carbon | Blackstone Laboratories

Oil Filter Inspection

Routine oil filter inspections are a useful tool in the aircraft owner’s diagnostic toolbox. We use spectrometers to test for metals on a microscopic level, smaller than you can see and smaller than an engine’s oil filter will remove from the oil. Larger pieces of metal that might not show up in spectral testing will be trapped in the oil filter. By checking the filter at each oil change, you’ll get a good idea of what normal is for your engine and be able to quickly identify any changes that might be the early signs of a problem.

Cutting the housing

In order to inspect the filter pleats, they must first be removed from the housing. While a hacksaw or angle grinder might get you there, we strongly recommend using a filter cutter to remove the lid of the filter housing. A filter cutter cleanly cuts the robust steel housing without producing metal shavings that might find their way onto the filter pleats you are about to examine. Plus, who doesn’t like a good specialty tool?

The AFC-470 from Airwolf Filter Corp is our go-to cutter here at the lab: http://www.airwolf.com/aw/products/oil-filter-cutter. This tool fits the filter from any Lycoming or Continental engine we’ve come across. Airwolf also offers a smaller cutter for Rotax engine filters. For those who might also want to examine filters from other engines, like their car or truck, filter cutters that cover a wider range of filter sizes are available from speed shops such as Summit Racing. (https://www.summitracing.com/parts/sum-900511)

Secure the filter lug in a bench vice. If the filter doesn’t have a lug, you can secure the lower section of the filter housing in the vice – just be careful to not crush the housing or it may trap the internal cartridge with the filter pleats. Poking a hole in the housing to allow oil to drain can also trap the internal cartridge, so we recommend avoiding that as well.
Place the filter cutter on the filter and gently tighten the cutting wheel. We like to take a conservative approach in cutting the housing, progressively tightening the cutting wheel over a few rotations, rather than trying to cut through in one pass.
Once the lid has been cut, the cartridge with pleats can be removed from the housing. It is also good to inspect the inside of the filter housing for metallic particles and other debris that may not be trapped in the filter pleats.

Removing pleats from the cartridge

You have two options at this point. You can use a solvent such as mineral spirits to wash debris from the pleats, leaving the cartridge assembly intact. The resulting solvent/debris slurry is then filtered for examination. In our experience, this flushing method may not always remove all of the debris from the filter pleats. We prefer to cut the filter pleats from the cartridge for examination by the following method.

Disclaimer: There is the potential to guillotine a finger or two during this process. Proper technique greatly reduces the chances of extensive cursing and an unplanned trip to the local emergency room.

Place the filter cartridge horizontally on the bench and hold with your non-dominant hand. Locate the filter pleat seam that adjoins the two ends, usually with a metal band or glue.
Hold the knife with your knuckles against the bench for stability. Starting at the seam and using only downward force, cut along the edge of the pleats opposite the side you are holding. We prefer to rotate the pleats into the knife blade, firmly holding the knife in a fixed position. This method, when done properly, protects your off-hand’s fingers from the knife blade, where the knife moves downward into the bench if it were to slip.
Flip the cartridge around and repeat steps 1-3 on the other side. You may have to make a few passes on each side to fully cut the pleats. Using a new razor blade helps.
Again locate the seam where the two ends of the filter pleats are joined together. Cut across the pleats on either side of the seam.
The pleats can now be removed for examination. If properly cut, the pleats will come out in one long piece with a clean edge on both sides.
The pleats will still contain a fair amount of oil at this point, making it difficult to see metallic debris. If time allows, you can place the pleats on paper towels to drain overnight. You can also squeeze the pleats like an accordion and mop up the oil that squeezes out with paper towels.

Inspecting the pleats and basic identification of common particles

Stretch the pleats out under a bright light or outside on a sunny day. Larger metal slivers will be obvious, but you may have to look quite closely to identify smaller particles. Here at the lab, we have a dedicated space with clamps that stretch the filter pleats out in one long section. You can improvise in the shop by placing something heavy on both ends of the pleats.

