Rotax 912 Gearbox

In spite of having my gearbox serviced at 600 hours the splines still wore out at about 800 hours. It was giving really bad vibration when throttling back. So in my case servicing on time did not save me from a big bill (~£1000). I have never let the engine idle too low and the carbs have always been in balance so there is no reason for my splines to have failed. The Rotax specialist who repaired my gearbox said that he had seen quite a few with failed splines.

I think that once your splines have started to wear it is too late, there is nothing you can do other than replace the parts. I don't know what the other failure modes are for the gearbox and in particular the ones that could be hastened by not servicing your gearbox on time.

Gearbox wear is caused by any number of things.  Prop overmass for the applicaiton, low idle, carb balence just to name a few.  On gearboxes fitted with the overload clutch, type 912 or type 914, there is a way to check this with the friction torque value you should do periodicly.  If low it shows the spring washers internally are worn and need replacement.  In my view adding shims to worn spring washers just is a band aid, I like to replace them and this can occur anywhere from 400 to 600 hours depending on the aircraft and how you fly it.  The injected engines you simply follow the time line for basic maintenace.  

The question is how to tell on engines without overload clutch and in between that arbitrary number you find in your maintenance manual.  The answer is very simple, listen to your engine.  On start and on shut off the engine has to do a dance inside the gearbox to absorb the tortional moment of the prop fighting the crankshaft for a smooth run.  This is due to the power pulse of the pistons from slowing down on compression to acceleration on power stroke.  The prop wants to keep moving once started, one of the Newtons laws working here, so we need to protect the parts from breaking.  The good part is once you get fast enough the tortional effect is gone and you don’t have to worry about the wear.  Low idle speeds are the most responsible for your damages.  Heavy props and low idle will kill the parts inside faster than anything.   if at your selected idle speed you hear that dance inside, speed it up.  If you have to go beyond reasonable idle speeds of say 1600 or 1800 RPM to get a smooth and quiet run, it suggests that you have worn springs inside.   Listen to your engine, if it is dancing at your idle speeds then you should address this by getting it re-shimmed and serviced.  

Rotax calls out for a MIN 1400 RPM idle.  This is the extreme low end of the idle range.  In my opinion it is better at 1600 or 1800, provided you are OK on approach to the runway at that RPM and have brakes that work.  Don’t worry too much on approach as the prop is loaded and the tortional issue is predominantly on the ground and not as speed.  Remember, never idle the engine in the range of the dance, it will kill the gearbox.  This same approach works for the injected versions except the type 915.  The 915 has a completely different method to damp the torsional moment in its gearbox.  Cheers

Thought i was on the over mass island by myself . Owned /maintained  several 9 series propelled aircraft  ,always noticed  the low rpm "rattle". Assuming this rpm range was hostile to all parts beyond the firewall particularly parts in front or forward of the prop flange  ,I would avoid allowing the rattle range at all times and really had made peace with this particular quirk that the rotax has. A billionaire friend of mine purchased  a new Stemme S12 a couple of years back, which is a fantastic aircraft  with a turbo charged 914 (a great engine as well). As a cfi and AP I fly and maintain the aircraft accordingly. The stemme  has what I have identified as a low rpm BACKLASH.  During the inflight factory provided training ,the instructors train the new billionaire owners to keep the idle rpm until oil temp is out of the yellow range. The metallic banging is horrific .During the training to attempt to smooth out the backlash with increase rpm would be reprimanded / not allowed.  The huge 7lb external clutch ,external alternator, 7' drive shaft to turn a very complex 600 part prop assembly  is a  huge over mass  issue coupled with bad training is taking its toll on the fleet. The external clutches are failing  inflight which causes the rpm  to go nuts and teardown inspection follows. With the past the flange weight tripled than other aircraft ,than on the stemme , so dose the damage.   The original reason given for allowing the backlash was to prevent unfiltered oil from bypassing to enter the oil galleys during low temps.  The good news is most Stemme owners dont own screw drivers , just call me when its fixed ( i wouldn't own a screwdriver either if I had that kinda money). To conclude the sermon- heavier the prop assembly the larger the backlash damage. So go forth my rotax brethren and backlash no more.

If like me, you're coming from airplanes with Lycoming or Continental (Ly/Con) engines, idling at 1800 rpm seems a bit, well, odd at best.  But when you stop and think about the 2.43:1 gear reduction ratio, the prop is turning at 740 rpm, whereas most of the old Ly/Con engines idled at 800 rpm...  Idling at 1400 rpm, the prop is only turning 576 rpm.  No wonder it gets rough in that range!  IIRC, the Rotax guidelines say to only use idle speeds below 1800 rpm for seaplanes, and only if idling at "normal" speeds causes problems with water-handling.

I have to keep reminding myself that even at max rpm (5800), the prop on a Rotax 912/914 is only turning 2,386 rpm, which is a lot lower revolutions than "cruise" settings for the old Ly/Con engines.  And at my typical 5300 rpm cruise, the prop would only be turning 2181 rpm.  

I just love this engine!  Starts quickly and easily, sips gas, and feels a LOT more powerful than the O-200 I had in my last airplane.

Hi,

Many thanks for all your comments and explanations.

Rotax Wizzard: quote: "the engine has to do a dance inside the gearbox to absorb the tortional moment..." and "Listen to your engine, if it is dancing at your idle speeds then...".

Please excuse my ignorance - but what do you mean with "dance"?