An essential part of engine monitoring programs
By Joe Escobar
Routine oil analysis has become quite the standard in our industry. Many of the engine manufacturers recommend regular sampling as part of a preventive maintenance program. The oil manufacturers have also embraced oil analysis as a way to detect abnormal properties in the oil. An effective oil analysis program is usually able to detect breakdown in internal components before an actual incident occurs. It can also detect contaminants present in the oil that may lead to premature wear of components.
Keep it clean
One of the most important steps in an oil analysis program is the actual sampling technique. Whether it is taking an oil sample or removing an oil filter for analysis, everything must be done to ensure that no external contaminants are introduced into the sample. Contamination can cause false readings at the lab, therefore a clean container must be used for every oil sample.
Not only can contaminants be introduced into the oil during sampling, they can be introduced into the engine oil itself during servicing. Care should be taken to ensure no contaminated equipment is used to service oil.
In addition, certain practices can lead to contamination. One reader shared the problem that his maintenance organization encountered. The oil analysis from the engines they serviced were coming back with small traces of iron shavings. The materials were analyzed and determined not to be coming from the engine. Further investigation revealed that the particles were being introduced into the system during servicing. A metal "church key" can opener was being used to open the oil cans, and small shavings were being sheared from the can's surface and the edge of the church key and introduced into the engine through the oil. An oil servicing unit with a filter was purchased, all aircraft had their oil changed, and the problem went away.
Types of oil analysis
Once the owner or operator chooses to perform oil analysis, they need to decide what type of analysis will best suit their needs. There are several ways that oil systems can be analyzed. The most common types of analysis are oil samples, oil filter analysis, and chip detector analysis.
A very common practice of oil system analysis is performing routine oil samples. Usually, the samples are taken within 30 minutes of shutdown. This ensures that any particulate contamination present doesn't get the chance to settle to the bottom of the oil reservoir prior to sampling. The oil is then sent off to a laboratory for analysis.
Most oil samples are tested using spectrum analysis. Jet-Care International of Cedar Knolls, NJ, is one lab that performs this type of oil analysis. In its test, an Inductively Coupled Plasma (ICP) Spectrometer is used to measure amounts of elements present in the oil. The sampled oil it receives is diluted. It is then mixed with an inert gas, which turns it into an aerosol. This gas is then induced to form a plasma at 9,000 C, which causes metal ions to take on energy and release new energy in the form of photons. A spectrum with varying wavelengths is then created by each element present. A machine quantifies the amount of energy emitted, and determines the concentration, in parts per million, of metals present.
While the oil spectrum analysis is a valuable part of any oil analysis program, it does have its limits. It is only effective in detecting particles smaller than 8 microns. Most oil filters are able to filter out particles above 10 microns. Therefore, to get a complete analysis, it is a good idea to perform an oil filter analysis in conjunction with the spectrum analysis.
Rory Hammond, owner of Aviation Laboratories in Kenner, LA, stresses the importance of oil filter analysis. "The oil filter analysis is able to detect and identify the larger contaminants that are present in the oil," says Hammond. "If an engine is experiencing wear that produces particles larger than 10 to 15 microns, a spectrometer analysis is not going to detect all of it, since most turbine oil filters remove debris down to 10 microns in size. Unless the customer includes an oil filter analysis, some types of problems may go undetected."
Performing an oil filter examination is a structured inspection. The lab technician removes the oil filter from the shipping container and plugs it to prevent any contamination from entering the interior of the filter. He then places it in a clean container, adds a solvent to it, then shakes it to dislodge any debris in the filter. The particle-laden solvent is then passed through a patch filter and the step is repeated. After these initial solvent washes, the filter and container are placed in an ultrasonic cleaner to remove any remaining contaminants. This solution is also passed through the patch filter. The oil filter is then carefully inspected for any debris that may still be present and it is removed as necessary. Now the particle analysis begins.
