Guys,
There exists a breach between the FAA and ICAO on this issue. Initially, when the FAA started the "GPS overlay" approvals, they required the underlying approach navaid to be tuned and monitored. This requirement went away in FAA World sometime ago, if the FMS meet TSO-specific requirements as an approach aid, the need to tune, monitor or even to have the underlying navaid was eliminated. The ICAO and EU countries, as I understand it, have kept the requirement to tune and display the underlying navaid, unless it is a RNAV approach.
In my plane, we can fly any NPA, using FMS approach, without the navaid tuned or displayed, as long as "APP" or "GPS APPR" is annunciated before the FAF. This is common in the US, but not so in many regulatory regimes.
This from FAA AC 90-94, 1994
(1) Phase I. This phase ended in February 1994, the date when
the FAA declared GPS operational for civil operations.
(2) Phase II. This phase began on February 17, 1994 when the FAA
declared the system suitable for civil operations. Certified GPS
equipment can be used as the primary IFR flight guidance to fly an
overlay to an existing nonprecision approach without actively
monitoring the applicable navaid(s) which define the approach being
used. However, the underlying ground-based navaid(s) required for the
published approach must be operational and the associated avionics
must be installed and operational. The avionics need not be operating
during the approach if RAIM is providing integrity. Pilots can tell
that Phase II applies because "GPS" is not included in the title of
the approach.
(3) Phase III. Phase III began April 28, 1994, when the first
instrument approach procedures were published to include "or GPS" in
the title of the published approach procedure. Neither the aircraft
traditional avionics nor the underlying ground station navaid(s) need
be installed, operational, or monitored to fly the nonprecision
approaches at the destination airport. For GPS systems that do not use
RAIM for integrity, the ground-based navaid(s) and the airborne
avionics that provide the equivalent integrity must be installed and
operating during the approach. For any required alternate airport, the
traditional ground-based and airborne navigational equipment that
defines the instrument approach procedure and route to the alternate
must be installed and operational.
This authorization is expressly limited to FAA regulated airspace. Also, the TERPs standard used to designed the approach was that for the ground-based navaid, not RNAV. So, one can fly a VOR approach, with approved equipment, as an FMS approach without use of the ground-based navaid in US airspace. It will not be an RNAV approach, it will be a VOR standard approach flown using FMS.
GF