Is it legal to use GPS to navigate VOR Airways? A previous question concludes that it is legal to fly an aircraft in the US with only an IFR-approved GPS/WAAS system and no other navigation equipment on an IFR flight plan. My question is about how the pilot is then allowed to navigate.
Specifically, must a pilot fly on GPS (T and Q) Routes? FAA Advisory Circular 90-108 discusses using RNAV systems on conventional routes, so it seems that a pilot could also fly on VOR airways, for instance. In fact, 90-108 specifies exactly what a pilot can use their RNAV system for:

Determine aircraft position relative to or distance from a VOR, ... DME fix, ...
Navigate to or from a VOR,  TACAN, NDB, or compass locator.
Hold over a VOR, TACAN, NDB, compass locator, or DME fix.
Fly an arc based upon DME.

Navigating on a VOR airway involves navigating to and from VORs, which is covered by point 2. However, the list does not include navigating to a DME/radial fix off a VOR. After taking off IFR the departure controller may clear an aircraft direct to a fix on the airway e.g. &quot;cleared direct BOONE&quot;. Could a pilot legally use only GPS/WAAS to fly to BOONE or any other VOR radial/DME point?
It's legal if you have a suitable and acceptable RNAV system as described by FAA Rules.
Also realize that it is only RNAV capability that provides for 'direct routing' without recourse to DR - Dead Reckoning - or Radar assisted DR needed by traditional Radio Aid based navigation.
When an RNAV system navigates to an out of service VOR, it is flying a computed course from it's position vis a vis the VOR to the VOR position. This is no different from flying from any point 'A' to any point 'B'. That is the whole purpose of the Advisory Circular being quoted. A few more extracts from the AC (quoted below) should make it amply clear.

FAA AC 90-108 extracts . . .



PURPOSE. This advisory circular (AC) is intended for the following purposes:



a. Provides operational and airworthiness guidance regarding the suitability and use of Area Navigation (RNAV) systems while operating on, or transitioning to, conventional, i.e., non-RNAV, routes and procedures within the U.S. National Airspace System (NAS). This guidance material applies to two broad categories:



(1) Use of a suitable RNAV system as a Substitute Means of Navigation when a very high frequency (VHF) omni-directional range (VOR), distance measuring equipment (DME), Tactical Air Navigation (TACAN), VOR/TACAN (VORTAC), VOR/DME, non-directional radio beacon (NDB), or compass locator facility including Locator Outer Marker (LOM) and Locator Middle Marker (LMM) is out-of-service, i.e., the Navigation Aid (NAVAID) information is not available; an aircraft is not equipped with an automatic direction finder (ADF) or DME; or the installed ADF or DME on an aircraft is not operational. For example, if equipped with a suitable RNAV system, a pilot may hold over an out-of-service NDB.




(2) Use of a suitable RNAV system as an Alternate Means of Navigation when a VOR, DME, VORTAC, VOR/DME, TACAN, NDB, or compass locator facility including LOM and LMM is operational, and the respective aircraft is equipped with operational navigation equipment that is compatible with conventional NAVAIDs. For example, if equipped with a suitable RNAV system, a pilot may fly a procedure or route based on operational VOR using that RNAV system without monitoring the VOR.



b. Qualified RNAV Systems. Describes the types of RNAV systems that qualify as “suitable RNAV systems”



DEFINITIONS. For the purposes of this AC, the following definitions are provided:



b. Area Navigation (RNAV). A method of navigation which permits aircraft operation on any desired flightpath within the coverage of ground- or space-based navigation aids or within the limits of the capability of self-contained aids, or a combination of these.


g. Substitute Means of Navigation. The use of information from an RNAV system in lieu of that from out-of-service conventional NAVAIDs and/or inoperative or not-installed navigation equipment compatible with conventional NAVAIDs.