What you need to know about oil analysis to protect your airplane

In February 2023, my friend Paul McBride wrote a very good column on oil analysis. He outlines some of the limitations of oil analysis and some of the advantages.

Since that time there have been a few more questions raised, so I thought I would address some of them.

First, oil analysis can be a very effective tool if used properly.

Many years ago, I worked with Weldon Garrelts at the former Institute of Aviation at the University of Illinois. They ran a test program on oil analysis in high-usage aircraft.

When an aircraft reached the time for an oil change, it was run up and then towed into the shop. They immediately removed the cowl and drained the oil. This can be a problem because most aircraft are built with the oil drain designed to inflict first degree burns on any mechanic who dares go near it. I guess it is a defense mechanism.

The mechanics at the university bent several tin troughs to place under the oil drain of the O-360 Lycoming engines so that with a quick drain they were able to get to the oil with a minimum of pain.

Once the oil started flowing, they would let about a quart or two run down the trough and then take an oil sample. By carefully tracking the results for each aircraft, they could note trends or changes in each and investigate and look for any problem.

Their system worked well because of three factors:

1. They used the same sampling procedure each time.
2. They sampled each aircraft at the same number of hours for each oil change.
3. They monitored the results of the oil analysis and looked for changes.

A lot of pilots choose to drain the oil after their aircraft has cooled down. The problem with this is that once the engine is shut down, moisture and particles in the oil start to settle.

Additionally, if you sample the first oil out of the aircraft, the results will be much different than later in the oil stream and probably not representative of what is going on in your engine. Also, there will probably be a significant amount of particles left in the engine.

Timing is Everything

The second problem can be the most difficult for a private pilot and that is time on the oil change. A typical private aircraft is flown about 100 hours a year. Engine manufacturers recommend oil changes every 50 hours with an oil filter or every four months.

This would mean an oil change at 50 hours in the summer, a 30 to 40 hour oil change in the spring or fall and then maybe 10 hours on the winter oil change if you are lucky.

If your engine is “making metal” it will do so in a linear scale. That means that all three of your oil changes will have a different level of contaminates. This makes trend analysis more difficult.

Speaking of trend analysis, a lot of people think that one oil analysis will tell you everything you need to know about your aircraft engine, plus the tire pressure and battery charge.

One analysis may indicate a gross problem, but it is doubtful.

Most engines will seek their own level of wear metals through their life cycles. That is why there are no maximum levels of wear metals to say this is good and this is bad.

While I was at Shell, I monitored one test on a Continental TSIO-520 engine that showed the level of iron in the oil was very high.

After a thorough checkout by Continental personnel, we continued the test but we could find no problem.

At TBO the iron was still high, but at teardown everything measured OK.

Buyer Beware

Oh, and if you think an oil analysis on a used plane before purchase will ensure everything is OK, think again.

If a plane starts making metal, some sellers will change the oil at, say, 40 hours and discover the problem. They then put in fresh oil but do not log it in.

The buyer checks the oil at what they think is a 50-hour drain and all is OK.

So, buyer beware.