Why is 915iS max RPM not electronically limited to 5800?

I believe it is because in order to be certified by FAA an engine cannot have anything on it that limits power output.

Does not Rotax prohibit in the Manual running engine above 5800 RPM? How is it different from electronically limiting engine RPM at redline?

It would just add un-needed complexity.

Would you also limit it to 5 minutes at 5800 RPM?

If you needed just another 30 seconds at 5800 to clear the trees at the top of a ridge are you going to be happy with the engine automatically throttling back???  

And then how long at 5500 rpm before allowing another 5 minutes at 5800 RPM?

Do we really need to open this can of worms?

An Un-Advertized RPM Limit is that you can run at 6200 RPM for up to 1 minute.  See LMM Section 05-50-00, Page 17.

Most aircraft have ground adjustable props that only reach 5800 rpm in a dive where the pilots normally throttle back anyway.

Adding an adjustable prop removes the LSA status of the aircraft then requiring a Private License with a 3rd class medical.

The pilot needs to take responsibility for the operation of the aircraft and not rely on automation.

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Having said all that; Maybe flashing the Lane lights alternating A and B would be a reasonable Overspeed Warning.

In theory it could limit it to a much higher RPM setting like 7000 RPM that would never be seen in flight even in the worst conditions which could protect the engine from over-revving if it's started without the prop or a spinner. The ULPower UL350iS (it's another pretty light engine with fuel injection, except it's direct drive and wet sump unlike Rotax and its heavier) has a limiter for those purposes and I'm sure it would be easy for Rotax 915iS to do that as well. It might even have that already for all I know (I didn't check). From a software point of view it would not add any undue complexity.

But limiting it to 5800 RPM would be a bad idea like Bill said. Software limits that keep an engine healthy might not keep a person healthy. That's a reason I still enjoy simpler carbureted engines is that they don't have FADEC so I can push them a bit harder. If my engine is overheating but I need to clear some trees then I want to be able to push it to the breaking point. In my perfect world there would be a switch to turn off the FADEC and go fully manual even if it means harming the engine, just for emergencies or if a sensor fails at an inopportune time. But I know that in the real world those kinds of failures are so rare that they don't really matter.

Update: The engine does have a speed limiter. I don't know what RPM it engages at.