Oil Analysis: What It Can and Can't Do
Oil analysis can help predict some types of problems, but it can't help with all of them. We clear up some misunderstandings and present some realistic expectations for using it as one of many tools that can keep your engine running well.
In the mid-1980s, my father-in-law Harry became the proud owner of his first diesel-powered pickup truck. A Michigan camping enthusiast, he was certain his new engine would not only out-perform any engine he'd ever owned, but it would last him forever.
Harry decided to use oil analysis to ensure he'd know about any potential problems in his V-8 diesel engine before they actually occurred. Harry's engine wore heavily at the cylinder area from the start, but it did so consistently for his regular reports at 3,000-mile oil changes, which he performed religiously. This went on for a decade, and there never occurred any one event during that time that pointed to a significant mechanical problem developing.
After about 150,000 miles, Harry's diesel wore out to the point that he didn't have enough power to pull the truck up a set of ramps to change the oil. He was irate, not only at the companies that made the truck and engine, but at oil analysis. He felt oil analysis had failed him. But did it?
There is a great deal of misunderstanding, even among the more mechanically astute aircraft community, about what information oil analysis can and cannot provide about the mechanical condition of an engine.
Most operators of piston-type aircraft engines do use oil analysis, since the health of their engines is vital to arriving at the intended destination and meeting the earth gently. As the old saw goes, takeoffs are optional, landings are mandatory.
To understand what oil analysis can and can't do, it's necessary to understand some of the mechanics behind the process. The basic tool in oil analysis is the spectrometer, which reads the wear metals (the metallic particles in the sub-visible size range) present in a sample of oil. Larger particles -- as they approach the visible size range -- do not make it into the spectrometer's plasma, and therefore are not read. If you have a mechanical part in your engine that is beginning to give off the larger, visible particles, they will collect at the oil filter or screen. These accumulations can and should be monitored by you or your mechanic, or they can be microscopically examined to determine where they're coming from and if they relate to normal or abnormal events.
Oil and oil filter element analysis work hand-in-hand to give a complete picture of your engine's metal-making tendencies. Many aircraft owners will submit the first oil sample from their engine after a prodigious amount of visible metal has been found in the oil filter media. But that's using the system backward. If your engine is leaving visible metal particles in the oil, it is possible (though somewhat rare) that the parts have already degenerated past the point of leaving wear metals that the spectrometer can "see."
All engines wear, and they all eventually wear out. Other variables aside, any given engine will wear an average amount in an average oil use interval. If a mechanical problem develops, wear will progress from average, to poor, to abnormal. The progression might occur over the course of several oil fills, or it might occur more rapidly than that. As wear progresses though the abnormal stage, large damaged particles will separate from the engine parts and begin collecting at the oil filter or screen.
There are some types of mechanical difficulties in aircraft engines that are easily seen in oil analysis. There are other problems that aren't. Poorly wearing friction bearings, for instance, will generate high lead in the oil from babbitt. But the lead is masked by lead left in the oil by aviation gas additive (100LL blow-by). There are other metals in babbitt that are easy to identify, but without lead to tie them together in a recognizable pattern, it is difficult to say whether the bearings are actually the source of the metals.
Mechanical parts may have internal flaws from manufacture that affect the part's integrity. As the part ages, heat cycles and use cause a weakening of the part and eventually, sudden failure. As the problem progresses, these parts will flex and cause an increase in wear metals in the oil, but the onset of failure is normally too rapid to make any type of oil or filter analysis useful.
Some types of problems can be detected through oil analysis in the early stage, and through filter analysis or observation at a later stage, as the problem progresses. Valve train wear -- including wear at the camshaft -- is usually discernable in oil analysis. As the poorly wearing valve train deteriorates, it tends to leave visible metals at the filter or screen. Poor wear at the interface of bushings and steel shafts can be also be detected early with oil analysis. This problem also normally progresses to visible metallic particles.
Some problems don't leave visible particles in the filter or screen, and oil analysis is a vital tool for finding these mechanical difficulties. Much of the high copper we see in oil, for example, is from brass oxides washing into the hot oil from new oil coolers. Through oil analysis, we can usually see the difference between brass and bronze wear. Poor wear at bronze valve guides in some types of engines is easy to detect, and when those metal particles become dislodged from their heat-shrink fit, they leave elements of bronze and high aluminum in the used oil.
