Reading Terry Yingling's guide to writing terminal procedures, I got the impression that PMDG aircraft will search the AIRAC data for any fixes that are not locally defined in a FIXES/ENDFIXES section of the procedure. However, when I actually tried to write a small procedure of my own, it seems that the FMC never looks at the AIRAC data at all when reading procedures, and every single fix used by the procedure must be defined within the procedure. Is this true?? If so, doesn't that mean that any change to a navaid requires a change to every single procedure that references it? Seems awfully inefficient.

True, all fixes used by procedures must be defined in the FIXES section; however, you may use external references to navaids by using the keyword NAVAID; for example, TURN RIGHT DIRECT NAVAID LAX. The FMC will look for the definition of LAX in the wpNavAID file.This scheme makes sense if you assume, as I do, that only fixes defined in the wpNavFIX file are those used by routes. This also is convenient in that you don't have to worry about duplicate fix names for the fixes you might create as a user for handmade approaches.Navaids rarely change location - when they are moved they are usually renamed (VORs anyway).I used to use the NAVAID keyword for all VORs and Localizers, but have since started declaring all fixes that my procedures rely on within the file to ensure they always work.