A clean oil filter, and what else?

[photofly]

Hurrah. Nothing but some crispy bits of carbon in my 25 hour old spin-on oil filter - just enough to reassure me that I was checking the right side of the filter paper. Nothing ferrous, and nothing otherwise metallic. So I figure I'm good for another 25 hours on my on-condition engine.

What else can I be checking in between annuals?

[crazy_aviator]

What is your hot, low idle oil pressure? it can be a good indication that the bottom end ( bearings ) are getting too tired ( worn out),,, too much wear and you can spin a bearing , leading to seizure etc. Its not uncommon for bottom ends to go well beyond 3000 hrs. Watch for case cracks, Cyl studs pulling/leaking.

[ahramin]

Are you sending your oil in for oil analysis?

[iflyforpie]

Instrument readings are a big one. Get to know within half a needle width what your normal oil pressure readings are, as different ones are a sign of bearing failure, sludge buildup, or filter bypass.

Spark plugs are an elementary maintenance item. I do mine every 50 to prevent excessive lead fouling (I lean aggressively, but still take out a ton of lead). Reading the plugs will give you an idea of what each cylinder is doing in terms of temperature, mixture, and oil consumption. It's also a great time for a bore scope to check those exhaust valves... especially since you can get good ones for about a hundred bucks. Continental has a great service bulletin on the TCMlink website for exhaust valves that applies to all engines.

Keep track of your oil consumption. Check your oil to ensure it is a nice brown or grey colour rather than black which means bottom end blowby. Run a magnet through the drained oil itself, as sometimes big chunks or heavy chunks can come out of there that don't make it into the oil pump pickup.

Pull the engine through for a poor mans leak down check, feeling for even pressure in each cylinder (mixture and mags off of course) and move it back and forth to listen for any unusual sounds that might indicate big end bearing failure or other bad things.

As an AME, I will keep an engine on condition provided it isn't making any metal and isn't leaking from the case (case split, case crack, through studs). Low compressions and oil consumption can be fixed with top overhaul, but that has to be weighed against the age and condition of the engine and the cost.

[Old Dog Flying]

I have a modified 0-235 C2C overhauled and mod under STC with 300 hrs SMOH. Did an oil change at 25 hrs and the filter, an ADC 20 micron screen showed plenty of aluminum as well as carbon. The interesting thing was the 20 micron screen in the neck of the big funnel through which the sump oil flowed into my bucket..it had a lot of aluminum as well..all signs of the piston pin plugs being shaved off.

We pulled #2 and it was OK but #1 had a 1.5" gouge on the forward side of the cylinder which was destroying the plug. When taken apart, the exhaust valve guide was trash. Rather than grinding the damaged cylinder oversize, we bought a very low time cylinder assembly which had a well documented history.

The little beast is running just fine now while we break-in the newly certified jug.

[Colonel Sanders]

Reading spark plugs is an excellent point. You
can learn a lot about how your cylinders are, by
looking at the plugs before they are cleaned.

Here's a set I pulled from someone else's engine:



It's so easy to quickly take a high-res picture
of the plugs, and so easy to transmit the picture,
I am a bit surprised that people don't keep a
picture of the plugs with their maintenance
records.

If you looked at several pictures over the years,
you could easily spot a trend, or a sudden change.

N.B. Any AME can glance at a photo like that, and
tell you a lot about your engine. In the example
above, by looking at the bottom plugs, cylinders
1,4,5 and 6 are in excellent shape, despite the
high time on the engine. There is a bit of lead
accumulation on the bottom plugs, so you might
nag the pilot/owner to lean the mixture a bit more
on the ground, and if he has to idle for a prolonged
time, do it at 1200 RPM.

Cylinder number 2 is showing just a little oil on
the bottom plug (top plug is fine) indicating that
the piston rings are just starting to wear.

Cylinder number 3 is a bit crappy. Although the
top plug is looking ok - so it's probably still flyable -
the bottom plug is showing signs of more oil in
the cylinder - probably worn rings (worse than #2)
which again is not surprising in an engine approaching
published flight time TBO.

If you were really bored some rainy day, you could
pull cylinder #3 and put a new set of rings in it, and
hone the cylinder. Most people wouldn't bother until
the top plugs showed some signs of oil, though, or
it's oil consumption increased, etc. Note that with
an oily cylinder from worn rings, you can often get
great leakdown numbers, because all that oil is sealing
the rings.

PS TBO - both flight time and calendar - is legally
and practically irrelevant to privately-owned light
aircraft, which are "on condition" as soon as the
engine is installed, either new or overhauled. An
engine with 100 SNEW (two years ago) can be
trash, and an engine with 5000 SMOH (fifty years
ago) can be running strong.

CAR Std 625 App C (7):

No hard time, including calendar time, between overhauls need be observed in the case of small aircraft reciprocating engines in non-commercial private operation.