"Don't split the case halves"

Ask the A&Ps

A light oil mist on his windscreen has one caller concerned he needs to split the case. Plus, Mike, Paul, and Colleen tackle cold cabins, tire changes, and horsepower calculations. Email [e-mail] for a chance to get on the show.

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Full notes below:

Grant has a Mooney M20F and he’s seeing a light mist of oil on the windscreen after flying. His mechanic thought it was coming from a case bolt, which he re-torqued. That didn’t solve the problem, and the mechanic said the proper way to fix it would be to split the case, which Grant doesn’t want to do. Paul said that’s not the problem anyway. He said misting oil on the windscreen is almost always the crankshaft seal or the prop o-ring, both which should be fixed together. The parts are cheap, and the job is relatively easy. Colleen just did this on her airplane and describes the process.

Joe’s kids are freezing in the back seat of the family Cherokee. They’ve put tape around some gaps, and while it’s helped a little, it’s still very drafty. Paul mentions the spar carry-through gap between the cabin and the wing. There’s supposed to be a piece of foam in the gap, but often when the wings are removed the foam isn’t replaced. The floor is lifted up, and you can look in the gap with a flashlight and mirror to see if the foam is there. The rear spar also has a gap where there should be rubber discs glued onto the box where the flight controls pass through.

Todd is curious how JPI derives the horsepower number. Mike said JPI keeps the math as a closely held secret. But he said there’s a right way to do it, and it's how Savvy does it. Power is regulated by air or fuel, whichever is in shortest supply. Rich of peak, you have more fuel than the engine can combust, so air is in short supply. Mass airflow is therefore the power determinate, which can be calculated with manifold pressure and rpm. MP is how much air goes into the cylinder, and rpm is how often that happens. Multiplying the two gives the mass airflow number. Power when lean of peak is a function of fuel flow. Fuel flow times a set number based on the compression ratio gives horsepower. He thinks the engine monitor doesn’t know whether it is rich or lean of peak, so it probably computes it both ways, and the lower number is the right one.

Seth has some dry rot on his tires and he’s wondering if that means he needs to change thems. Desser says they need to be replaced when the cord is showing, and his mechanic says the dry rot means it should be changed. The hosts agree with the manufacturer that there’s no minimum tread depth, and to keep going.