While it may not have the sex appeal of politics or religion, oil analysis will spark some lively debate if you put together a group of pilots or mechanics. Some people shrug it off as an expensive exercise in futility that tells little about the inner secrets of engine wear. Others swear to their engines health if they get good sample results and get ready to shoot it if they dont.
In fact, the true value of analysis is somewhere in the middle. It cant tell you how long your engine is going to live and its far from a sure-fire way to predict catastrophic failure. On the other hand, experienced eyes can spot wear trends that can catch minor problems before they turn into big ones. Whether thats worth the cost of routine analysis is up to the person writing the checks.
If youve ever burned Christmas wrapping paper in a fireplace or worked with powdered metals over a Bunsen burner, you know that metals put off colored light when they are burned. This is particularly apparent with a metal like copper that, when introduced to a hot flame, will turn the flame blue. Virtually all metals produce this effect, though the colors may not be visible to the naked eye.
However, a spectrometer can measure the light given off by burning a sample and detect the unique light signature that each kind of metal has. When several metals are present in a sample, a computer can identify each of the metals being burned and tell how much of each is present. This procedure is called spectroanalysis. Because of whats happening inside the engine, spectroanalysis takes on an important role.
The typical aviation engine is made of various metals, with each metal used for a purpose that best suits its characteristics. For instance, light aircraft engines generally contain pistons, cylinder heads, and a crankcase made of aluminum; cylinder sleeves, drive gears for the magnetos and accessories, valves and valve springs made of steel; lead/copper bearings around the crankshaft and rod ends; and other metals in other applications.
Wear is an inevitable byproduct of engine operation in spite of any oil or additive around. As the metals wear, the oil flushes the wear particles away and off to the filter. The filter removes only large particles, on the order of 5 to 25 microns, leaving the much-smaller wear particles suspended in the oil during operation. Although the smallest bits of metal pass through the filter, they do eventually fall out of the oil and collect in the oil pan if the oil isnt changed first.
If all conditions were to remain exactly the same, wear would remain constant. In a theoretically perfect world, the crankshaft would follow exactly the same path on each rotation, the cylinder walls would have exactly the same amount of pressure exerted on them by the piston rings on each pass, the mag gears would stay in exact alignment with the same amount of pressure being applied to the teeth through each rotation. Unfortunately, engines dont work that way.
Crankshaft bearings wear a little on each rotation, gradually increasing bearing clearances through millions of cycles. Pilots abuse their engines by ramming in the throttle and holding 1,500 RPM immediately on startup. This pounds the rod and main bearings and scrapes the poorly lubricated cylinder walls. Some engines get operated in arctic cold and tropic heat while others sit for months between operations. All of these things change the rate at which wear occurs within the engine.
Getting a handle on that wear is the whole point behind oil analysis. New oil will begin to collect metal particles immediately and these particles will continue to build up in the oil until the oil gets changed. If no extraordinary events occur between oil changes, an oil sample taken at 15 hours operation will have roughly half the metal particles of a sample taken at 30 hours operation.
The measurement used in oil analysis is a ratio of the particular metal per given amount of sample oil, usually expressed in parts per million. For reading an oil analysis report, the important thing is not so much the method of measurement but that this method remain the same. Your results should always be recorded in PPM.
Oil Analysis Program
Oil analysis is like a relationship – the only thing you get from a one-time deal is more questions than answers. Oil analysis rarely identifies any engine problems with a one-shot effort, principally because engines are operated so differently and in under such varied conditions it is impossible to say what a good reading or bad reading really is. To get any use out of an oil analysis program, you need to analyze at least three samples to determine a trend. The trend is the tool you use to determine engine health.
Monitoring the trends of the metal content is something easily done by the average owner/pilot. Simply put, you watch for a steady or downward trend. If the trends are climbing a small amount in certain areas, you should be able to come up with a decent answer by remembering an event that would cause it. For instance, an engine overhaul will initially result in high numbers as the engine breaks in, but these numbers should significantly drop with each oil change until they eventually stabilize. A prop governor or the installation of a new alternator may see initial wearing-in of gears and other components. Winter operations will usually see increased wear rates somewhat proportional to the level of cold youre dealing with. With patience, proper operation and frequent flying, the numbers generally will improve.
Choosing a Lab
Oil analysis labs are available in larger communities across the country. Most of these labs are geared to industrial and fleet automotive applications and can be located by calling heavy equipment dealers in your area. While the process is the same whether theyre analyzing airplane oil or truck oil, accuracy may be in doubt and youll likely be dealing with someone who knows almost nothing about the peculiarities of aircraft engines.
