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Thread: gear box rebuild cost

  1. #11
    Senior Member
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    Default Re: gear box rebuild cost

    Quote Originally Posted by jiott View Post
    As further clarification of my "on condition" statements:
    First, Greg is correct in that the manual states 1000 hours (not the 800 I was recalling) for 912ul/uls with overload clutch and less than 30% use of 100LL.

    My use of "on condition" maintenance comes from closely following Mike Busch's excellent webinars on the EAA website. This guy is a nationally recognized and awarded AP/IA mechanic and owner of a large maintenance company for mostly certified aircraft. He is a strong proponent of "on condition" maintenance where appropriate. First of all it doesn't apply to ac in commercial use, and doesn't apply to items likely to catastrophically fail like rubber and hose and belt replacement. The items it does apply to are things that rarely fail catastrophically, but start giving warnings well ahead of complete failure, like cyl heads, piston rings, carbs, many bearings, etc. Don't take my word for this but watch his webinars in the EAA webinar archives. "On condition" is not for everyone, but requires either a knowledgeable hired mechanic watching things, and/or an owner that is very comfortable mechanically and knows the tests and warning signs to watch for.

    In my own case I feel the gearbox is one of those items that will give warning (sure there is a small risk of sudden failure as is true of most items) if you pay attention and know what to look for; this also true of carburetors in my opinion. That said, I will definitely replace rubber parts near the 5 year recommendation, tear down the carbs, maybe even overhaul the gearbox during this convenient downtime since it will be near the 1000 hours. Everything has to be done with common sense rather than blindly following a book, although following the book is never bad. I believe following Mike Busch's philosophy gives the best combination; after all, going "on condition" may cause you to repair some item far sooner than the book says.
    Jim, I am familiar with Mikes webinars. My recollection is that he recommends "on condition" overhaul or repair when there is a reliable method of inspection, test and or data analysis to determine if a non critical componeent can likely go longer than manufacturers recommendation for overhaul or repair.

    Personally (read - my opinion), I don't think "smoothness" qualifies as an inspection or test. Further the Rotax requirements for both the gearbox and carbs are to "check" i.e. inspection. It does not require replacing parts unless out of spec. It is an inspection - which Mike promotes.

    Additionally, if any of you have an SLSA, Mikes opinion is that nothing applies except what the manufacturer states. Go here: https://www.savvyaviation.com/home/r...ikes-webinars/ and search for the webinar "TBOs and other maintenance intervals" at 45:23 he discusses this topic.

    To be clear, these are my opinions of what works for me, I am simply encouraging other readers to do their own due diligence as to what works for them and not me or Jim or even Mike.

    Each of us has their own tolerance for risk and how we approach risk management. Do what works for you, you are ultimately responsible for your decisions!

    YMMV, Greg

  2. #12
    Senior Member jiott's Avatar
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    Jun 2010
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    Default Re: gear box rebuild cost

    It is interesting that SLSA's are required by the FAA to follow manufacturer's recommendations for maintenance intervals. It is a weird anomaly in the FAA rules that Mike Busch thinks is strange and not reasonable. Certified ac are not restricted like that. Also, I don't believe Mike promotes "inspections" that require total disassembly if there is no reasonable indication to do it, even if maintenance manuals recommend it.

    Regarding Rotax gearboxes with overload clutch, I do faithfully do the clutch friction torque test/measurement every 100 hours. In addition I disagree with Greg and strongly believe smoothness during starts and shutdowns and at low idle, as well as sounds and any kickbacks can tell a lot about the health of the gearbox. My own 700 hours and discussions with Rotax trained mechanics as well as posts on this forum seem to all indicate that the gearbox nearly always gives some of the above indications well ahead of total failure. I would welcome anyone correcting my thinking on this who has had actual experience otherwise. In all respect, Greg I am not aware that you have ever flown behind a Rotax 912 or ever built or maintained a Kitfox; If I am wrong about this I apologize.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

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