Did you know things live in jet fuel? These are the fun things you learn being a mechanic. Mike, Paul, and Colleen enlighten us on this strange fact, and tackle questions on leaning, burned exhausts, puddles of oil, and how not to set your airplane on fire when preheating. Email podcasts@aopa.org with your question for a chance to be on the show.
Full notes below:
Chris owns a Piper Comanche with 1900 hours on the engine, and it that has developed an oil leak of one quart every 2.5 to 3.5 hours. Fuel tank sealant and an air/oil separator didn’t work. Paul says there’s no way he is leaking that much oil from a leak in the crank case. He suspects another issue. Mike said Lycoming has a procedure to slather a sealant on the case seam. Since there’s a lot on the belly Mike thinks it might be going out the breather. He suggests a ring wash.

Lance flies a Cessna 185 and is worried about his leaning procedure. He’s recently replaced all six cylinders and is concerned his leaning procedure may have caused some problems. He’s worried that by learning until roughness and then enriching slightly he’s in the “red box.” Paul said if the engine is running rough, at least one cylinder has quit firing, so he's very lean at that point. Mike said the GAMI spread should be less than a gallon per hour. The GAMI spread is the difference in fuel flow between when the first cylinder peaks and when the last cylinder peaks. Assuming he only enrichens enough to stop the roughness, and no further, and his GAMI spread is good, he’s likely fine. If the CHTs are all below 400 degrees F and he’s less than 60 to 65 percent power, he has nothing to worry about.

Chris number two asks about corrosion in fuel tanks. He works on turbine aircraft and often sees a brown staining on fasteners inside fuel tanks and he’s wondering whether it should be replaced. Paul said it’s only when the materials starts to degrade when you need to worry about it. He said a good rule of thumb is that if the part number imprint is still there that it can be left alone.

Brad is wondering about pulling the prop through prior to starting. The general idea is apparently to move the oil around. Colleen said there’s no truth to any of it. You’ll scrape the oil off the cylinder walls if there’s any left. You can either preheat to heat the oil up on cold days, or you can pre-oil the engine if it’s been sitting a long time. Paul warns against pulling the prop through for safety. There’s no value in turning it backwards, and turning it forward can start the engine if you aren’t careful.

Barry has a Tiger that had a muffler that lasted a long time when he ran rich of peak. Since running lean of peak or near peak his flame cones have burned out within 400 hours. Colleen talked to a local exhaust rebuilder who said he believes running lean of peak is burning up exhausts. It’s not because of the exhaust gas temperature, per se, but more likely the oxygen content of the flame at those mixtures.

Jeff had a student with a Cessna 172 that he thinks has a tachometer from a Piper. It was replaced in 1993, and had been correctly signed off every year since. He’s wondering how that happens and what the pilot’s responsibility is in this case. Mike, Paul, and Colleen agree that an IA wouldn’t necessarily catch it on the inspection, although instruments and markings are part of the annual inspection checklist. Paul said he’s mostly concerned with the redline, and not necessarily the green arc.