The accuracy may be questionable because different applications have different tolerances. A breakdown in a backhoe isnt as critical as a turbo Bonanza. Periodic calibration is necessary to determine if the machine is putting out accurate numbers, yet some shops calibrate infrequently or run too many samples between calibrations.
Calibrating the test equipment is done by running a sample with a known amount of metal in it through the analyzer and seeing if the machine comes up with the correct numbers. Typically, aircraft-oriented operations will calibrate their test equipment often. The lab I use calibrates daily and runs less than 20 samples a day. Automotive labs may do a daily calibration but run a hundred samples before re-testing. If you elect to use a local lab, you should establish this important point beforehand.
Another important service provided by many labs is a written report of what may be wrong with your engine should a trace metal show out of limits. This is one area where the quality of the lab truly becomes apparent.
Automotive labs providing this service to airplane owners may suggest several things not applicable to aircraft engines or may flag metal levels that may be quite explainable given the season or operating conditions. Put simply, what happens in an industrial engine and what happens in an aircraft engine just arent the same.
A few labs around the country specialize in aircraft engine oil analysis. Probably the best known is Engine Oil Analysis of Tulsa, Okla. The owner, Howard Fenton, has been in the business of sampling aircraft oil for longer than most of us have been alive, much less flying. For $15, Fentons company will provide sample kits, burn the samples, and provide an assessment of your engines health that is as good as anyone in the business. Moreover, hell track your analysis results for you and let you know if something needs your attention. It sure beats standing in your mechanics hangar staring at a set of numbers and wondering what may be going wrong.
If you decide to perform our own oil analysis, its important to accomplish several steps to get successful results.
Sometime before your next oil change, contact the oil analysis lab youre interested in working with and determine if they fit your requirements. Remember that inaccurate results will do you no more good than no analysis at all. All labs should provide you with a sampling container and give you some idea how to perform the sampling. If you select a distant lab, make sure you give yourself sufficient time prior to your oil change for the materials to get to you.
Once you receive your sampling materials, be sure you keep the sampling container clean prior to its use. Any dirt or foreign matter introduced into the container will show up on the analysis, making it inaccurate. This means you shouldnt throw it into a dirty toolbox or toss it under the seat in the pickup. If it comes in a protective cover, keep the cover on until youre ready to use it.
If you have an A&P change your oil, let him or her know ahead of time that you want a sample taken and make sure the sample is taken properly. If youre doing your own oil changes, proceed as follows:
First, warm up the engine oil, preferably by flying the aircraft. Since wear particles do settle out of oil over time, this will allow the oil to circulate and mix, giving a more uniform distribution of wear material in the sample.
As soon as practical, begin draining the oil into a container but dont collect a sample right away. Drain plugs/valves are located at the lowest part of the oil system and can accumulate lots of goop thatll come out early in the oil drain. Remember, youre only interested in what is suspended in the engine oil. Waiting a few moments also clears out any accumulated dirt in the oil drain tube (if used) that might also contaminate the sample.
Because petroleum products have been linked to health conditions ranging from rashes to cancer, be sure to wear rubber gloves and eye protection. When approximately half of the oil is drained, uncap the sample container and place it into the flow of draining oil until it is almost full. Carefully cap the bottle before wiping it or your fingers off. This will keep dirt on the rag or your hand from getting into the sample.
After completing the oil change, follow the labs instructions for submitting the sample. If you plan to track the sampling in your logbook, be sure to leave room in the log entry for the results when they get back to you.
Some Final Thoughts
Oil analysis can go a long way toward giving you greater confidence in your engine. Since it depends on metals being worn away, it cant be considered a catchall engine health procedure. You still have to change and inspect filters or inspect oil screens. It likely wont catch hidden flaws in a crankshaft or rod that may suddenly decide to fail.
What analysis will do is show you when a piston ring has broken and started scraping its way inside a cylinder to a failure a few hours down the road. Itll let you know when youve got an induction system leak thats allowing dirt into your engine, decreasing its life by hundreds of hours. Itll even show you if your flying habits are improving, your engine is being flown often enough, and tell tales on others who might be flying your plane.
And no price can be hung on the feeling you have when youve launched into solid IMC after reviewing your latest analysis results.
-by Clint Lowe
Clint Lowe is an A&P mechanic who works on light planes and F-16s, He also owns and flies a 172 and a Cessna 206 jump plane/air freight hauler.