Thomas Borchert wrote:

Sam,


When you
select a LOC or ILS, you instead receive an advisory message cautioning
you that RNAV is for monitoring only.



Yes, but you can (and should, per the manual) activate them. So the
question remains, what exactly does that do? In my experience, it starts
both sequencing of the waypoints in the procedure and the change in CDI
sensitivity. And from my experience, same goes for an RNAV approach. The
430 just continues to fly the original flightplan if you don't make it fly
the approach - by activating it.



I agree that the manual says to activate a previously loaded approach.
But, it is overkill and unnecessary.

En route to terminal sensitivity occurs when passing within 30 miles of
the airport reference approach, whether or not an approach is loaded.,
*provided* the destination airport is the final waypoing in the flight plan.

Once flying an approach it will activate (approach sensitivity and
approach RAIM) when reasonably on course and passing inbound 2 miles
prior to the FAF.

You can hit the "activate approach" option all day long once on an
approach flight plan and inside the IAF, and nothing will happen until
you reach 2 miles from the FAF. And, the same will happen at 2 miles
from the FAF without ever pressing the "activate approach" option.

Try it in the trainer on any RNAV (GPS) IAP (or overlay IAP).