You can make a plane fly higher by using the setpos command and setting the height part higher, the big problem is it won't turn as it needs to drop a wing to do so and the script won't allow this.
If all you want is to fly in a straight line use this:
_obj = _this select 0
_height = _this select 1
#loop
_objpos = getpos _obj
_objx = _objpos select 0
_objy = _objpos select 1
_objz = _objpos select 2
_obj setpos [_objx,_objy,_height]
goto "loop"
and call it with this [unitname,500] exec"scriptname.sqs"
I wrote this from memory as I don't have OFP on this PC so it may not be perfect.