Jose wrote:
In order to be approved as an IFR installation the 430 has to drive an
external CDI or HSI where the G/S will be displayed.


What is the reasoning behind this requirement? (especially in light of
glass panels cockpits)

Jose

A glass panel cockpit is different than a non-glass cockpit with a GPS
box that is typically mounted off to the side. The external CDI or HSI
is in the pilots primary field of view. Plus, some approved GPS devices
have marginal displays at best. The Garmin 500/530 has a very good
display but there is no requiement that you select that map page. Also,
if that map page became your primary display you would then be unable to
select a different page during the approach, such as the flight plan page.

A full-press glass cockpit, however, separates functions where
appropriate and integrates them when appropriate. Thus, the primary GPS
box in this case is like an FMS screen in a transport aircraft and the
display array in front of the pilot is the "super HSI, moving map, ADI"
that more than satisfies the separation requirement from the primary
database box and the tactical display.