 18 years now, I have a very good sense of what my filter contents usually look like. In my view, any substantial increase in metal above what has been the norm deserves a closer look.The first step in that “closer look” is to rinse the filter medium in a clean jar or can using clean solvent or mineral spirits to wash the particulate matter out of the filter medium. Then slowly pour the now-dirty solvent through a large, clean coffee filter. This will allow you to examine the particles much more clearly.Next, pass a strong magnet underneath the filter paper to determine whether the metallic particles are ferrous (iron or steel) or non-ferrous (aluminum, chrome, tin, bronze, etc.) A small amount of non-ferrous metal is not unusual; ferrous particles are of greater concern.Non-ferrous metal can often be distinguished by appearance or other simple tests. Bronze particles have a characteristic yellow color. Chrome flakes are shiny, sharp and very hard. Tin is dull and melts at a very low temperature. Aluminum will fizz and dissolve when exposed to dilute sodium hydroxide (lye), including common household drain cleaners like Drano and Red Devil brands.

What To Look For In The Filter

Carbon particles. A certain amount of carbon in the filter is normal, and turbocharged engines generally exhibit more carbon than do normally aspirated ones. An unusually large amount of carbon in the filter suggests that oil is getting excessively hot and coking. This can be caused by several things. One is excessive blow-by past the rings, and is usually accompanied by elevated oil consumption and marginal compression readings in one or more cylinders. Another cause is one or more badly worn exhaust-valve guides, and is usually accompanied by carbon build-up under the cylinder rocker covers, heat-damaged valve springs, and/or valves that move more than a very small amount in a “wobble test.” Yet another cause of carbon occurs in turbocharged engines that are shut down without a reasonable turbocharger cool-down period.Iron or steel. Iron and steel are readily identifiable because they are magnetic. Any significant quantity of iron or steel particles or flakes in the filter is cause for concern. Generally, you should not fly the aircraft until the source has been determined. If the cause is not readily apparent, you may want to consider sending your filter contents to an expert for microscopic examination, which often can pinpoint the source. The most likely sources are cam lobes, tappet (valve lifter) faces, and cylinder walls (if the barrels are steel) or piston rings (if they are chrome), but there are also a lot of other steel parts in the engine (accessory gears and shafts, starter adapters, etc.) that can generate ferrous particles.Aluminum. These are silver-colored, non-magnetic particles that dissolve when exposed to a dilute solution of lye. Small amounts of aluminum are normal in some engines, but significant quantities warrant further investigation. Possible sources of small aluminum particles include fretting crankcase halves (check torque on spine bolts and through bolts), a loose valve guide (check rocker boxes for metal), and piston-pin plugs (check with borescope). Larger aluminum chunks suggest burned pistons, possibly caused by preignition (check with borescope).Chrome. Chrome is shinier than aluminum and much harder, and is often found as flakes rather than particles that feel sharp to the touch. Any amount of chrome in the filter is not normal except possibly during break-in. The most common source is from chrome-plated piston rings abraded by a rough or pitted cylinder, or from chrome-plated cylinder barrels that are developing a problem. (Check the cylinders with a borescope.) Another source is abnormal wear of chrome-plated exhaust-valve stems, particularly if the engine has hardened Nitralloy exhaust-valve guides (introduced in the early 1990s by TCM and subsequently discontinued).Brass/copper/bronze. Identified by distinctive yellow color. In TCM engines, the presence of long bronze slivers often indicates failure of the starter adapter spring. Smaller particles may come from worn bushings, or older aluminum/bronze valve guides.

Dry-Particle Analysis

If you find metal in the filter that prompts concern something might be coming apart inside your engine, it’s often a good idea to package up 50% of the filter contents in a plastic bag and ship it via overnight express to a laboratory for microscopic dry-particle analysis.Both TCM and Lycoming operate metallurgy labs that will perform microscopic dry-particle analysis of filter contents from their engines. Superior Air Parts and Engine Components, Inc., do as well. There are also a number of independent labs that will do this, and that might be a better choice if you think a warranty claim might be involved. One of these is Howard Fenton’s “Second OilPinion” in Tulsa, Okla.An expert can often tell from the size, shape and appearance of the particles or flakes, as seen under the microscope, whether they came from a spalled lifter, a damaged cam, an oil-starved main or rod journal, a defective gear, or a scored cylinder barrel.

Spectrographic Oil Analysis

While oil-filter inspection is usually the best way to determine if something is seriously coming apart inside your engine, spectrographic oil analysis can be thought of as an early warning system capable of giving you advance notice of certain kinds of incipient problems, often long before they reach the safety-critical stage.The key to understanding oil analysis is that it focuses on only the tiniest of wear metal particles that are so small that they can’t be trapped by your oil filter or even seen readily under an optical microscope. We’re talking here about particles that are 5 microns (about 0.0002″) or smaller in diameter. (A micron is one-millionth of a meter.)Unlike the larger particles visible during oil filter inspection (typically 0.001″ or larger in diameter), it is perfectly normal for an engine to continually shed micron-scale particles during normal operation as the unavoidable byproduct of wear. The quantity of such micron-scale particles produced during normal operation varies widely from one engine to another.The purpose of spectrographic oil analysis is to establish a historical trend of wear-particle production for a particular engine, and then to provide warning of any significant departures from that historical norm. Let’s look at a specific example, an IO-520 engine in a Cessna 206 that was placed on an oil analysis program right after major overhaul:

