Revealing the health of your engine through oil analysis and other tools
Major league baseball players who average two hits for every 10 at bats aren't likely to remain long in the major leagues. This batting average of .200 represents a level of hitting performance that is often referred to as the "Mendoza line," after either Minnie or Mario Mendoza. In engine oil analysis there's no equivalent of the Mendoza line. Put simply, there aren't any limits published by any engine manufacturer in regard to how much iron, or copper, or chrome, or other contaminant is too much.
The important factor in using oil analysis is not the raw numbers indicating the quantity of metals in the sample — which are reported in parts per million (ppm) — but the amount of deviation from the engine baseline established by at least three previous samples.
Oil sampling is one engine diagnostic tool and it's the most easily misunderstood. Let's visit the other diagnostic tools before we look at the art and science of oil analysis.
The diagnostic tools
There are six widely accepted tools for gauging an engine's health. The first and most well known of the diagnostic tools is the compression test. Engine manufacturers, especially Teledyne Continental Motors (TCM), have issued multipage service bulletins focused solely on this subject. This subject was previously covered in " Airframe & Powerplant: The Healthy Engine," April 2004 Pilot.
The second tool is oil filter and screen inspections. It's very easy to inspect a screen, but disassembling a filter to check for contaminants takes at least one special tool — a can cutter — and 15 to 20 minutes to do the job properly. If the engine is healthy, then the debris captured in a filter or screen inspection will be carbon flakes. Sometimes there are a very few tiny flakes of metal. If the engine is getting sick, or is just being broken in after an overhaul, then more debris will show up in the filter. Debris found during these inspections is readily observable with the naked eye.
The third tool is oil consumption, or more properly, oil consumption per flight hour. It's important to keep a record of how often and at what tachometer (or Hobbs) time oil is added between oil changes. Modern aircraft engine oil systems are very effective, so when an engine starts using more oil it's usually — but not always — a signal that the engine is tiring. Unlike modern automobile engines, aircraft engines are designed to use oil. Healthy engines use oil. Both TCM and Lycoming publish oil-consumption limits that are based on a formula involving horsepower output per hour of flight. A perfectly healthy Lycoming or Continental four- or six-cylinder engine may use as much as a quart every two flight hours.
Changes in flight patterns will influence oil consumption. An airplane that's been a cross-country cruiser with one takeoff and one landing for every two, three, or four hours of stable flight will use more oil when it is used to teach cousin Jack — or Jill — how to fly. Touch-and-go landings require full-power engine operation, which increases both the combustion and crankcase pressures.
The fourth tool is trend monitoring. Trend monitoring requires collecting data on fuel consumption and oil temperatures and pressures, and downloading engine-monitor data and plotting these parameters in the monitor software program. An owner who understands the operation of an engine monitor, and who takes time to download and chart data such as cylinder head temperatures, exhaust gas temperatures, manifold pressure, rpm, and oil temperatures and pressures is doing a lot to keep track of his engine's health.
Open wide
The fifth tool is the borescope inspection, in which a borescope is used to visually inspect the inside of the engine. Borescope inspections are performed by inserting one end of the bore-scope — which looks like a metal tube that's about twice the diameter of a common pencil — through a spark-plug hole. The end that's inserted into the cylinder has a lens and a light and the other end has a viewing port.
TCM insists that cylinders with compression test numbers that used to be considered too low to be airworthy must now be borescoped — and visible damage present — before cylinder removal is justified. The visible damage usually consists of the burned edge of an exhaust valve or the loss of the cylinder-wall crosshatch pattern, which indicates excessive cylinder bore wear.
The last tool is oil analysis. Some owners believe in it; others think that looking for contaminants that are too small to be seen by the human eye is a crazy idea. I've seen entire engines torn down based on a bad oil-analysis report with no finding of damage. Other engines have destroyed themselves soon after a passing analysis report. A good friend of mine who is very conscious of the theory of oil analysis and conscientious in taking oil samples failed to grasp the significance of elevated chrome readings in a series of analysis reports. Upon cylinder teardown he concluded that the chrome was coming from one exhaust valve stem. As the stem wore, it became so loose in the guide that the valve was no longer held in alignment with the valve seat. The valve overheated and burned. Experiences such as these reinforce the idea that oil-analysis reports are used to augment other more easily interpretable diagnostic findings, not the other way around.
What do Lycoming and TCM say?
TCM initiated its TopCare HealthCheck program to "provide a preventive maintenance checklist to utilize in identifying and correcting factors, which if left unaddressed can lead to reduced cylinder life." In its booklet Tips on Engine Care, oil-analysis trend monitoring is discussed.
Lycoming's general maintenance tips booklet Lycoming Flyer Key Reprints says, "If oil is analyzed, it should be done at each oil change in order to establish a baseline. Analysis is a tool which only [author's emphasis] gives useful information when a dramatic departure from an established norm occurs." The booklet also says, "It is a good tool only if properly used. Like any other tool, it is only one of many things that must be used to determine engine health."
