As an airplane owner, you have a substantial investment in the engine of your aircraft. In some instances, the value of the engine (or engines) may exceed the value of the rest of the airframe.
Little in the way of maintenance on an aircraft engine could be considered inexpensive, especially when compared to their automotive counterparts. An oil change on an airplane will run all of a hundred bucks or more (if done properly) compared to about twenty bucks at your local automobile drive-thru oil change shop. It is for this reason that you should consider employing every tool and trick available to you to help you anticipate and hopefully preempt a major problem with your powerplant.
One very effective tool available to every aircraft owner is the spectral oil analysis. This process requires that an oil sample is vaporized and resulting emissions spectrally interpreted by a computer to determine the percentage of content of specific elements.
I won’t bore you with all of the techno-jargon but I’ll ask you to accept that this practice is in widespread use throughout commercial aviation, especially in the turbine world. Now while it sounds complicated, it is not. There is, however, a trick to getting useful information from the effort.
First you must understand that oil analysis is nothing more than a sophisticated form of trend monitoring. Many General Aviation engine monitors, like the JPI I have in my twin, come complete with a data port and software that allows me to download trend data as recorded by the analyzer in each of the parameters that it monitors to my computer for long-term trend analysis.
On turboprop engines we typically perform trend monitoring on indicated engine operating parameters such as ITT (inlet turbine temperature), oil temp, oil pressure, percentage of power (N1), fuel flow and prop rpm, at a specific altitude and outside air temperature. These are recorded every five hours of flight time, or once a day.
This data is used in conjunction with S.O.A.P.S. (Standard Oil Analysis Program) as part of a life extension program commonly used on commercial turbine engines.
In the case of oil analysis, you must first establish a baseline: what is normal for your engine?
It is also important that the oil sample itself is taken at the same relative timeframe in the oil change cycle, (I do mine every 25 hours) and I go so far as to warm the engine and drain the oil and take the sample in the same way every time. (I remove the drain plug, allow the oil to drain for five seconds, then take my sample.)
It is also important to use the same company for your analysis over an extended period of time because there are actually two different methods of generating an analysis and they may yield different results. By the third sample, you should have an established baseline for your engine.
A significant change that is reflected in a subsequent report would be cause for further investigation. It is also important to track the amount of oil added between oil changes because it will affect the results.
There is a place on the order form you send in with your sample for this information. It also asks for grade and manufacturer of the oil you use and whether or not you use an oil additive such as AvBlend.
What will oil analysis do for you? In the long run, it can alert you to impending internal failures that would have no other outward signs. For example, high Pb (lead) levels would be indicative of a sticking or broken piston ring. High Al (aluminum) may indicate piston scuffing associated with cold weather starts or piston pin plug wear, commonly found with certain Lycoming installations. Elevated Cu (copper) content may indicate an impending bushing failure, while a higher than normal Ni (nickel) number may indicate a valve guide issue.
Though these are the most common sources of these metals, they are not the only sources. On Continental engines, excessive copper could be the result of a failing starter gear, and high aluminum on some engines might be caused by wrist pin failures, associated with an overheating incident.
On the other hand, it is not uncommon to see high Fe (iron), especially after a long period of inactivity (like winter), but on some engines it could be due to rocker shaft wear. Don’t be surprised to find Si (silicon) levels spiking especially if you have flown off of dirt or grass runways or sandy airports like those found in Florida and the Bahamas.
A spike in one metal or another is not a cause for immediate alarm, but it should start you thinking about looking for other telltale signs and to exercise a little extra caution in your operations until the next cycle. Two unusual reports in a row is usually an automatic call to the engine shop for me until I can determine the cause of the anomaly.
But remember, oil analysis is just a tool. There are other important things that can and should be done. Second to ritualistic oil and filter changes is cutting and inspecting the oil filter after removal. If your aircraft doesn’t have an oil filter, just a finger screen, you might consider an aftermarket filter assembly. The important part is to inspect the paper pleats for metal particles.
If you really want to be ultra-thorough or you are looking for a potential problem, take the inspected paper pleat and put it in a glass jar filled half way with Avgas. Close it up, shake vigorously, and sit it down on top of a common earth magnet for about 20 minutes or so.
The magnet will attract all of the ferrous metal particles, and gravity will work on the non-ferrous stuff. Engine manufacturers have a standard for volume of metal particles allowed, though it is a tough standard to impose because as I understand it, the standard doesn’t specify the amount of time it can accumulate in, but it is implied to be 25 hours. The maximum acceptable amount? A quarter of a teaspoon.
I’ll admit that I don’t do the glass jar thing every time. If I find a large amount of material in the filter or if I’m hunting a specific problem, I will go the extra mile and do the glass jar trick.
I have heard all of the arguments for and against this type of work. Some “experts” claim that that the particle size that oil analysis can detect is less than five microns, while the smallest size particle an oil filter can trap is about 30 microns, thus leaving a large range of particles that can go undetected, thus negating the value of the analysis and inspection.
My personal view of this is simply this; the law of averages suggests that in any failure mode there will be particles of varying sizes from the very small to pieces large enough to read a part number off of, thus creating the opportunity to detect the impending failure, one way or another.
It is a little like playing the lottery. Here in Florida, the state lottery slogan is “you gotta be in it to win it.” Well, if you plan to catch an impending engine failure, you need to be looking for it in the first place.
Since a typical oil analysis costs about 15 bucks, it sounds like cheap insurance to me.
Michael Leighton is a 3,600-hour CFII/MEI/ATP and holds a type rating in CE-500 series business jets. He is a current and active flight instructor and a former FAA Accident Prevention Counselor. He operates a Part 135 on demand Air Carrier Company in South Florida. Send questions or comments to editor@www.piperflyer.com.