A strong magnet (covered with a plastic baggie or cling wrap) will remove ferrous particles from the pleats. We also suggest checking the pleats themselves with a magnet. Severe steel wear may generate enough small ferrous particles to make the pleats react to magnet.
Aluminum has a bright, silver appearance and will not react to a magnet.
Copper-containing alloys, such as brass or bronze, vary from a light straw to copper color and will not react to a magnet.
It is also common to find carbon, especially in the filters from turbocharged engines. Carbon is black, hard particles that can be broken apart between your fingers. A large amount of carbon might indicate excess blow-by, but what counts as excessive is unique to each engine. Regularly checking the oil filter will give you a good idea of how much carbon is normal for your engine. You might also find carbon with steel embedded in it, so it is good to check carbon particles with a magnet.
Small bits of sealer material may also be found, especially after repairs. We generally don’t worry about this sort of non-metallic debris.
You might also find lead deposits from fuel blow-by. These particles have a bright, foil-like appearance that can look very much like a metallic wear particle. These deposits can be distinguished from metallic wear by their soft and “smudgy” texture. It is worth mentioning that these deposits are not lead from the wearing surface of a crank or camshaft bearing.

Steel sliver

Aluminum flakes under magnification

Brass/bronze under magnification

Carbon deposit

Sealer material

Lead deposit

Evaluating Filter Debris/Conclusion

In some cases, a filter will contain so much metal that a looming problem is almost certain. But it is more often the case for the findings to land in an ambiguous gray area, where the severity of the metal is situationally dependent. You can expect to find some metal and other debris in the filter from a fresh overhaul, for example, where the same findings would be unusual in a routine filter inspection for that same engine at 500 hours since major.

Lycoming offers good guidance on the identification and evaluation of filter debris in Service Bulletin 480F. In our opinion, a lot of the information in that bulletin can also be applied to Continental engines. Blackstone also offers a filter and filter screen examination service as a compliment to oil analysis – but we recommend doing routine filter screenings yourself to get familiar with what’s normal for your particular engine. Save your money for flying — check your filter yourself!

Under Pressure! (Part 1)

About a year ago I started a flying club at my local airport with three other pilots – that was a long and interesting process that I won’t go into here – but after a few months of trying to get the right people together then struggling to buy an airplane in a hot market, we were able to purchase a 1965 Cessna 172 F with the Air Planes 180hp Lycoming O-360-A4M upgrade, with about 1300 hours SMOH. The pre-buy inspection went well and I had the honor (with another member) of flying our new ship home to Illinois from Florida.

As a 12-year analyst at Blackstone Laboratories, of course I did an oil and filter analysis as part of the pre-buy inspection. With that data in hand and considering how active our new aircraft was, we decided to proceed with Lycoming-recommended 50-hour oil changes right off the bat.

The first few oil analysis reports were excellent, and the oil filters came back nice and clean. The first time we pulled the oil suction screen we found a small amount of fibrous material in it, but the previous owner confessed that they hadn’t ever pulled the screen, so we assumed the debris was probably years – if not decades – old. We didn’t worry too much about it. When the next screen came back clean, save for a mere spec of carbon, we forgot about the fibrous debris entirely and happily went about enjoying our new bird.

Lost oil pressure

That is… until March 29th, when one of our newer student pre-solo members texted that, 10 minutes after takeoff, the engine lost oil pressure. He and the instructor landed without incident and reported everything looked ok, hoping it was a bad gauge.

The questions from the other partners streamed in: where were you? How was the engine running? How was oil temperature? The student pilot clarified: they took off to the north, climbed to 5,000’ to work on maneuvers, and when they did the cruise checklist they noticed the low oil pressure. He reported returning to the airport immediately with no issue. The engine ran great the whole time, and oil temperature was fine. No apparent oil loss.

I was out having coffee with a friend at the time, but my heart sank and I couldn’t focus on anything else, so she and I ended up going to the airport, with an A&P friend on the phone to look for something obvious, like an oil leak at the oil pressure gauge or where the line comes through the firewall. I didn’t find anything, but I wasn’t too discouraged: I knew we had a lot of things to troubleshoot before suspecting engine damage, so I tried not to jump to any conclusions.

