This month Mike, Paul, and Colleen tackle questions about inexpensively improving aircraft paint, cylinders that are too cool, low compression readings, and a magic floatplane that burns less fuel with its floats fitted. Plus, the ultimate stumper, an airplane that can't turn left. Submit your questions to [email protected] Full notes below: Eric has a Pitts with an unusual problem. When he turns left the engine starts to sputter, and when he turns right the rpm increases. They initially suspected the flop tube in the header tank. Although it looked to be in good shape they replaced it anyway. They also overhauled the wobble pump, and carefully inspected all the fuel lines. There were no obvious problems. Colleen reached out to Pitts experts. They suggested making sure the fuel divider was properly bled. Eric is pretty sure it’s not vapor lock. “It has to be the flop tube,” Mike and Colleen say. Another theory Eric and his mechanics came up with is the fuel controller. They sent it for service, and the shop found that the throttle body mounting hole had a little rounding from vibration damage. They thought that maybe turning opened it to allow in more air, leaning the mixture. Once he gets it back, Eric is planning to do a test flight. Doug is looking at his paint job, and while he knows that paint is an important protector, he’s trying to avoid a costly and invasive full repaint. Paul said you can absolutely go in and touch it up yourself. You can use a touch-up brush, sprayer, or anything you like. Colleen said that the vast majority of the work is in prep, and that the paint color will never match because the original paint will have faded. “Be careful and measured,” she said. Mike said to make sure the aluminum is clean and corrosion-free prior to applying the paint. Paul suggests Scotch Brite and Alumiprep (https://www.aircraftspruce.com/catalog/cspages/alumiprep.php) and Alodine (https://www.aircraftspruce.com/catalog/cspages/alodine1201.php) to make the primer stick. Get an automotive paint person to help blend the colors, he said. And Mike says to make sure you don’t sand through the thin layer of Alclad. Andy wonders if his IO-540 is running too cool. His cylinder head temperatures only top out at 310 degrees, which he's reading on an Electronics International engine monitor. Mike said EI makes two probes for their engine monitors, and the P-100 always reads low because there’s an air gap at the cylinder head probe that makes it artificially low. The only downside of running too cool is potential lead scavenging. Mike said to pull the spark plug and use a borescope to look at the exhaust valve stem. If there isn’t deposit build-up, then there’s nothing to worry about. But lots of lead means you have to somehow increase the temperature. Tom is worried about his compression readings. His O-320-D2G had a cylinder of 38/80 only 18 hours after overhaul. Two more cylinders are reading a little low now at 100 hours. He thinks all of them are leaking a bit, and have gotten slightly worse. He expects them to get to the low 60s, and he’s worried that when he wants to sell, buyers will baulk at the numbers. So he’s wondering if he should just replace the cylinders now so he never has to worry about it. The hosts all agree he is overanalyzing and should monitor as normal, keep flying, and work on the cylinders only when it’s required. Jeff has a Cessna 180 floatplane and a new fuel flow meter has him thinking. He’s wondering if the performance charts are based on tach time or clock time. Paul assumes that because the tach was the only instrument they had when they certified the airplane, it has to be tach based. “They had clocks when that airplane was built,” Colleen says. She and Mike are sure it’s clock time. Jeff also wonders why his float supplement shows fuel flow lower than the original handbook. With a difference of only .2 gallons per hour, Mike says they are effectively the same number, and not to worry about it.

You may think an annual inspection includes a verification of the accuracy of your fuel gauges, but you'd be wrong. Nor does the IA check the accuracy of your tach or any other number of instruments. This information comes to a pilot undergoing a first annual. Mike, Paul, and Colleen also tackle oil loss and temperature issues, why we stop our engines with the mixture, and the difference between a major and minor alteration. Submit your questions to [email protected] for a chance to be on the show. Full notes below: Chuck flies a 1941 Interstate Cadet with a Continental 85-8F engine. He had a prop strike four years ago, was inspected and got some new rings, gaskets, and a few other parts. He’s noticed he goes through a quart every four or five hours, compared to 15 hours prior to the overhaul. Mike suggests there may have been a problem when honing the cylinders, and Colleen suggests borescoping to see what the honing looks like now. Paul said a normal honing crosshatch is about 30 degrees, but that on a teardown they use a ball hone that doesn’t make a regular pattern. If it’s at all shiny it means they’re glazed. But ultimately they all agree it doesn’t matter and that it’s an acceptable amount of oil loss. Tom is a new mechanic who is looking for advice. A friend asked him to install a new stall warning horn to augment the stall warning light in his Cherokee. Paul suggests looking at a newer version of the airplane with a horn and a light, and submitting that information as approved data to the FAA. FAR 1.1 defines a major versus minor alteration. Advisory Circular 43-2B is acceptable methods for aircraft alterations, which Collen calls the bible of alterations. Mike makes the point that it’s a mechanic’s decision as to whether an alteration is major or minor. A major requires someone else’s signature, such as a field approval or FAA sign-off, while minor is only the mechanic’s signature. Jay flies a Mooney with M20F with an IO-360 with a JPI engine monitor. The oil temp probe is located near the first cylinder. On climb it hits 230F and trips the over-temp alarm. In cruise it stabilizes at 195F. The analog probe shows 30 degrees cooler. The hosts aren’t at all surprised that the probes are different. Colleen mentioned that when she changed engine monitors they moved the probe and her oil temps have gone down. Mike said our goal is to usually measure the oil temp at the coolest location. Peter has a Cardinal and is going through his first annual. He is wondering where the accountability is for previous mechanic oversights. His IA has found things like airworthiness directives that haven’t been complied with, a bucking bar left in the airplane, and so on. Mike makes the point that there is a difference between airworthiness discrepancies and things that are more discretional in nature. And all three hosts caution that each annual and pre-buy inspection are unique, and that it’s impossible to snag every discrepancy. Walter asks why we don’t just turn off the key when stopping a traditional piston airplane engine. The hosts agree that it is solely a safety consideration, and meant to get gas out of the system. That way an ungrounded mag can’t refire the engine and create a safety issue with the propeller. Rolf has a TB-20 with an IO-540 engine that’s never been overhauled and is trouble-free. Last year he noticed on the engine monitor traces that the number 1 and number 2 EGTs were very low at low fuel flow. His shop measured the dry tappet clearance and found the intake valves were well in excess of the limit. They were able to bring them within limits, but the EGTs are still bottoming out. Mike makes the point that the valve clearance limit is only to ensure that the amount of play in the system so that the hydraulic lifter can “take it up.” The hosts agree his engine is in fine shape, and not a safety issue. They do recommend he prepare for an overhaul in the future based on the symptoms.