Many aviators may consider the process of changing the engine oil as a rather simple task deemed necessary to the routine maintenance of the aircraft. Drop the old oil out and refill the new liquid gold colored oil and a filter inspection thrown in for good practice. While this oil change process may seem somewhat elementary upon first glance, there are also several analytical factors to consider while performing this task. Once you understand what to look for, the oil change activity can become an excellent opportunity for you to examine the internal health of the engine. Even if you are not directly involved in the hands-on experience of changing oil, your technician should be able to interpret several oily clues (while considering service severity and environmental conditions) as a preventative maintenance exercise.
The primary purpose for changing engine oil is to remove the suspended contaminants from the engine. Our previous lesson on aviation oil revealed that the common Ashless Dispersant oil used in general aviation not only cleans the engine but also effectively holds the contaminants in suspension until the oil can be completely drained out. AD engine oil absorbs and suspends debris and corrosive matter remaining as a by-product of the internal combustion process. Regular oil changes serve to simply get rid of these unwanted nitric and sulfuric acids, water, lead salts, carbon, and even metal wear particles (not captured in the pleated filter element). The oil will appear visibly dirty proportionate to the amount of time it has been left in service. Operating beyond the recommended oil change interval compromises the oil’s ability to do its job as advertised. Oil screens should be inspected every 25 hours whereas the spin-on filters are replaced on a 50 hour cycle.
Aside from the obvious lubricating function of the oil working inside the engine, the oil filter and/or screen traps the suspended debris thus preventing carbon deposits and other particles from continually cycling through the lubrication system. An engine that has been equipped with an external spin-on oil filter must be replaced with a new one at oil change. A separate suction screen must also be removed, inspected and cleaned. The suction screen is strategically located upstream of the oil pump in order to prevent unwanted debris from entering the lubrication system. Maintainers will inspect the integrity of the screen for damage or distortion as well as inspecting for foreign particles and routine cleaning.
Access to the oil filter in the engine compartment may differ depending on make or model and how much room there is to actually work the filter around wire bundles, hoses or other obstacles. This process often requires the aid of a custom made funnel and container or some other effective method to catch any residual oil from the engine and/or filter during the replacement procedure. This can be a messy step if you are not prepared. It is always best to have shop rags handy as well just in case things don’t go exactly as planned. It will be inevitable that some amount of oil may be spilled onto the floor at some point, so be prepared to clean up the mess with a good oil absorbent product. Even a small puddle of oil on the floor can become a serious slip hazard when you least expect it.
The spin-on type oil filter should be cut open with an appropriate can-cutting tool so that the paper element can be removed and inspected for ferrous and non-ferrous contaminants to determine their internal origin. A proper oil filter cutter will neatly open your filter for closer inspection without introducing any other foreign debris from the can cutting process. Most classic aircraft engines were only fitted with a factory installed oil screen but many have been upgraded to the external spin-on filter which have become more standard on all modern aircraft. External oil filters are available in different shapes and sizes and certain filters are designated for certain applications as determined by the aircraft manufacturer. Although they look identical to automotive oil filters, these aircraft filters must be installed to a specific torque value and lock-wired for safety.
Textron Lycoming also recommends engine oil change at maximum intervals of 50 hours or 4 months. This calendar rule is considered to be more significant for inactive aircraft. Low time oil may appear to be relatively clean even though it has been compromised with residual moisture and acids from the natural condensation process. Infrequently flown aircraft do not get the opportunity to get rid of the moisture and acids, which continue to contaminate the unchanged oil even in a state of dormancy. The cleansers and acid neutralizers in the oil additive formula tend to weaken over time even during normal service and can only be replenished by changing the oil. Operational concerns with more active engines depend upon regular oil changes to prevent carbon build up in vulnerable areas of the cylinders and drive train. Lack of oil control in the cylinder increases the chances for increased oil consumption, plug fouling and intrusive deposits within the combustion chamber area. Frequently flown aircraft get the oil temperature above 180 degrees more consistently therefore burning off any moisture and acids which are dissipated out of the engine vent (or breather tube). Regular flying activity further promotes the preventative maintenance regimen by agitating contaminants back into suspension until they can be drained out.
Changing the oil after the aircraft has been flown or at least had an extensive ground run increases the percentage of engine contaminants being worked back into suspension. A basic oil change only involves replacing the oil and filter whereas a more intensive oil change may coincide with a scheduled aircraft inspection. This permits the maintainer to also drain residual oil from the oil cooler and associated hoses. Oil that has done its job will tend to be visibly darker in color revealing its ability to suspend all of the contaminants that we mentioned earlier. You should preferably see dark grey or brown colored oil as opposed to black oil which has been clearly overworked. Another cause of overly darkened oil may be traced to combustion gas blow-by which could be an indicator of oil control issues within the cylinder(s). Changing the oil at the recommended time and calendar intervals can easily become the least expensive method of preventative maintenance for a hard working aircraft engine.
Spectroscopic oil analysis is a procedure whereby a small sampling of oil is taken during the draining process. The sample is sealed in a clean container and sent to a lab for analysis. This scientific analysis will identify a fairly broad spectrum of contaminants measured in parts per million (ppm). The levels of various engine metals can then be traced to specific areas of the engine to determine the likely source of normal or premature wear. Utilizing the benefits of oil analysis is best referenced as an ongoing trend monitoring program for a specific engine. Having an oil quick drain installed in place of the factory installed oil drain plug will also speed up the oil change process with less of the mess and less fuss.
A thorough engine ground run is always necessary after the oil has been changed to ensure that there will be no suspect leaks under normal operating conditions. Use a good light source to look around the engine compartment after the ground run to verify that everything is still dry (Useful tip – ensure that any evidence of previous oil leaks are cleaned thoroughly during the oil change). The oil level may need to be adjusted soon after this phase since the new oil has been further distributed throughout the oil galleries, oil cooler and filter. All oil change activity should be approached with the same diligence as maintaining the rest of the aircraft. Any minor omissions may prove to be disastrous when even the most elementary tasks are compromised by human factors.