Filter pleats with slimy debris embedded

Initial shock and investigation

The following day, our club maintenance director – under the watchful eye of our A&P – started working through the troubleshooting list. We quickly determined it wasn’t the gauge, and it wasn’t the line, so the easy, cheap fixes were off the table.

Now concern was starting to mount; the deeper we had to dig, the deeper our pockets were going to have to be. We’re a new club, already operating on a shoestring budget, so we didn’t have time or money for costly repairs. But we had a problem to solve.

The next step in our troubleshooting process was to start looking for a problem. We drained the oil, pulling an oil sample in the process, and we removed, cut, and inspected the oil filter. The only abnormalities were some clumpy, slimy looking lumps (see figure 1) and a giant piece of carbon (figure 2). The carbon was large enough to wonder if something similar had gotten stuck in the oil pressure relief valve, so we pulled the oil pressure relief valve off hoping for a big piece of carbon to be our easy culprit.

Much to my dismay, the oil pressure relief valve was totally clean.

Looking at the screen

Later that night, with the oil drained, the filter cut and the oil pressure relieve valve not the suspect, I pulled the oil suction screen. My heart sank. The screen was completely blocked by fibrous

Debris on the end of a fingertip

material. It looked like the same stuff we found in our earlier oil change, only a whole lot more of it. The pickup screen was so caked full that I had to use a screwdriver to pry the debris out of the screen (figures 3a and 3b).

That’s when concern really started to set in: we actually did lose oil pressure, and there could be some serious damage here.

A few days later the oil analysis and official filter report came back: the oil report was unremarkable, but the oil filter report was disheartening. Our engine had gone from making “no appreciable metal” in 50-hour oil change intervals to making “non-ferrous metallic flakes at an approximate rate of 20 pieces per filter pleat” in a matter of 20 hours. Great. Now what?

Fact-finding

The filter report stirred up more questions than it did answers. The next step in the process was to figure out how much damage was done, what that fibrous material was, how it got there, how much remained, and, most importantly, what to do next?

The fibrous material turned out to be the easiest question to answer – it was a paper towel. When examined under a microscope, it had the very distinctive dimpled pattern you see on “quilted” paper towels (see figure 4). Once we identified the contaminant, the next step was figuring out how it got there.

Fluffy stuff that came out of the suction screen

Interestingly enough, the previous owners provided the answer in the photographs they shared with me during the pre-buy. A couple of photos showed the process of swapping the original O-300 with the O-360. In figure 5 you can see a couple of paper towels sticking out the back of the engine where the magnetos should be. I believe that some (or all) of that paper towel somehow ended up getting stuck in the engine – either someone turned the prop and the gears pulled it in, or some of it got wet with engine oil and broke off into the engine.

**You did what?**

There’s another interesting twist to this story: as we were trying to figure out how long the engine ran with no oil pressure, we looked at the flight track on FlightAware. As reported, the plane took off to the north, climbed to about 5,000’ turned around then did a 360’ turn, presumably to lose altitude. Then there’s a normal approach to landing and…a go-around.

Wait, what?

That’s right – a go-around. With no oil pressure measuring on the gauge. When we confronted the CFI about the flight track, he confirmed: the student pilot was flying, and he came in too high and too fast to salvage the landing, so a go-around was initiated. *face palm*

Why didn’t the CFI take control of that flight and land? Because he assumed it was a gauge problem. Oil temp was fine, the engine was running well, so he didn’t worry about it.

Ground up paper towel under a microscope

The good news was that the CFI confirmed that oil pressure wasn’t lost entirely (as the student pilot had reported). It was only lost when at idle. If there was some power, there was some (albeit not much) oil pressure reading on the gauge. With that information, and knowing the engine hadn’t seized entirely, we were hopeful the engine damage was limited.

Moving forward

So, we’ve finally found the culprit, and now the long road to recovery begins. The main takeaway from this part of the story is, if you see anything unusual at all – anything – get on the ground as soon as possible. Don’t assume a low oil pressure reading is a bad gauge, and don’t fly when something might be wrong. Don’t dilly-dally, waiting for traffic to clear. Declare an emergency if you have to and diagnose with two feet firmly planted on terra firma.

Check back in the next newsletter as we discuss the confusing, gut-wrenching process of figuring out whether we could salvage the engine!