Sample Date
Total Hours
Parts Per Million

At 10 hours, the original break-in oil was changed, and was relatively high in wear metals — particularly aluminum (Al), chromium (Cr) and iron (Fe) — just as you might expect. Some metal particles were also seen in the oil filter, but we know that this is also normal during the initial break-in process.The engine oil was then changed approximately every 50 hours, the filter inspected, and an oil sample sent to the lab for analysis. No significant metal was found in the filter after the initial 10-hour break-in interval, and there were no other signs of engine problems.The oil analysis results also looked perfectly normal during the first year. Wear metal levels came down quickly and stabilized within the first 100 hours, establishing a good baseline for comparison of future oil analysis results.However, the sample taken on 4/30 at 245 hours total time revealed a big increase in iron (Fe) to 92 parts per million (compared to mid-40s during the preceding 150 hours). Aluminum (Al) and chromium (Cr) also increased, although not quite as dramatically.What’s going on here? In this case, the smoking gun can be found in the oil analysis results themselves: specifically the three-fold increase in silicon (Si) from historical levels. When silicon is found in engine oil, it’s usually caused by abrasive, silica-laden dirt getting into the engine. (High silicon can also be caused by silicone sealants or fragments of silicone gaskets getting into the oil, but typically that occurs only after major engine maintenance.)Tipped off by the high silicon readings in the oil analysis, the owner of the 206 alerted his mechanic, who checked and found the inside of the engine’s induction ducts coated with a gritty-feeling substance. The induction air filter appeared to be in good shape, although the mechanic replaced it anyway on general principles.After a bit more investigation, the mechanic discovered a broken spring that prevented the alternate-air door from sealing properly. This had been allowing the engine to breathe unfiltered air, which accounted for the grit in the induction system and the high silicon level in the oil. The abrasive grit, in turn, accounted for the higher-than-usual levels of wear metals. The bad alternate-air door spring was replaced, the airplane was returned to service, and the next oil sample 50 hours later showed wear metal and silicon levels returning to the historical norms.

Interpreting The Reports

This example underscores several very important concepts about spectrographic oil analysis. First, the numbers seen in the report on the 4/30 sample at 245 hours were not inherently remarkable, and might very well have been perfectly normal for some other engine. The only thing that made them noteworthy in this case is that they were significantly higher than in the previous three or four oil samples taken from this engine. That’s why you really can’t tell anything from one or two oil analysis reports; it’s a trend-monitoring tool, and you need to give it time to establish a trend before you can derive useful information from it.Second, the accelerated wear rates caused by dirt ingestion through the faulty alternate air door were still quite modest. Had the engine not been on oil analysis and the air leak been missed at annual inspection (which could easily happen), the engine would undoubtedly have continued to run fine for hundreds of more hours before any major symptoms of the accelerated wear showed up (most likely as bad compression on one or more cylinders). The oil analysis provided an early warning of a problem that would typically have been found much later through other means.This is rather typical of the kind of benefit you can expect from doing regular oil analysis. Finding the problem early undoubtedly saved the 206 owner a lot of money in the long run by possibly sparing him the expense of a mid-TBO top overhaul. It probably didn’t do anything dramatic like save his life, because the dirt getting into his engine and causing accelerated wear was not likely to make his airplane fall out of the sky. But it sure paid for itself many times over.In the absence of oil analysis, do you suppose the broken spring would have been picked up at the next annual inspection? Maybe. Maybe not. Things like this are missed at annual all the time.Occasionally, an oil analysis report will come back showing a huge increase in wear metals, enough to create concern of some sort of impending catastrophic failure. Usually, however, such reports will be accompanied by visible metal in the oil filter that would also provide warning that something serious is going on.Usually, that is, but not always. There are plenty of cases where a major internal engine problem was missed during filter inspection but caught due to oil analysis. Even more common are cases where a major problem never showed up in oil analysis (because it was making only big chunks of metal) but was caught during filter inspection.If you care about the health of your aircraft engine, you really need to do both … religiously.

What The Numbers Mean

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Here’s an oil analysis report on the left engine of my Cessna T310R after an unusually long oil-change interval. (The engine was then 150 hours past TBO — it’s now at 450 past TBO and going strong.) Blackstone Laboratories emails me these reports as PDF documents. (Click for larger version.)

