Guidance on using RNAV systems, what the AIM refers to as GPS (non-WAAS) and WAAS, is scattered around various publications. AC 90-108 addresses using GPS in place of equipment that you probably no longer have in your panel—ADF and DME—and for determining fixes from cross-radials. Basically, you can use GPS to determine distances and fixes on an approach but you may not use it as the sole means of lateral guidance for a localizer based approach or VOR approach past the FAF. The exception is when the approach is labelled “… OR GPS”.
NOTE: This AC does not address the use of RNAV systems on RNAV routes and RNAV terminal procedures. The current edition of AC 90-100, U.S. Terminal and En Route Area Navigation (RNAV) Operations, applies to those operations. This AC also does not address the use of RNAV systems on instrument approach procedures (IAP) titled, RNAV (GPS) and GPS. The current edition of AC 90-105, Approval Guidance for RNP Operations and Barometric Vertical Navigation in the U.S. National Airspace System, applies to those operations.
7. USES OF SUITABLE RNAV SYSTEMS.
a. Usage of Suitable RNAV Systems. Subject to the operating requirements in this AC,
operators may use a suitable RNAV system in the following ways.
(1) Determine aircraft position relative to or distance from a VOR (see first note in subparagraph 7b), TACAN, NDB, compass locator (see second note in subparagraph 7b), DME fix; or a named fix defined by a VOR radial, TACAN course, NDB bearing, or compass locator bearing intersecting a VOR or Localizer (LOC) course.
(2) Navigate to or from a VOR, TACAN, NDB, or compass locator. (3) Hold over a VOR, TACAN, NDB, compass locator, or DME fix. (4) Fly an arc based upon DME
8. USES OF SUITABLE RNAV SYSTEMS NOT ALLOWED BY THIS AC.
An otherwise suitable RNAV system cannot be used for the following:
a. NOTAMed Procedures. Unless otherwise specified, navigation on procedures that are identified as not authorized (“NA”) without exception by a NOTAM. For example, an operator may not use a RNAV system to navigate on a procedure affected by an expired or unsatisfactory flight inspection, or a procedure that is based upon a recently decommissioned NAVAID.
b. Substitution on a Final Approach Segment (FAS). Substitution for the NAVAID (for example, a VOR or NDB) providing lateral guidance for the FAS.
c. Lateral Navigation on LOC-Based Courses. Lateral navigation on LOC-based courses (including LOC back-course guidance) without reference to raw LOC data.
There is an update in the AIM which allows to fly the final approach segment or VOR, TACAN or NDB approaches with GPS lateral course guidance provided that the underling navaid is monitored.
AIM 1−2−3. Use of Suitable Area Navigation (RNAV) Systems on Conventional Procedures and Routes
2. These operations do not include lateral navigation on localizer−based courses (including localizer back−course guidance) without reference to raw localizer data.
5. Use of a suitable RNAV system as a means to navigate on the final approach segment of an instrument approach procedure based on a VOR, TACAN or NDB signal, is allowable. The underlying NAVAID must be operational and the NAVAID monitored for final segment course alignment.