How to Spawn object only once

I have this plane that flies (doh' <img src="smileys/smiley9.gif" border="0" align="middle" /> )   

Want this plane to shoot only one missle when it closes within a distance of another object on map.

What I did is

Event ( system -> distance(plane.X,plane.Y,myobj.X,myobj.Y) < firedistance)   then   

Action ( plane spawn missile, ... missle set angle to plane.angle)

Now what this obviously does is .. as long as the distance is less .. it will keep spawning missiles.

I only want it to spawn once.

I tried to use "trigger once" .. but that makes only the first plane to fire a missle.. and the next ones dont.

Please advise :)

add "for each" condition to range check. should work

(

For each "Plane"

system -> distance(plane.X,plane.Y,myobj.X,myobj.Y) < firedistance)