The flight director is only giving guidance based on the MCP/nav modes-
LNAV, VNAV, VORLOC, G/S(APP), etc
The NPA is only bringing you to a position from which you can complete a visual manoeuvre to land.
If, for example you were following a VOR radial that is very much offset from the runway course, the lateral FD command would be contradicting the actual need to fly towards the runway. So it is now providing bad information. Get rid of the FD.
If you were descending in VS for example, to achieve MDA, once visual with the runway, your vertical profile needs to be consistent with the PAPI or whatever visual cue is needed to touch down in the right place. So the pitch FD that you were using is no longer relevant and may be providing contradictory information. Get rid of the FD.
When you hit the toga buttons, now the FD pops up again to achieve commanded pitch for the V2+ speed, and limited bank to 400' in the initial go around manoeuvre.
I think that covers most of it, with the usual caveats.
Last edited by Shiny side down; 7th Dec 2012 at 03:48.