The data shown on oil analysis reports vary somewhat from one lab to another, so you generally should pick one lab and stick with it. These days, I use Jim Stark’s Blackstone Laboratories in Fort Wayne, Ind., and have been very impressed with the service. Jim is an A&P and he and his staff (many of them family members) really know aircraft engines. I particularly like Blackstone’s quick turnaround, easy-to-read report format, and the fact that they routinely send the reports via email (as PDF documents) so I don’t have to wait for the snail-mail.Most of the information contained in the oil analysis reports by Blackstone and other labs shows the results of spectrographic elemental analysis of wear metals and other elements of interest. These include:

Aluminum: High levels of aluminum generally come from abnormal wear on piston skirts, piston pin plugs, and fretting crankcase halves.Chromium: Normally from abnormal wear of chrome-plated piston rings. If chrome-plated cylinders are installed, may also indicate cylinder barrel wear.Copper: Typically from bronze bearing shells and bushings. Occasionally from the core of a deteriorating oil cooler.Iron: Iron is the principal wear element in most piston aircraft engines, since most of the major wear components are made of steel. The iron comes mainly from steel cylinder walls, but can also come from cam lobes, lifter faces, crankshaft and camshaft journals, and gears.Nickel: Used in high-temperature alloys, such as exhaust valve guides. Also found in nickel-based cylinder coatings (e.g., ECi CermiNil and Titan cylinders).Silicon: Usually indicative of abrasive silica from dirt getting into the engine despite the induction air filter. Can also come from silicone sealants and gaskets or from glass beads (usually only after overhaul or major engine maintenance).Tin: Typically from the Babbitt alloy layer used on the main and rod bearings. Bronze parts also contain tin alloyed with copper.

For all these elements (and several others of marginal interest), Blackstone includes not only sample results (measured in parts-per-million), but also what they call “universal averages” and “unit/location averages.” The “universal averages” attempt to show normal values as derived from Blackstone’s entire database of engines of my particular type (TCM TSIO-520), while the “unit/location averages” are historical moving averages for my particular engines. While the universal averages are interesting, there is so much variation from one engine to the next (even if they’re the same make and model) that I tend to pay much more attention to the unit/location averages.In addition to this elemental analysis, Blackstone and some other labs also include data on:

Insolubles: Shows the quantity of insoluble solids found in the oil. High levels of insolubles are usually an indication of excessive blow-by past the piston rings, or alternatively an indication that the oil got too hot and oxidized.Viscosity: Shows how much the rated oil viscosity has deteriorated during the course of the oil-change interval. Very low viscosity can indicate fuel contamination of the oil, while higher-than-normal viscosity can indicate that the oil became overheated.Water: Shows the amount of moisture in the oil. High levels of water generally mean the aircraft isn’t being flown enough, and suggests that the oil should be changed more frequently.

Getting Consistent Numbers

Oil analysis is a trend-monitoring technology where you’re watching for unexplained departures from a historical norm. Therefore, it’s essential to make sure that the numbers you get from one oil sample to the next are comparable.I’ve already mentioned one important element in this regard: Pick one lab and stick with it. Due to differences in equipment and procedures, the results you get from one lab will not necessarily be consistent with those from another lab. If you have to change labs for some reason, wait until the new lab has analyzed three or more of your periodic oil samples before you start drawing any major conclusions from the data.Another important point is to try to take the samples at a consistent time interval, like every 30 or 50 hours. Oil that has been in service longer will naturally have higher wear metals. Realistically, of course, it’s sometimes necessary to go a bit longer or shorter than usual between oil changes — and when that happens, simply keep this in mind when interpreting the analysis results.Although less important, it’s also a good idea to pick one kind of engine oil and stick with it. Changing from one kind of oil to another can sometimes cause odd fluctuations in the oil-analysis results. This can be particularly true when changing from mineral-based oil (like Aeroshell W100 or Phillips 20W-50) to semi-synthetic oil (like Aeroshell 15W-50 or Exxon Elite 20W-50), but can also be true in other situations.For example, I recently got back a report from Blackstone that showed a big jump in phosphorus levels from what I’d been used to seeing. That concerned me for a moment (what the heck does phosphorus mean?) until I realized what was going on: Turns out that my local oil jobber had run out of Aeroshell W100 oil, so I wound up using a few quarts of Aeroshell W100 Plus as make-up oil. The main difference between W100 and W100 Plus is that the latter has an antiwear additive called triphenyl phosphate (TPP), and that’s what was causing the higher phosphorus numbers on my oil report.Finally — and perhaps most important — you should be very consistent about how you take your oil samples that you send to the lab. For best results, samples should be taken immediately after coming in from a flight, when the oil is still hot and particulates have not had a chance to settle out.There are two main methods used for obtaining oil samples: catching a sample as the oil is being drained from the engine, or aspirating a sample through the oil filler using a long tube and vacuum bulb. Both methods work fine, but pick one and stick with it. I personally prefer catching a sample while I’m draining the oil.With both methods, it’s important to avoid getting sludge in your oil sample. If you use the catch-while-draining method (as I do), try to catch your sample about midway through the oil draining process, avoiding the first or last oil that comes out of the pan. (This is much the same as what the doctor tells you to do when collecting a urinalysis sample.) If you use the aspiration method, make sure the end of the pickup tube does not touch the bottom of the oil pan while you’re drawing your sample.Oil-filter inspection and spectrographic oil analysis are both important tools for monitoring the health of your powerplant. Don’t leave home without them.See you next month.

Want to read more from Mike Busch? Check out the rest of his Savvy Aviator columns.