I have been trying to add an autothrottle to a turboprop aircraft and finally ran across the solution. I did not create it on my on but only found the solution at a glance from someone else's suggestion from somewhere. In order to have the autothrottle work on a turboprop aircraft like the Saab 340, you have to modify one or two lines of code in the aircraft.cfg folder. Change this line to autothrottle_max_rpm=7000 located under the [Autopilot] listings. I have checked this solution on other aircraft and it works quite well, certainly better than none at all. It may or not be necessary to enable the autothrottle arming required. 0 for not required and 1 for required.