Precache on start gametype e.g. maps/mp/gametypes/_hardpoints.gsc init()
precacheModel( "vehicle_f15" );

Change to setModel( "vehicle_f15" ) in the maps/mp/gametypes/_hardpoints.gsc doPlaneStrike() function.
// Spawn the planes
plane = spawnplane( owner, "script_model", pathStart );
plane setModel( "vehicle_f15" ); // change this
plane.angles = direction;

