Originally Posted by
jiott
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.