All approach procedures stored in your Garmin GPS nav database are created using GPS waypoints and courses. Some of these are “legal” GPS approaches and some are not, and can only be used for monitoring. Those airport approaches that use ground based equipment to define the courses, such as non-directional beacons, ILS localizer and glideslope antennas, or VOR stations, cannot be executed using your GPS receiver for sole navigation unless there is an official “GPS overlay” of those approaches. The title on those approach plates must have GPS in it, such as “NDB or GPS-B”. For those, you can fly them either way -- using GPS waypoints and courses, or using the signals from the ground equipment to drive your CDI. Let the controller know which one you’re doing by how you request it.
Those approaches without this legal overlay have “unofficial” GPS overlays of them. If you call up one of those approaches you’ll get a message that it is for monitoring only. What you are monitoring is the course error for the “GPS version”, as I call it, for that approach. So, if you want to do an approach using ground based equipment you must instead monitor the needle(s) that show the course error based on signals from that equipment. For NDB, VOR, or ILS approaches the primary navigation instruments (respectively) are the ADF needle, the VOR driven CDI needle, or the LOC/GS needles on a CDI or HSI.
But suppose you want to fly the “GPS version” of those approaches? How can you do that legally? Very simple – monitor the correct needle (above) for the particular approach but use the CDI that’s driven by the “GPS version” of the approach to drive your autopilot. Clearly your autopilot is flying the GPS version in this case, but if you’re monitoring the raw data from the ground equipment for any course decisions you are legal. For a VOR or LOC/GS, that requires a second CDI, driven by the ground signals, since the first CDI is driven by the “GPS version”. For an NDB approach you need an ADF indicator, and if you have one you’ll probably be much better off flying the GPS version.
When do you have to monitor the primary data? It’s required only during the approach phase, between the FAF and the MA. All courses leading up to the FAF, as well as the missed approach course, are in TERM phase. GPS navigators approved under TSO C-129 or C-146 are legal for ENR and TERM phase operations (for secondary and primary navigation respectively).
You can do an ILS this way too, if you’re willing to hand fly the vertical while your autopilot uses the GPS version for the horizontal (in NAV or GPSS mode). The idea here is to eliminate the scalloping often found on the LOC course from multipath interference.
In general, use the “GPS version” of all approaches until you’re a few miles from the FAF. At that time, change autopilot modes from GPSS to APR/ALT for VLOC approaches, unless you want to continue with the GPS version and can simultaneously monitor the raw data for that approach. For GPS approaches continue to use GPSS mode unless there is vertical guidance for the approach. Then, switch to APR and select ALT to arm the GPS glideslope. Change back to GPSS for the missed approach after your climb is established.
If, like me, you have a digital autopilot with GPSS and GPSV modes, you can use that on GPS approaches with vertical guidance, since the WAAS version of the 430/530 produce both pitch and roll commands on the approach.