Gearbox Problems 912iS

Thanks for the information. We always index mark them when we disassemble them. It is obvious a problem with either this Prop + Engine + Gearbox setup. I am not totally blaming Rotax but why did they not just use the Gearbox out of the Carb versions (912uls). Its alway a pity you couldn’t just bolt on a 912uls Gearbox. I have seen a couple of Tow Plane (Glider Tugs) Gearboxes using the same DUC Windspoon Prop with  high hours (1500-2000 hrs) and there is very little wear on the Dogs. Having worked on the 912iS for the past 7 years I don’t think it is a poor design and it should run smoother than it does. This is our 2nd aengine, the first one did 1240 hrs, although it ran Ok but we were advised by Rotax that a change of engine might cure some of the problems we were having. The 2nd Engine has done 1400 hrs and mechanically it is still Ok, good compressions and no metal in the filter. The Electronics is rubbish with failures of Ignition Coils, Spark Plug Caps and ECU’s. The heat in the engine bay probably doesn’t help, its well cooled in flight but its after landing waiting for the next Sailplane (Glider) to Tow when it heats up.

Hi Malcolm

Good to know you index the parts. The issue with the wear as you see it is nothing to do with the prop or engine, it is the parts themselves.  

The 912 ULS, and any of the other carb versions with an overload clutch,  have a 30 degree freeplay on the dog to gear.  This helps with the initial torsional movement in starting or shut down.  The problem Rotax had was that with the new ignition in the injected engines they have 2 crankshaft position sensors, CPS, mounted on the ignition housing.  The proximity of one to the other made it impossible to use the 30 degree freeplay.  What was found was the only way to get the engine to start they needed to remove the freeplay altogether.  This is why the injected engines have a zero degree freeplay and all you can do is remove the gearbox at periodic times as shown in the MML and you no longer could do a friction test on the gearbox spring wear inside.  The result was a noticable rougher start on the iS model engine from the standard gearbox.  

It gets a bit more confusing in that because of the new dog with zero freeplay the splines on the propshaft were not getting enough lubrication.  In effect we used to see seizer of the dog (inside the overload clutch to shaft) to the splines due to a lack of lubrication.  Some 5 years ago Rotax developed the spray system inside the gearbox to solve this issue and it has worked very well.  

The original secondary coils on the iS, mounted on the intakes, had crap connections that would vibrate and loosen at low hours.  The solution was a new coil and connector.  Any engines however that have vibration these areas still suffer so they need to have constant monitoring.  The location is also an issue for some installations.  Extra heat, heat soak while sitting on the ground, can cause the temperatures to go well above the range they can handle.  Be aware to have some way to  vent heat from that area.  The newer PVL caps seem to work well so far so hopefully that will be OK.

Ground heating, relative to what you might have seen in the old carb versions, is most definitely higher.  The injected engines do not have fuel in the intake manifolds or airbox, therefore they always will run hotter on the ground running while you are waiting to go.  Many OEM have installed small 12V cooling fans like we all have in cars and trucks these days.  The small weight gain is worth the price to allow longer ground running with temps in better control.  

Good luck on your engine,, hope some of this  helps.

Cheers mate

“The result was a noticeably rougher start on the iS model engine from the standard gear box”. 

Early on I had instances where starting the engine was rough enough to displace a spring held exhaust joint.  The manual states the throttle should be advanced 2-3 cm for starting, but also states that should be about 55% throttle position.  There is a complete graph that shows throttle position vs starting behavior.  I was using the 2-3 CM method of setting the throttle for starting.  I later realized my flight display (G3X) has a small TPS indication at the top of the engine strip.  Before starting it indicates throttle position and after start that changes to just an ECO mode indicator.  It is designed this way to assist in proper engine starting for the iS. I have found a significantly smoother start if I use 60% throttle when cold, and 50% when warm.  On my plane this takes much more throttle movement than 2-3 CM.

If you get this settings correct, you will have to REDUCE the throttle immediately to 2100 RPM when the engine fires.  Remember, there is no choke on the iS engine, but the more you advance the throttle the longer fuel injector pulses are, so this makes the mixture richer. You are essentially commanding the ECU to add more fuel.

I think this is relevant to this conversation because much of engine wear happens in the first second or two of operation when there is little oil pressure.  I think this also applies to the gearbox, and it seems logical that a rougher start would be harder on the gearbox.  

Thanks, this all makes sense and it is the first time anyone has written down an explanation. We have gone back to a 0-360 Lycoming powered aircraft. We would consider a Rotax Carb Engine but we would never touch an injected one. We do 300-400 hrs a year so we do hammer them. Although we will keep a close eye on the 915iS engined tow planes, there are approx. 5 in the UK and more in the pipeline.

Thanks,

Malc

Just a note.  They type 915 and 916 have a completely different design in the gearbox.  They absorb the torsional moment with a torsion shaft and not a dog and ramp system like the others.  The allowable prop mass inertia is also much higher so it can accept bigger props. 

Cheers