Spectrographic oil analysis has become very popular with the owners and operators of
general aviation piston powered aircraft. This process was developed over 35 years ago by
the railroads as a method of identifying wear characteristics in large diesel engines.
Eventually oil analysis was adopted by the military and then by commercial and general
aviation.
Engines are designed and manufactured utilizing various materials and alloys. The
engine lubrication system is designed to provide either pressure or splash oil to the
areas of the engine that are subjected to frictional loading. These areas, in the course
of normal operation, undergo minute, continuous wear. During operation submiroscopic
material is released from these contact surfaces and are suspended in the lubricating oil.
Spectrographic oil analysis identifies these materials and the level of concentration.
There are two accepted methods of performing oil analysis, atomic absorption or atomic
emissions. Either method, atomic absorption or atomic emission, will identify the presence
of submicroscopic material that is suspended in the engine's oil. The oil analysis report
will identify the material suspended in the sample and the quantity of that material in
parts per million (PPM).
Atomic absorption will identify suspended wear material that is 5 micron in size or
smaller, while atomic emissions will identify suspended wear material that is 10 micron in
size or smaller. Since engine oil analysis is used as a tool to establish an engine's wear
trend and identify deviations from the established norm, either method is acceptable.
However, it is important to realize that the oil analysis program must utilize the same
laboratory for all sample analysis. Comparing an oil analysis report from a separate
laboratory will have little meaning if each laboratory uses a different analysis method.
Spectrographic oil analysis is an additional tool which can, in some cases, assist in
the identification of an internal engine problem. If samples are properly taken on a
regular basis, it should provide the owner/operator and oil analysis technician with
information relative to the normal or abnormal wear that occurs during the course of
engine operation. Deviations from an established wear trend pattern should alert the
owner/operator and oil analysis technician to the need for further investigation.
Spectrographic oil analysis must be accomplished according to a set protocol to provide
any useful information. Even if done properly, spectrometric oil analysis will rarely
provide any prior indication of a fatigue type failure. A single spectrographic oil sample
will usually not provide a high level of useful information since there is no established
wear trend data on that specific engine for comparison. A proper spectrographic oil
analysis program should begin with the first engine oil change. To establish a wear trend
data base for an engine will require that at least 3 oil samples be analyzed. As the
engine accumulates operating time and additional oil samples are analyzed a more
definitive wear trend will be identified. Unexplained deviations from the engine's normal
wear trend pattern should be investigated using accepted, conventional inspection methods.
LIMITATIONS
Variations in operation, utilization and maintenance may be reflected in the parts per
million content reported. Deviations from standard or previously used sampling procedures
may result in variations in the parts per million content of the sample report. Seasonal
changes may also result in variations to the parts per million content in the sample
report.
GENERAL PROCEDURES
To establish a data base for comparison and for the oil analysis reports to have any
meaning the oil samples must be taken on a regular schedule using the same sampling
technique and laboratory. The engine must have been operated long enough to obtain normal
operational temperatures and the oil sample taken within 30 minutes after engine shut
down. The tube or funnels used to drain the oil from the oil sump must be clean and free
of any foreign material or residue. If the oil sample is taken from the oil as it drains
from the sump, allow approximately 1/3 of the oil to drain prior to taking the sample. If
the sample is taken via the oil filler or other location using a sampling tube it is
critical that the sample not be taken from the bottom of the oil sump, but at a location 2
to 3 inches above the bottom of the sump. Under no circumstances should an oil sample be
taken from the oil filter canister.
OIL USE
Questions regarding oil use are among the most frequently asked of factory
representatives. As with many aspects of proper maintenance practice, there is no simple
answer. However, understanding the many issues that affect oil performance and selection
may be useful. What follows is some common sense data which I hope will be beneficial.
Oils approved for operation in TCM engines are found in TCM Service Bulletin SIL99-2.
For TCMLINK Aviator and FBO Services members this bulletin is accessible on line.
Approved oils must conform to MIL-L-22851 for Ashless Dispersant oils, or MIL-L-6082 for
Non-Dispersant Mineral Oils. Approved oils must also conform to the requirements of TCM
Material Heat Treat Specification MHS24.
Oils are approved and added to SIL99-2 when the oil manufacturers certify that these
requirements are met. These specifications largely assure acceptable load carrying
ability, temperature stability, corrosion protection and flow characteristics. In other
words, they facilitate the operation of the engine under normal operating conditions.
