In our last Greasy Bits we noted engine oil must be periodically changed because it gets dirty and some of its chemicals are consumed. And that the oil filter is also replaced during these oil changes.
We’ll leave the complexities of engine oils to chemists, but review oil filtration as it’s a primary maintenance task. Let’s start by noting older aircraft engines might not use a replaceable filter but simply push the oil through a metal screen. These screens are not as effective at filtering oil as a replacement-type oil filter, but they definitely reduce operating cost as the screen is simply cleaned in solvent and reinstalled in the engine during an oil change. Given typical pavement runways (not dirt) and frequent oil changes, common aircraft engines seem to survive pretty well with just an oil screen. But converting from a screen to a replaceable oil filter is definitely an upgrade and lengthens the hours between oil changes.
Oil filters are overwhelmingly a pleated paper element inside a metal shell. Today the paper filtration media and outer metal housing are almost always built into an assembly: the oil filter. The whole thing is replaced during an oil change.
Unlike automotive oil filters the aviation variety tends to have a hex for wrenching the filter on and off, plus a tab for attaching safety wire.
Part of an oil change is cutting open the old oil filter and inspecting the pleated paper to see what’s in it. Of course there are many things in oil that can’t be seen, which is why there is oil analysis. That’s sending a sample of draining oil to a lab for spectroscopic identification. Done regularly, oil analysis is great at setting your engine’s baseline oil contamination. Should a trend or large change occur you’ll notice at the next oil analysis.
Typical oil change intervals are 25 hours with a screen and 50 hours with a replaceable filter (Rotax allows 100 hours given unleaded gasoline and synthetic oil). There’s a time interval as well: four months. It’s often observed in the breach and many owner/pilots simply change oil once a year as they fly so few hours. This tests the oil’s additives anti-rust properties but is common practice.
In any case, loose tolerance air-cooled aircraft engines dirty their oil far more than modern car engines. Thus, aircraft oil change intervals are much shorter than in cars. Don’t skip on changing your aircraft’s oil.
After you cut open the oil filter (using a proper tool so it doesn’t create metal fragments that contaminate the filter), when you inspect the pleats you always find something – they are never perfectly clean and that can be normal. Lycoming SB480F is worth reading for guidance on how to interpret what you find.
…or if flying behind a radial, change interval isn’t as critical since you’re really only renting the oil… 🙂
That said, oil analysis in recips is a bit random for my taste as there are external contaminants that muddy the result, for jets completely agree since oil stays much cleaner clear of the combustion process.
Participating in oil changes to see how much carbon, shiny bits (magnetic or not) or serial numbers and the deltas from one change to another is the real benefit, all part of investing the time to know your acft and what it’s telling you.
<< oil analysis in recips is a bit random for my taste as there are external contaminants that muddy the result >>
Agreed, and after several years finding both false positives (UOA said there was a problem but there wasn’t) and false negatives (there was a problem that UOA didn’t detect), I finally decided UOA was a waste of my time & money. I still of course inspect the screen and filter pleats. YMMV.
I’ll join the agreement. Engines don’t quit because of what’s in oil analysis, they quit because of what’s in the filter/screen pleats.
That said, OA can be useful as a precursor to something more serious.
You may already be aware, but I learned something about the limitations of oil analysis when I read this article: https://www.wingmanaviation.net/post/a-clean-engine-oil-analysis-report-can-be-a-false-peace-of-mind-and-here-s-why
Rotax filters have no means of safety wiring them in place
For aviation use there had to be a torque value and since you can’t put a torque meter on an oil filter wrench, there had to be a hex.
Notice that oil filters NEVER come loose on cars when installed per the instructions. Neither do they come loose on airplanes. If you are reusing a used filter for some reason and oil got on both sides of the square gasket, they will come loose and must be safety wired.
Also note that only a small amount of the oil is going through the paper filter since the internal bypass valve is always wide open when the engine is running.
Why would the oil filter’s bypass relief valve always be open? It opens based on the differential pressure – not the absolute pressure. The differential pressure is too low to open the bypass unless the filter is clogged, or the oil is really thick (such as a cold start). See FAA 8083-32B page 6-7.
Why would it? Because these engines flow much more oil than automotive engines because they are largely oil cooled. The designers knew that, that is why they were designed to only use screens, not paper filters. Measure the pressure differential across the filter and you will see.
Circa 1980 a Porsche race car came into the shop where I was working. In place of the rt seat it had a paper oil filter mounted there. It was the size of a large fire extinguisher! It’s bypass valve was probably not open.
True, so I apply a drop of torque stripe. I’ve never had an oil filter, tightened 3/4 turn from initial contact, come loose on any car, motorcycle, or Rotax engine.
Glad to see you are using gloves to change filter. Not only is the oil itself toxic but those using leaded fuel the oil contains lead which is toxic to adults in very low does.