Every engine has its own wear signature, which is determined by variables such as frequency and duration of each flight, the pilot's operating practices, the elevation and weather patterns at the airport where the airplane is located, and the habits of the maintenance personnel.
Here's how it works. Oil-sample labs sell sample kits either through aviation parts supply houses or directly. The kits typically consist of a small plastic jar, a shipping container, an information sheet, and instructions about taking a sample. Oil samples are usually taken during oil changes. The small bottle is filled with oil drained from the engine and tightly capped. The information sheet contains questions such as what type of oil is used, the number of hours since the last change, how many quarts of oil have been added since the last change, and whether the engine has a filter or screen. The blanks are filled in and it and the sample are sent to the lab.
Within a short time the lab sends back the results. Many labs now send the test results attached to an e-mail as a portable document format (PDF) file. When the lab detects what it believes to be an abnormal variation from the engine's norm, labs will suggest a maintenance course of action.
The lab report contains columns listing common wear metals such as aluminum, copper, chrome, nickel, iron, tin, silver, magnesium, and lead. There also will be a column for silicon (dirt). Some more sophisticated labs also may report oil viscosity, water, solids, and pH.
Too many variables
To increase the accuracy of the analysis, owners must alert the lab of any recent engine work such as changing a cylinder, and any change in activity such as a period of inactivity. Since sample reports are in ppm, consistency in sampling methodology is critical.
Oil samples must be taken while the oil is hot. Let the first third of the oil in the sump flow past before inserting the plastic sample jar into the oil stream. This prevents the heavier contaminants, which have settled to the bottom of the sump, from skewing the result.
There is a slight chance of contamination if the oil is drained by opening some type of quick drain. This is especially true if the quick drain has a permanent hose attached. It's also true if the sample is taken from the end of a hose that has been temporarily attached to the quick drain to direct the oil into a bucket outside the engine compartment. Most maintenance shops have a couple of hoses that are used for this purpose. There's no way to get the hoses contaminant free. So some care is required to get a true sample. Blackstone Labs sells a pump for $25 that enables samples to be drawn up an inexpensive, easily replaceable tube that's inserted down the oil dipstick tube. This permits sampling without the added expense of draining the oil sump.
In addition to sampling errors there are other factors that can skew the results. Some oil-filter manufacturers advise lubricating the oil-filter gasket with Dow Corning DC-4 compound to prevent it from sticking, thus easing removal when it's time to change the filter. This compound contains silica. If too much is applied to the gasket the excess may be washed into the oil system.
If the shop or owner adds a makeup quart of oil that's different from the normal brand of oil used, this will have a bearing on the baseline. Mike Busch of the Savvy Aviator reported that he saw a jump in phosphorous levels during one report. He later traced the deviation to the fact that he added a few quarts of Aeroshell 100 Plus oil to his engines instead of the usual Aeroshell 100 because his local jobber had run out of 100; 100 Plus contains the anti-wear additive triphenyl phosphate (TPP).
The point of the last paragraph is to emphasize the variety of factors that can sway oil-sample readings. Fortunately sampling labs are well experienced with these factors.
The labs
Reputable oil-sample laboratories have all developed engine-specific average-wear metal profiles. If the sample turns up values that fall outside the profile, the lab will notify the owner, or the maintenance facility that sent in the sample. If the values significantly vary from the norm, the lab will suggest that further diagnostic work be done on the engine. This usually means the removal of the valve covers for an inspection of the valves, rocker arm bushings, and valve guides for wear; a borescope inspection of the cylinders; or an oil-filter or screen inspection. If these inspections don't reveal any abnormalities, the normal course of action is to either shorten the oil-change intervals or continue sampling at shorter intervals.
Labs maintain running records of each engine and print all the previous sample results with dates and engine hours during sample periods in adjoining columns. These records have some value when an aircraft is sold because the log of reports is evidence that the owner has gone the extra yard to stay on top of the engine's condition. This body of data is especially valuable when the sample dates are evenly spaced, showing that the airplane engine is operated on a regular basis. Regular usage is the most important factor in keeping airplane engines healthy.
So is keeping the engine oil as clean as possible. Dirt that gets into engine oil acts as an abrasive and accelerates wear. Dirt shows up in oil samples as silicon. High silicon readings tell the owner that dirt is finding its way into the engine through a worn inlet air filter, or through a tear in an inlet air duct, or past a poorly sealing carburetor-heat flapper. Maintaining the integrity of the inlet air system is critical for long-term engine health.
Is oil analysis worth the money? A typical sample kit from Wear Check or Blackstone Labs costs between $16 and $20. Regular 50-hour samples over the life of a 1,500-hour TBO (time between overhauls) engine will add $600 to the cost of operation. This is quite an expense for the weekend flier but it's cheap insurance for owners of complex higher-powered engines.
If an owner elects to continue running an engine beyond the manufacturer's recommended TBO, a long history of healthy oil-analysis reports will add confidence to that decision. Oil analysis is a valuable diagnostic tool, especially when the samples are carefully taken and the engine is flown often.
E-mail the author at [email protected].