A quick look at SIL99-2 will show a rather lengthy list of approved oils. Actually,
there are only a few types of oils with the many specific brands listed being licensed
production of similar products in various countries throughout the world. In general, you
will find mineral oils used for break-in and straight-weight oils, semi-synthetic
multi-viscosity oils (such as Shell 15W50) and natural base multi-viscosity oils (such as
Phillips 20W50) used after break-in. Lubricating oils have been continuously improved over
the years and all perform the basic lubrication process. With the exception of the recent
experiences with full synthetics, most aviation oils have demonstrated acceptable
technical performance.
However, as is often the case, "the devil is in the details." For most
pilots, the most important factors to be considered in selecting an appropriate oil may be
more related to the operational, environmental and other factors affecting the oil rather
than the technical process of lubrication.
The factors to be considered in determining the best oil for you will depend upon your
specific circumstances and, therefore, may change depending on changes in the location of
your aircraft and the manner in which you operate it. To make the best selection, some
awareness of issues that need to be considered concerning oil selection can be helpful.
Here are some points you need to keep in mind when selecting an oil for your aircraft:
FREQUENCY OF USE
Frequency of aircraft use is one of the most important factors in determining how your
oil will perform. Frequently flown aircraft (at least one hour per week) with twenty five
(25) hours, or four month interval oil changes, will make most oils look good. Frequent
flying keeps the oil covering on internal parts fresh. Debates on the tendency of
multi-viscosity oils to "run" from engine parts after a period of time become
immaterial. The adverse impact of water content, acid formations and contaminants are
minimized. TCM regularly evaluates returned engine cores and it is generally easy to
identify those engines that have had regular utilization with frequent oil and filter
changes.
Conversely, engines flown less than 100 hours a year or irregularly are strong
candidates for corrosion formation. When camshaft, lifter, and cylinder wall corrosion is
found, low utilization aircraft are often involved. Simply because an airplane or engine
is new doesn't provide extra protection against corrosion. A new airplane flown
infrequently in the distribution network pending sale, or a newly installed rebuilt engine
which is test flown then allowed to remain inactive are prime candidates for rust
formation unless properly preserved with preservation oils.
When the aircraft will not be flown for extended periods, the use of a preservation oil
on the cylinder walls is required. Simply put, preservation oil is formulated to adhere to
the surface of cylinders walls for an extended period of time.
CREATING A PROTECTIVE COATING OF OIL
Between periods of use the oil must maintain a protective coating on lubricated
surfaces. The oil is originally deposited during the operation of the engine. If the
protective coating is not maintained, light surface rust will begin to form on steel
surfaces within days. Left unattended for longer periods of time, the rust will begin to
damage the base material. In such situations, when the engine is started, abrasive rust
particles are circulated in the engine. Frequent oil and filter changes is one way to
minimize their influence. In general, the thicker the oil, the better the quality of the
protective coating. This would seem to suggest that the use of a straight-weight oil would
be preferable if this were the only characteristic of the oil that was relevant;
unfortunately it is not.
STARTING YOUR ENGINE
Significant engine wear can occur at startup, before the oil circulation system has had
a chance to pump oil to the engine parts. The thicker the oil, the longer this will take,
resulting in less oil on running surfaces during the startup sequence. Preheating during
extremely cold temperatures is one of those "ounces of prevention" that can keep
cold starts from damaging your engine. Cold temperatures not only slow the initial
movement of the oil but the differential thermal expansion of engine materials (aluminum
and steel) decreases engine clearances, creating the potential for more accelerated wear
during cold startups. Unfortunately, the ability of the oil to provide a protective
coating and to circulate quickly at startup are conflicting needs at cold temperatures.
MOISTURE FORMATION
The next time you're thinking about the beauty of the dew formation on a cool evening,
remember that a similar event is taking place in your engine. The heating and cooling
cycles that your engine components go through night after night condenses water vapor on
exposed surfaces and results in increased water content of the oil. Regular engine
operation at least once per week with flight duration of at least 1 hour per
flight will help to evaporate the moisture from the oil. Ground running is never a
substitute to proper operation of the engine for the following reasons: Short duration
ground run only serves to heat the engine components and actually promotes the
condensation process. Second, during a ground run the engine is not operated at the
necessary temperature for the proper duration to boil off the water in the oil. Such a
ground run increases the potential for water formation and corrosive attack.
