I was merely using the "oriented plane" to extend the notion of a two-dimensional trajectory into three dimensions. In fact, all the coordinates are being calculated parametrically as a function of time, so by no means does it have to be a plane. In fact, if you add something like wind that affects the X and Y coordinates (remember Z is affected by gravity), then you will get a three dimensional curved trajectory.

So to answer your question, it was always being solved in true 3D Here''s where I think I might have confused you - it''s just that the X and Y velocities were calculated such that the trajectory was planar. But by all means, they don''t have to be calculated as such. If you think of wind as a "gravitational" force that can act on X and Y, then you can get all sorts of wierd trajectories going