In almost every discussion I have had on used oil analysis for aircraft engines, one of several myths come up as to the value — or lack thereof — of the process.
Myth 1 is that a single used oil analysis will tell you everything about your engine and point out any problems or potential problems. Unless you have a very serious problem, a single oil analysis will not tell you much about the condition of your engine. Analysis works well as a trend monitor, but is of limited value as a single diagnostic test.
Myth 2 is that the reported values are absolute values and should be about the same for any engine. Most engines settle in to a certain level of each contaminate throughout their lives.
However, these values can vary if you do something unusual. For instance, if you fly into a dirt strip several times in the summer, that can affect the silicone level in your oil.
I saw one TCM 520 engine that averaged around 100 ppm iron from the start. This engine was inspected early on and at TBO, and no unusual wear was noted. This may have been very unusual, but it makes the point that the absolute value is not as important as the change in the wear level.
Myth 3 is that you should start analysis from the first oil change for a new or overhauled engine. The problem here is that during the break-in process your engine will be making a lot of iron from the new cylinders. This level should go down after 50 or 100 hours, so wait to start your analysis program until then.
Also, if you replace a cylinder mid-time on your engine, the iron and silicone from the gasket sealer can be elevated in your next report.
Myth 4 concerns whether or not wear metal levels are linear with hours. This is kind of, sort of, true but not completely.
This is important for the typical pilot who flies about 100 hours a year and changes oil the recommended three times per year or every four months. Say they fly 45 hours on the spring and summer oil changes, but only 10 hours on the winter oil changes. Are the three oil changes going to have similar wear metal reports?
Probably not. The 10-hour oil change will probably have mostly lower values, but maybe not 4.5 times less. In fact, the iron level could be higher because of rust activity during the idle time. So, you need to compare similar conditions when doing a trend analysis.
Myth 5 is that you can use different labs and get comparable results. There are several different systems to measure wear metals and they do not usually give identical results, so always use the same lab, and take the sample the same way each time.
Myth 6 is that oil analysis is a good way to ensure that the engine on a used airplane you are planning to buy is in good condition. That is a big maybe.
First, you do not have anything to compare the report to. Second, you do not really know how many hours are on the sample. And third, you do not know how the sample was taken. The seller could have just used a thief to pull a sample off the top of the oil level after the engine had been sitting for days and the oil is not mixed up. The report may give you some information, but it could be very limited in its accuracy.
So, after all of that, most of you are probably asking if oil analysis is really worth it? Well, I believe it is if you consider the cost of a new or rebuilt engine, and the safety and peace of mind that it can give you.
But, and this is a very important but, you need to be very particular and methodical about how you take the samples and monitor the results.
I’ve been doing oil analysis for the last 10 years, and it has shown stable low results vs other TCM engines.
However, it did not indicate worn exhaust valve guides, which showed up as 0/80 compression and a huge hiss from the exhaust.
It didn’t indicate broken rings on number 3 cylinder, but a borescope showed scoring, and the compression check had a large amount of air leaking in the crankcase filler opening.
A friend has been doing oil analysis on his piper twin, but never had cut open the oil filters….until an A&P did, and found 2 tp of metal in it…the cam and lifters were badly spalled..!!
So, oil analysis has some use, but other investigations are more critical in determining the state of an engine.
Increasing silicone results from oil analysis pointed me to cracks in my 172 airbox.
I never get to COMPLETELY change the oil, because the drain plug is not always at the lowest point.
Also, residual oil remains in the oil cooler & lines, so there is some carryover results from the previous oil analysis.
Also, darned near impossible to visually check cam lobes and lifters on a Lycoming unless you pull a cylinder first.
Good information, Ben. Thank you
I think the only real value of oil analysis is to tell the operator of bearing problems.
Cylinders have compression tests and borescope tests. Oil won’t show a rotocoil going bad on the exhaust valve. And I’m of the camp that believes rotocoils should be replaced every 500 hours on big-bore Continentals.
Cams and lifters can be inspected directly or by borescope. Not many do this and it SHOULD be done on a periodic basis based on how often the engine runs.
It won’t signal a connecting rod about to punch through the case.
The only time I’ve heard of oil analysis as the leading indicator of a failure is bearing failure. Everything else was a distant secondary indicator, and as you pointed out in your article, unless you are fastidious in your record-keeping and keeping all of the variables accounted for, it can be misleading.
I’ll personally start oil analysis when I want to start monitoring the bottom-end. I’ve got 1500 on my Continentals and will start oil analysis around 2000 hours. Hopefully it will remain stable until I hit 3500 hours, but I won’t start to think about an overhaul until I start seeing elevated ferrous content.
According to this analysis (https://www.machinerylubrication.com/Read/36/oil-analysis-vibes ) vibration is a better predictor. I have an Insight G4 and I hope to find a good methodology to use with it’s vibration sensor. (I’ve discovered that every engine has a unique vibration signature, so like oil analysis, you have to keep a trend.)