ACID FORMATION
When water is introduced into the oil, another undesirable event occurs. During normal
operation, the oil picks up various by-products of combustion. When mixed with water,
acidic compounds are formed. The acids that result are capable of etching metal and
providing a point for further corrosion to take place in conjunction with the water that
has absorbed into the oil. Frequent oil and filter changes can be used to minimize these
effects. Again, under normal operating conditions, as with the water content of the oil,
these compounds will be boiled off from the oil.
WHERE YOU LIVE
Obviously, the formation of moisture is more intense in areas of high humidity. When we
examine rust related complaints, we find that many are in coastal areas. During our
inspections at various fly-ins, we see very little rust from aircraft which are based in
areas of low relative humidity. If you live in a high humidity area, your oil will be more
likely to absorb more water from condensation. How does this relate to oil selection? Oil
experts point out that a multi-viscosity oil can absorb more water in solution than a
straight-weight oil as a result of operating cooler. If you're in a coastal area, fly
infrequently, and like the good flow of a multi viscosity at startup, good practice would
be to perform frequent oil and filter changes to minimize the potential for the harm that
any water absorption might cause.
PUTTING ALL THE PIECES TOGETHER
When you look at all of the ingredients to a proper oil selection, it is easy to see
why there is no simple answer. With the excellent technical quality of all aviation oils
it would appear that environmental, utilization, and other external factors may hold the
key to proper oil selections.
For my airplane, my thought processes would go something like this:
Regular and frequent oil change intervals provide the best proactive choice for the
individual pilot. My airplane would see regular 25 hour oil and filter changes, or a
maximum of four months to be conservative. The higher hours for oil change intervals that
are found in the manuals work fine if the aircraft sees regular and frequent use. With the
frequent oil and filter changes, I minimize the potential for destructive corrosion
resulting from water absorption or acid formation in the oil and help those situations of
high contamination (dirt & dust) or high temperature installations.
The engine should be broken in on a recommended mineral oil. This procedure has
historically provided consistent ring seating. The oil should be changed to an approved
ashless dispersant oil when oil consumption stabilizes, or at a maximum of 25 hours. An
early oil change with a new engine or following major work also removes initial break-in
material.
The oil level should be maintained near the full level. A reduced oil quantity means the
remaining oil is handling an increased heat load. Oil cooler baffle sealing should always
be checked to provide proper cooling.
When operating the engine, be sensitive to the need to develop an oil temperature above
170 degrees Fahrenheit for approximately an hour to boil off the water and acids in the
oil as a means of minimizing corrosion risk.
To guide the long-term oil selection and engine monitoring process, I would begin a
disciplined oil analysis program. Again, our TCMLINK Aviator Services program provides a
convenient and cost effective method for the owner/operator to track his oil analysis
results. Followed with discipline, oil analysis can help you monitor engine wear trends
somewhat more scientifically as compared to reliance on "hangar talk".
When living in a coastal or high relative humidity environment and flying infrequently,
I would pay particular attention to frequent oil and filter changes and the need to follow
preservation procedures. Again, if the engine will not be operated for periods in excess
of thirty days, use a preservation oil as recommended in TCM Service Bulletin SIL99-1.
Evaluate the overall oil consumption with respect to the engine installation. In
general, look for stability of oil consumption rather than level as a health indicator. If
you have a highly loaded installation or turbocharged engine, you may want to evaluate and
reflect on the oil consumption rate between a multi-viscosity oil and a straight-weight
oil. Generally the use of multi viscosity oil will result in a lower oil consumption
level. It is not harmful to change the basic type of oil used in the engine and to
evaluate the result.
I would listen to what works in my geographic area for people with similar utilization
patterns. Some local shops go as far as to recommend specific oils. Two large regional
engine shops, for example, warrant their products only if they are operated after break in
on natural base oils.
I hope these comments will provide some food for thought at your next oil change. There
is no easy or universal answer concerning oil choice because there are many variables that
affect your choice. As with many maintenance items, a little awareness and involvement on
the part of the pilot, coupled with simple trend monitoring of consumption and oil
analysis data can form the basis for a more scientific method to manage the life of your
engine.
