Tecnam 912 service Bulletin to disable soft start

Rotax released this Service Bulletin (Attached) on November 22, 2024, to address issues caused by not following the proper procedures outlined in the Installation Manual, and it specifically mentions the Advanced Start Module (ASM) system on page 22 of the SB. 

Although the Service Bulletin does not explicitly mention it, this SB was released in response to issues related to Tecnam aircraft.

On the Rotax-Owner website, you can find Rotax Service Bulletins by navigating to the Support / Bulletins tab on the top menu bar and selecting Service Bulletins.

It's important to regularly check for updates on these types of bulletins to stay informed about critical safety and maintenance information.

Thank you Rotax Owner, I forgot that was about the soft start problems Tecnam were having.  

Cheers

Note.  Flight training where someone actually turns off the engine in-flight will result in the soft start to activate is my understanding.  That should be avoided.  

Rotax Wizard wrote:

Thank you Rotax Owner, I forgot that was about the soft start problems Tecnam were having.  

Cheers

Note.  Flight training where someone actually turns off the engine in-flight will result in the soft start to activate is my understanding.  That should be avoided.  

Why? I shut down a 912S with soft-start modules quite often in my motorglider (otherwise, it wouldn't be a motorgliider).. With the engine stopped, a re-start is no different than on the ground, actually easier since the airflow helps the starter turn the engine.

Perhaps one could have a problem with a pure windmill start. The old modules shift timing based on rpm, so a diving start is no problem (and I've done it several times) since the timing is retarded appropriately. The new ones depend on a trigger from the starting circuit, so diving only would start the engine with the normal 26 degree advance. Perhaps that could cause an issue and should be avoided. But even if the battery to too dead to turn the starter, it takes like 3V to retard the timing, so pushing the start button plus diving should start the engine safely too.

I once encountered an issue with the soft-start engaging in flight. It was a Bristell, I think, with a 912is. The owner had rerouted some wires and somehow was getting enough interference in the "starter" wire to trigger the retarding function. 

Page 23 of SB-912-079 has important information: "Intentional activation of the easy-start function is only possible when a minimum voltage of 2.8 V is applied at the advanced start terminals and they must therefore be grounded when not intended to be activated."

When we deactivate the soft start: If soft start inputs must be grounded, then I think that these inputs are not sufficiently pull-down and/or not filtered properly. Otherwise, it would not be specifically stated that they must be grounded.

Let's consider the cable between the start relay and the soft start input. The end connected to the start relay is grounded because the relay's coil impedance is low. However, if the soft start input is not sufficiently pull-down, it may activate the soft start when an induction voltage exceeding 2.8 volts occurs on it. This is because the cable will act like an antenna.

The fact that Tecnam's bulletin states that the problem is observed at high speeds also supports the argument I tried to explain above. At high speeds, the generator produces a higher voltage than at lower speeds. This results in more electromagnetic pollution and creates more induction on the cable connected to the soft start.

The solution may be to use a grounded shielded cable between the soft start - start relay. If this is not enough, it may also be appropriate to connect each soft start input to ground with a suitable (pull-down) resistor or by reducing the input sensitivity with voltage divider. If the high frequency signals induced on the cable are also causing the problem, this can be also filtered. I could only come up with so many ideas without knowing the internal circuit of the modules.

In reality, there may not be such a problem in electronic modules. The problem may be due to Teknam's usage, assembly or the parts it uses. My comments, ideas and solution suggestions above are my hypothetical thoughts on the problem Teknam stated in its relevant bulletin.

However, considering that it is not possible to start the engine under 10 volts of battery voltage, I believe that the threshold value of 2.8V selected for soft start is low. If the soft start input had been designed with a threshold value such as 5-6 volts, I think the inputs would have been more resistant to external effects.