"John Harper" wrote:
..
My understanding is that the only difference between
APP and NAV modes is whether the GS coupling
is enabled.

Nope. On an S-TEC 50, the only difference between APP and NAV is the
degree of sensitivity to CDI deflection: APP is more sensitive. The 50
does not do VNAV; there's now way it could "pick up the glide slope."

In my airplane, I set the S-TEC 50 to APP and the KLN-90B GPS CDI output
to 1.0 mile. This setup keeps me on course with a cross track error of
50'.
--
Dan
C172RG at BFM