FYI. Choppers are a pain in the butt, so let me throw out a few hints that will save you some trial and error.
setspeedmode ONLY takes effect after the chopper starts another waypoint. So if he is on his way to a way point and you do set speedmode "limited" from a script, the chopper will not slow down until it starts its next waypoint.
If want you create waypoints with a script, use the "move" command. - like: chopper1 move [234,6664,0]
setspeedmode will NOT work with the domove or commandmove commands. You need to use waypoints, hence the move command. ;D