Tach cable replacement
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We recently replaced the engine in our Cessna 172 after an “Inspection and Repair as Necessary.” A few test and breakin flights later, upon starting the engine, the tachometer did not register. Our mechanic thinks it is the cable. The mechanic is unavailable for a couple of weeks, so we are wanting to get the cable and have it ready to go when the mechanic returns. I am running into more questions than answers! First, it seems that those cables are sold by length, and I cannot find them listed by aircraft make and model. Well, OK, we can probably deal with that (though we aren’t going to remove the cable so that we can get a truly accurate measurement.) Next, most of the cables are not “FAA PMA approved.” How does that work? I thought that just about everything on a non-experimental airplane had to have FAA approval. Thanks for any enlightenment on that issue.

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Univair tachometer cables are PMA approved: https://www.aircraftspruce.com/catalog/inpages/univair-tach-cables.php?clickkey=4551 . I don't know if they have one for your 172. You aren't prohibited from using non-PMA parts as long as the Type Certificate or Supplemental Type Certificate doesn't require them. See https://www.faa.gov/documentlibrary/media/order/8110.42c.pdf . However, PMA approval is a good thing in that it generally means a higher quality part that is subject to FAA scrutiny of the design and construction. A PMA part must meet or exceed the standards for the original (OEM) part. A TSO part must only meet the minimum FAA standards established by the TSO.

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Andrew - Thank you for that information. It is very helpful.