I get many questions from pilots about how they can use an IFR-approved GPS while flying departures, airways, arrivals, and approaches that are based on or include navaids—VORs, DME, localizers—and sometimes even NDBs. The current guidance from avionics handbooks, instructors, and FAA publications, such as AC 90-108 and AIM 1-2-3, isn’t always easy to understand, and some details have evolved as new avionics have become available.
Note: FAA has released Draft AC 90-119 Performance-Based Navigation Operations, which updates guidance on this topic. When it is published, the new AC will also replace several existing AC and drive changes to the AIM and other sources of information.
The video presentation below takes a close look at the FAA guidance on the topic, including details in the draft AC 90-119 and uses specific examples to help you understand how you can use a suitable RNAV system–for most GA pilots, that means an IFR-approved GPS–to fly all or parts of departures, airways, arrivals, and approaches that are based on navaids.
For more information on this topic, see the following posts here at BruceAir:
- Draft AC 90-119 Performance-Based Navigation Operations
- Use of Suitable Area Navigation (RNAV) Systems on Conventional Procedures and Routes
- Setting the CDI on a Conventional Approach (The “Kill Switch”)
- Use of IFR GPS on Conventional Approaches
To see more presentations for pilots, check this playlist at my YouTube channel, BruceAirFlying.
BruceAir, LLC 
If 90-119 defines that you only need to be on the localizer (green needle) for the FAS, then I don’t have to switch the CDI source until the FAF. Is my understanding correct?
We have to see the language in the final, published version of the AC. But if it reflects the draft, then technically, yes, you only need the “green needles” for the FAS, the segment from the final approach fix to the MAP. In the real world, however, you will probably intercept the localizer outside the FAF–at least within the approach gate, so you’d want to ensure that the LOC is tuned, identified, and showing the correct course before you reach the FAF.