CNS-80 VNAV

After upgrading to software version 2, the CNS-80 offers a new
display page, resembling an HSI with a VDI pointer at its left.
The unit's computed VDI presentation is active for many GPS
approaches, as well as for all LNAV and LPV approaches.

However, I have not (yet) found in the pilot's manual any
explanation of when VDI drive is withheld from an external HSI.
By flying several different kinds of approaches, I seem to see
that the external VDI will be active *only* when VNAV minima apply.
Only on precision approaches having DH instead of MDA,
my autopilot could arm and capture the glideslope,
exactly the same as an ILS.
On non-precision approaches my external VDI needle remained
driven out of view, even though its glide-slope flag was pulled,
and as a result I could not arm the glide slope for automatic capture.

(Unexplainably, the CNS-80 annunciated LNAV/VNAV
on approaches which actually were non-precision LNAV.)

I'm guessing that the external VDI drive was withheld precisely to
prevent glideslope capture on coupled non-precision approaches.
I don't know the reason behind this, or even if that's the rule.

Of course, LNAV coupling works for all approaches, as expected.
---JRC---