That's an unfortunate setup. On my GNC 300XL setup the VOR/GPS switch
is overriden by tuning in an ILS freq. I can't display the GPS CDI when
an ILS freq is tuned.

I think it was an option during the installation but 'recommended'

Peter R. wrote:
Gary Drescher wrote:
Not at all suggesting that this was the case, but at our local flight
school that has all late model C172SPs equipped with B/K KLN-94 GPS's and
moving maps, a number of students over the last few years have been known
to forget to toggle the NAV/GPS switch from GPS to NAV when transitioning
from en route to ILS approach.

As you know, when this happens the VOR1 instrument would show the CDI alive
(but really tracking the GPS's course for the localizer). The glideslope
needle, however, would be flagged and remain perfectly centered, as if the
pilot were flying a perfect glideslope.