[AlexPeterson]

Wikipedia defines "total acid number", TAN, to be the amount of potassium hydroxide in milligrams that is needed to neutralize the acids in one gram of oil. In other words, it indicates how acidic the oil is. Obviously, the more acidic the oil is, the more corrosion that will happen. The analyses of my oil typically run around .6 to 2.0.

I'm curious if there is a generally accepted limit for recip aircraft engines. I could imagine a lower limit for infrequently flown airplanes, higher for frequently flown, but I don't know.

 

[hydroguy2]

Don't know if there is a acceptable limit for recip engines. BUT...

We use Mobilgear SHC 320(synthetic) in our hydroplant gearbox. We do monthly oil analysis monitoring. Our target TAN is <2.0. Since a higher TAN is bad from a corrosive standpoint, Another # to watch is water content.

An oil change of 1500gals is ~$45,000, so we watch all our numbers closely. TAN increases linear over time. So as we approach TAN=1.7, we drain out 300gals and add 6 new barrels. This drops the TAN to 1.1 or so. We'll do it again in a few years. Make-up oil is ~50gal/yr.

Best thing is to keep the system clean and moisture free and the other numbers take care of themselves. We filter with a 6micron filter.

OK, thats "Hydro Oil 101"