Try this out:-
>>>>>cut>>>>>>
goto "init"
#loop
~20
? !alive _plane: _plane = "a10" createUnit [_pos,grpplane1,"",1.0,"captain"]; _plane setdir 0; _plane setvelocity [0,300,0];_plane flyinheight 100
goto "loop"
#init
_plane = _this select 0
_obj = _this select 1
_pos = [getpos _obj select 0, getpos _obj select 1, 300]
goto "loop"
>>>>>cut>>>>>>
name the above script "planespawn.sqs" and save it into the mission folder.
change the "a10" to the class name of whatever plane you want (retaining the brackets).
enter the following in the init line of the starting plane in the editor:
>>>>>cut>>>>>>
grpplane1 = group this; [this, planespawn] exec "planespawn.sqs"
>>>>>cut>>>>>>
place an object on the map where you want the planes to spawn at and name it:-
planespawn
The planes will spawn 300m above this point with an initial velocity of 300km/h in a northerly direction.
You can test the script by standing near the planespawn object and getting your original plane to fly into a trigger set to detect all with:-
this setdammage 1
in the init field. (don't move into the trigger area and set it so your original plane doesn't start in it either). You should get a new plane within 20 seconds of the demise of the first.
see how this goes.