Tecnam 912 service Bulletin to disable soft start

Rotax itself may not have much info on this but the information in the TECNAM service bulletin is EXACTLY what I have experienced several times in the past 10 years on my RANS S7 with the 912ULS.  I could never quite figure it out and it was very intermittant but it is exactly as described in this bulletin.  Engine starts normally, with softstart disengaging, runs fine, runs UP fine but at some point in the flight the soft start reengages and when I land and pull back the throttle the engine idle RPM drops several hundred RPM and almost dies.  If I shut the engine off and restart it, the engine starts fine and runs normally as the soft start logic is reset.  It may be dependent on specific engine configurations or specifc possible electrical interference and it is hardly noticable at cruise RPM but definately something that exhibits itself exactly as outlined in the TECNAM service bulletin.

Hi Mike

Rotax released the following Service Bulletin in response to issues reported with Tecnam aircraft and the release of Tecnam's own SB (see attached).

Take a look at page 22. Section 3.10 – Ignition System: Easy-Start Activation, Trigger Coil Gap, and Temperature Influence.

Let us know if you have any questions after reviewing it.

Well for what it’s worth, whether applicable to this issue or not, one should bear in mind it’s possible to inadvertently adjust the 912 carburetors during a synchronization such that it starts and idles well, takes WOT just fine, returns to idle from low to mid range power applications, BUT- a rapid reduction to the idle stop from higher power settings (in my case it was above 5,000 rpm) will cause the engine to stumble or even quit. It will then restart as normal as well.

Jim Isaacs wrote:

Well for what it’s worth, whether applicable to this issue or not, one should bear in mind it’s possible to inadvertently adjust the 912 carburetors during a synchronization such that it starts and idles well, takes WOT just fine, returns to idle from low to mid range power applications, BUT- a rapid reduction to the idle stop from higher power settings (in my case it was above 5,000 rpm) will cause the engine to stumble or even quit. It will then restart as normal as well.

Can you elaborate on how this can happen and how it can be avoided?

I caused this to happen with a combination of too low, too lean idle speed and idle mixture screw settings during mechanical synchronization, and the off-idle adjustments I subsequently made masked this until discovered during my ground run post-maintenance operational tests. I now accept my sooty spark plugs as just part of the Rotax design and leave the mixture screws alone after confirming they are 1.5 turns out from fully closed, and I’ve stopped trying to guess what the final step of mechanical synchronization -“true running of the engine” - actually means or how to find it or measure it. As long as you leave the mixture screws at 1.5 turns out and set idle to at least the minimum listed in the OM of 1,400 rpm- I set mine to 2,000 rpm fully warmed- and do not reduce the idle speed screws further during the synch process what happened to me should not occur at the fault of the carburetors. Always perform a full operational check on the ground after synchronization to include smooth and brisk throttle input engine response from idle to maximum static rpm (as in a go around or balked landing) and maximum static rpm back to idle rpm (as in performing a simulated engine failure, for example).