One thing I don't understand is why Rotax engines don't have mixture "knob" yet still can climb to 12000 feet while delivering reasonable power (could climb more) - I have been demonstrated this (to my amazement) in an ultralight (Eurofox). Obviously there is some kind of mixture control, else the amount of petrol would stop the combustion at FL100 air pressure (about 75% of sea level), but I'm not aware of any FADEC or similar to do the job.