Try something like this:
Make sure the plane has a name, say, aPplane, then add this to the beginning of the camscript:
aPplane setvectorup [0,0,1];
apPlane setpos [(getpos aPplane select 0), (getpos aPlane select 1), 200];
_vel = velocity apPlane;
_dir = direction apPlane;
_speed = 100;
apPlane setVelocity [(_vel select 0)+(sin _dir*_speed),(_vel select 1)+ (cos _dir*_speed),(_vel select 2)];
Now the vector up of the plane should be level, at a height of 200 meters, and given a little "push" of speed to make sure it keeps on flying even if the player was just about to crash it or whatever.
Try it!
Wolfrug out.