912S Worn Prop Shaft

Hi Daniel. 

You should have found in your annual/100 inspections what the gearbox friction torque was.  Normally if low the gearbox must be taken off and re shimmed regardless of hours.  1000 hours on the gearbox without any checks would be excessive especially given that your aircraft are fitted with constant speed propellers.  The type of load in the gearbox with constant speed would most certainly damage the splines if the gearbox was worn and below the friction setting it needs to be.  Check your manuals and see what the recorded friction torque numbers were in the log books, this will tell you when it is starting to go bad.

There is one simple way to know if you need to reshim a gearbox without going to all the trouble of checking friction torque.  Simple get the engine warm and then bring it all the way down to idle speed, say 1600 crankshaft speed.  if you can hear the gearbox "clatter", that metal sound that happens when the dog and ramp system is trying to absorb the torsional moment between the prop and the crankshaft, then you have to reshim it.  At idle there should be no abnormal noise or banging inside the gearbox.  That type of noise should disappear just after start up,, if not you are wearing the gearbox parts excessively.

Cheers

Hi Daniel,

I have had the same problem even though I had my gearbox serviced at the correct intervals including having new shims fitted. It was a very expensive repair, about £1000 for a new prop shaft and dog gear. When I saw the wear on the old parts I was surprised how bad it was. The Rotax service centre who repaired the gearbox for me said that they were seeing an increasing number of worn prop shafts.

I don't know what caused the problem with mine since it has not been left idling too low. I've never run it on AVGAS, it has always had MOGAS. Perhaps there is a material problem or an oil problem. I had no symptoms until about 10 hours before it was bad enough to do something about it. Perhaps something happened to the surface treatment of the part.

Kevin

Hi Kevin & RFSC,

Thank you for your experiences sharing. 

Indeed we've doing the Friction Torque each 100H and the trend checked is that the Frition Torque started with 40 ft/Lb and start reducing until reach 24 ft/Lb with 900H after the 1000H inspection.

We only use AVGAS and we operate our aircraft in Flight Training environment with a lot of runway circuits per day.

Talking with a UL A/P with a lot of experience I was advised to disassemble and wash the gearbox each 500/600H to prevent this phenomenon but my question is if the engine parts are prepared to work with AVGAS 100% of the time!

Thank you for any comments/suggestion you may have.

Cheers,

No, the Rotax 912 series engines are not designed to use mainly AVGAS, they are designed to run on unleaded MOGAS. That said, Rotax have published information on the actions to be taken when running on AVGAS. The high lead content of AVGAS causes some problems in the 912 and you will require additional service and maintenance compared to an engine run on MOGAS.

There is a useful guide here with some photos showing the consequences of running on AVGAS https://www.yumpu.com/en/document/read/3935997/looking-after-your-rotax-912-series-engine-conair-sports-ltd

Daniel, if you have let the friction torque get down to 24 ft-lbs, that is very low, and you should certainly be expecting problems.  

If you run 100LL full time, you should definitely take the gearbox off and have it internally inspected/overhauled after 600 hours.