"When it starts to talk to you it's time to replace it"

Ask the A&Ps

How do you know when to replace certain parts? They begin to talk to you. Find out what, why, how, and when when Mike, Paul, and Colleen tackle this and other questions. Email [e-mail] for a chance to be on the show.

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Full notes below:

John is part of a club that owns a Cessna 172. Another member told him he shouldn’t move the rudder by hand when he preflights. Mike said to make sure he pushes it at a rivet line to ensure he doesn’t bend the sheet metal, but otherwise it’s ok. Paul describes how the system is built. He said in a Skyhawk the rudder pedal tension is created by springs that pull them toward the firewall. There’s not a cable tension spec. The steering bungees are between the rudder pedals and the nose gear. If you get in and push the pedals when stationary, you are putting stress on the bungees.

Jarek wants to know if the FAA has lost its mind. He is wondering about the FAA’s AC 20-105, which says that engines that run past TBO have a higher rate of failure, and that engine overhauls at TBO are usually cheaper than those that are run beyond. Mike said Lycoming had previously issued a white paper that said that the typical lifespan of a crankshaft was 14,000 hours. Camshafts have corrosion issues, and don’t cause failures, and sometimes gears need to be replaced. But rarely does time impact safety. There are people inside the FAA who disagree with the policy, Mike says. On one hand they require manufacturers to set a TBO. On the other hand, they tell Part 91 operators that they can ignore it. Typically engines that fail that have run beyond TBO tend to be not related to age, but because of maintenance resulting from the fact that the engine is older. In fact, it’s usually the opposite, Mike says. Lower time engines typically fail because of infant mortality.

Aaron and a partner recently purchased a Seneca with turbo Continental TSIO-360-KBs. They installed new intercoolers and the manual revision suggests new power settings. He’s ignored the chart and is flying around 30 inches lean of peak. Mike said the chart is interesting if you are operating rich of peak, but irrelevant lean of peak. The power output of the engine is determined by whichever component is in shortest supply. Rich of peak, you have more fuel than you need, so power is controlled by air through the throttle and the prop. Lean of peak, fuel is in shortest supply, so power is controlled through the mixture. He would run a slightly reduced throttle, just to avoid overworking the turbos.

Rory has a Cardinal that he bought 12 years ago, and he hadn’t ever changed the ignition wires. So he started to wonder about when and why to change them. The hosts agree that if a wire fails it feels like a stumble or mis-fire because it just takes one spark plug offline. They say to run them on condition, and only change them when they start talking to you through mis-firing.