I recently received this email.
We have been using AOA for oil analysis for many years. We are strongly considering a change to Blackstone, but some of our greybeards are questioning whether we ought to just continue cutting filters open and skip the oil analysis altogether.
For what-it's-worth, here is my reply:
It depends on your objective.
- Oil filter examination is used to detect particles from damaged and failing components, while,
- oil analysis is used to monitor wear rates.
There is some overlap in that a high wear rate might eventually lead to metal particles and metal particles circulating in the oil might increase wear rates. Here are some examples:
Example 1. Early camshaft and lifter spalling generates small, dark colored metal particles that are filtered out of the oil and deposited into the filter. They are not detected by oil analysis because they are in the filter and not in the oil and because oil analysis does not detect monolithic particles (above 3 microns). Eventually, damaged surfaces cause high wear rates that begin to appear in the oil sample. It's a guess with oil analysis where the wear source is located but the distinctive particles in the oil filter immediately identify the source.
Example 2. Dirt in the oil increases wear rates so the first item one looks at in an oil analysis report is the amount of silicon (silica or dirt) in the oil. High silicon causes abrasive wear and high wear metal readings. Reduce the dirt by inspecting the induction system, alternate door inspection, etc. Dirt is easy to monitor with oil analysis but cannot be monitored by oil filter examination.
Example 3. A bearing is failing in fatigue (see Flightsafety report) and depositing babbit chunks in your oil. Oil Analysis is clean.
Example 4. AOPA convention in Kansas City has airplanes taxi across grassy area. Thin layer of grass with a base of fine sand. Over the coarse of the event, sand kicked up by taxing aircraft deposit grit and sand in the engine cowling. When the pilot pulls open carburetor heat the sand and grit gets sucked into the engine. Several aircraft owners discover extensive engine damage after they fly home but are unaware why. Oil analysis revealed very high silicon in the oil. Later pieces of sand were found in cylinder carbon deposits and identified as the type of sand at Kansas City.
Both oil analysis and oil filter analysis are just one of several tools used to monitor and detect problems in the engine.
