Ask Paul: What’s best? A flush or overhaul?

Q: I recently found paper towel bits and pieces in a Lycoming engine in an aerobatic airplane. The pilot said he saw the oil pressure go to “zero,” so he reduced power, and landed safely. Would you suggest a flush or overhaul? What’s the best way to go about this?

ED NELSON, via email

A: Well Ed, it looks like someone had a “quality escape” that they failed to alert you about. I’ll have to admit paper towel bits don’t concern me as much as pieces of a shop towel would, but it’s still something that has to be taken care of.

I’m very pleased that the pilot was paying attention and did the proper thing by reducing power and landing safely once he observed the oil pressure go to “zero.” I guess that begs the question as to whether the oil pressure remained there for the rest of the flight and, if so, how long was it from when he noticed it go to zero until he got the plane safely on the ground?

There are several things that must be taken into consideration in a situation like this. Was this the first flight following a routine maintenance event, including an oil and filter change, or was more extensive maintenance done where internal components of the engine were exposed and the paper towel could have gotten inside without being noticed? Under these circumstances, I would inspect the oil filler tube area as best possible with an inspection light in order to determine if there is any additional debris in that area.

Since you didn’t mention having checked the oil filter and suction screen, I assume that was completed and, if so, what was found in those two areas, if anything?

If it were my airplane and not knowing just how much foreign material might be in the engine, I’d probably remove the oil sump and do a good visual inspection of that, especially around the suction screen area. I would also highly recommend you disassemble the entire aerobatic inverted oil system and inspect it closely.

This may be sufficient up to this point, but you may have more of a story than you indicated in your email. If there is any doubt whatsoever in your mind that this engine suffered a long duration with “zero” oil pressure or metal contamination was found in the oil filter when it was properly cut apart and inspected, then you’ve got to dig a little deeper.

If oil starvation is suspected, you may want to removed the #1 cylinder (leave the piston in the cylinder so you don’t have to hone the cylinder and install new rings) and remove the connecting rod from the crankshaft. This will allow you to inspect the connecting rod bearing for any signs of oil starvation or impregnation of the bearing from foreign material. Since the #1 connecting is the farthest from the oil pump, if oil starvation did occur, it may be detected in the condition of this rod bearing and/or crankshaft journal.

I’d also check the prop governor screen for any possible contamination. Needless to say, if things in this area look bad, then either a repair as required or an overhaul may be in order. I’m sure you’ll know by the time you’ve completed the aforementioned.

Let’s say after checking all of these things everything appears to be OK. I think I’d still consider flushing the engine while the oil sump is removed. This may be done using an approved safety solvent, then blowing clean shop air throughout the engine. Once this is completed, you may put everything back together following the manufacturers recommendations and properly service the engine with oil. I would suggest you pre-oil the engine before the first run-up.

Having flown inverted with both Patty Wagstaff and Sean D. Tucker in the past, I can tell you I’d want to be darn certain that the entire oil supply system was working like it’s supposed to and I wouldn’t want to be worrying if there still might be a piece of paper towel in there floating around, like in your case.

Paul McBride, an expert on engines, retired after almost 40 years with Lycoming. Send your questions to:


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