A.Oil leaks are sometimes considered a fact of life in general aviation airplane engines. Compared to the radial engines that preceded our horizontally opposed Lycomings and Continentals, the leaks we see today are nothing. Seriously, though, there are many factors at play here. You could have a legitimate small oil leak that is causing the oil inside the cowl. You could also be seeing the remnants of the last oil change in which the mechanic didn't quite manage to clean up everything afterward.
Was the oil clean or dirty? Clean oil generally comes from a bottle and could be the result of another renter's not-so-good aim in filling the crankcase. Dirty oil is likely coming from inside the engine. Remember, too, that oil is like blood, and a little bit looks like a real mess.
Unfortunately, many engines just leak. And when you don't own the airplane, it's hard to keep track of an engine's traits. A little leakage is acceptable in many cases. I recall a service bulletin on Lycoming TIO-540s used in Piper Navajos that says a certain amount of oil seepage through the crankcase is normal. I'll have to admit that the majority of cowlings I've stuck my head into have had some small amount of oil in there somewhere.
What can you do? You did the right thing by asking the mechanic about it. Keep an eye on its trend. Is it getting worse? Have people been adding more and more oil to the engine recently? One of the telltale signs that an engine is going downhill is a rapid increase in oil burn. Ask if the FBO has the engine's oil analyzed at the oil change. These reports offer a unique glance into engine health. Otherwise, the only way to find out where it is coming from is to clean the engine thoroughly and fly it to trace the leak.
Peter A. Bedell
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