PHP Code:
//Assume you have leader 'plane' and you want to setPos 'wingman'.
_dist = 150; //Distance from leader plane
_offdir = 90; //Relative direction from leader. (90+ here)
//Find compass direction to spawn from leader plane
_compassDir = ((getDir plane)+_offdir) mod 360;
//Find absolute coordinates by adding relative to leader plane
_newX = (getPos plane select 0) + (sin _compassDir * _dist);
_newY = (getPos plane select 1) + (cos _compassDir * _dist);
_newPos = [_newX, _newY, (getPos plane select 2)];
//Move wingman plane.
wingman setPos _newPos;
Not tested but pretty sure it would work.
Don't forget to give the plane velocity so it don't drop to the ground.
If you want to know the math more in depth then of interest is: "unit circle", "sin" and "cos", in general basic "trigonometry".