Detecting Cylinder Wear
A common complaint about oil analysis is that it can't tell you what the problem is. But that's not what it's supposed to do. If the problem can be detected with a spectrometer, then oil analysis does tell you when parts are not wearing normally. It's up to the mechanic or owner to determine why that is.
Poor cylinder wear, for example, is a very common problem in aircraft engines that can be detected early through oil analysis. Oil analysis can tell you that the cylinders are wearing poorly. It's up to the owner or mechanic to determine why that may be. In our experience, cylinders wear poorly for one of five reasons: the type of cylinder being used, quality control at the point of manufacture, incorrect setting of the ring-gap tolerance, the way the engine is flown, or detonation from metallic deposits.
If your engine is equipped with factory steel cylinders, and they are set up properly and operating in the normal heat range, the wear metals generated by the piston/ring/cylinder interfaces will read at average levels for that type of engine. If you have another type of replacement cylinders in place, you can probably expect above-average chrome and/or nickel to appear in your oil analysis report. If we know the metallurgical make-up of your cylinders from the onset, we can tell you if your cylinder wear properties are normal. If we don't know your cylinder types, we are guessing. When assessing cylinder wear, it helps to see the wear trends from multiple oil samples.
Cylinder assemblies -- even of the same brand name -- tend to vary widely. Even if the cylinders are being made consistently well, the repair shop mechanic who installs them still needs to set the ring end-gaps correctly for normal wear properties. A mechanic who takes a cylinder set out of the box and installs it on your engine without qualifying tolerances is not doing you a favor.
The qualification of ring end-gaps is a vital measure for normal cylinder assembly wear. The tolerance must be set perfectly because at operating temperature, the rings expand to close the gaps. If the end-gap is too wide, the gaps won't close completely, and blow-by and/or excess oil consumption will plague the engine for the life of the cylinder. If the gaps are set too tight, the rings will bind in the cylinders at operating temperature, causing excess wear at rings, cylinder walls and in the ring land area of the pistons. The engine will run okay under either circumstance, but if the ring end-gaps are not properly set, the cylinders will require premature replacement.
If you were fortunate enough to get mechanically normal cylinders at your last top overhaul and the mechanic set them up properly, there is still a possibility that cylinder assembly wear will be excessive if one or more of the cylinder head temperatures is too high. Poor fuel distribution -- most common in carbureted engines but possible with any type fuel system -- can cause a temperature imbalance resulting in one or more overly hot cylinder heads. So can problems with engine baffles and cowls that don't direct enough air to the cylinder's cooling fins. All the problems mentioned here are solvable with a little detective work. If none of these problems is the cause of excess cylinder wear, then the way you use the engine is probably the culprit. Perfectly normal engines used for glider or banner towing, skydiving, or crop dusting operations wear more heavily at the cylinders than do identical engines used for normal flying duties, due to rapid alternating temperature extremes at cylinder heads.
An air-cooled aircraft engine leaving seriously high amounts of cylinder assembly wear metals in the oil can cause an associated problem to develop: possible premature engine failure due to pre-ignition from the presence of metallic deposits in the combustion area. For the same reason you can't use automotive-type engine oils in aircraft engines -- the additives are not ashless and will leave deposits in cylinders -- high levels of wear metals will cause the same problem. Metals don't burn. When we see an engine generating excess wear from cylinders, we suggest shorter oil use intervals. The shorter interval won't improve wear characteristics, but will limit metallic (and deposit-forming) accumulations in the oil.
Oil analysis can help predict some types of problems, and can't help with others. If you use oil analysis and your engine is wearing at an excessive rate, there is a reason for it. A poorly wearing engine will wear out more rapidly than an average-wearing one of the same type. Whether oil analysis helps you see the problem coming depends largely on the information you provide with your sample, the frequency with which you sample your oil, and the additional work you and your mechanic do to find -- and fix -- a problem before it leads to a failure. Oil analysis is just one tool that can, in conjunction with filter analysis and vigilance on your and your mechanic's part, keep your engine running well.