Analyzing the particles
Hammond states that performing an analysis on a patch filter entails a triage of steps. First, the filter pad is visually inspected to get a brief overview of what is present. Any large particles are separated when possible for later analysis. Chemical spot tests can then be done to aid in material identification. Once the larger materials have been separated and materials have been identified using chemical spot tests when possible, the electron microscope analysis begins.
The electron microscope is a high magnification microscope able to magnify up to 20,000 times - far more magnification than is possible with an ordinary optical microscope. An X-ray spectrometer in the microscope aids in the identification of the debris. So the technician is able to determine from the spectra present the exact Aerospace Material Specification (AMS) number of a particle - thereby determining exactly what alloy it is. The technician is not just able to determine that there is aluminum or stainless steel present, but exactly what alloys or grades they are. This specific identification comes in handy in determining origin of the particles.
There are four criteria that are evaluated when analyzing the particles present in a sample. These are type, form, amount, and condition. By using all of the information from these four evaluations, the inspector is able to determine debris origin.
- Type - This is the specific identity of the material. Determining the identity is the first step in evaluating particulates.
- Form - This is the specific form of the particles. The forms can vary from fine wear, machining chips, and slivers to chunks and platelets. The form of the material is crucial in determining its origin. For example, carbon steel platelets can indicate bearing wear, whereas carbon steel fine wear can be indicative of shaft wear.
- Amount - This is the amount of the material present in the test sample. It can be expressed by weight of the material or by particle count.
- Condition - The most important factor on particle condition is oxidation. If the particles exhibit signs of oxidation, they probably are not recently generated particles. Particles that were generated in the past but are no longer being produced can lead to different disposition when considering corrective action.
Check that chip detector
The chip detector can be a valuable tool in monitoring engine health. Just because there aren't any large chunks present doesn't mean that the fuzz present can't be analyzed. The process of removing the debris is fairly simple. It involves using a strip of tape provided by the laboratory to pull the debris off the chip detector. Although this tape looks similar to transparent tape, it is different in its composition with lower adhesive properties. This allows the technicians at the laboratory to easily remove the particles from the strip and analyze them using an electron microscope as discussed earlier.
What to do with all of this data
After the debris is identified, the lab enters the data into a database. It is then compared to previous analysis results. It may also be compared to similar engine models on other aircraft. This is where the experience and knowledge of the laboratory personnel are important. Different engines have different material makeup, and presence of a certain metal in one type engine may be a minor indication whereas in another engine, it may be cause for concern. It is important for an oil analysis laboratory to have the knowledge base and the close working relationship with the engine manufacturer in order to establish baseline figures and analyze unusual analysis results.
Many companies have established their own methods of tracking the results they get back from the oil analysis lab. Jet-Care offers an electronic tracking software called Engine Condition Health Online (ECHO) in order to help customers track oil analysis results. In ECHO, Jet-Care sends the software out to the customers, who install it on their computers. Then, whenever an analysis is performed, the results are e-mailed to the customer. This allows for a very fast exchange of information from the lab to the operation. Engines can be tracked and any unusual trends can be identified.
It's not just a one-time shot
Oil analysis determines the amount and type of particles in the oil. Something that needs to be remembered is that any type of oil analysis is not very valuable on a one-time basis. Oil analysis relies on being able to determine increases in any particular particle count or an introduction of new particles in order to determine the health of the engine. In order to reap the benefits of oil analysis, there needs to be an established sampling or inspection interval.
Anson Mount, chief of maintenance for Klein Tools corporate aviation department, uses oil analysis as a predictive tool for all their aircraft. "Oil analysis can tell you a lot about what is going on with the engine," he shared. "But it's something that must be done on a regular basis. A one-time analysis is not going to tell you anything. We have our oil sampled on a regular 25-hour interval."
Oil analysis has become an important part of engine preventive maintenance programs and has been embraced by engine and oil manufacturers alike. With its ability to detect wear and contamination, it is a valuable part of any engine monitoring program.