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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:

  1. Determine aircraft position relative to or distance from a VOR, ... DME fix, ...
  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.

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. "cleared direct BOONE". Could a pilot legally use only GPS/WAAS to fly to BOONE or any other VOR radial/DME point?

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    $\begingroup$ Can we stipulate that the discussion assumes that it is an IFR-Approved GPS in view? Clearly, the answer would change if navigating by a handheld or iPad GPS. $\endgroup$
    – Ralph J ♦
    Jan 20, 2021 at 2:20
  • $\begingroup$ In fact, an airplane without RNAV capability cannot easily navigate "direct to BOONE" or any VOR radial/DME point without resorting to Dead Reckoning. $\endgroup$
    – skipper44
    Jan 20, 2021 at 3:19
  • $\begingroup$ The question would benefit from clarifying that it is about IFR flight. $\endgroup$
    – quiet flyer
    Jan 20, 2021 at 13:42
  • $\begingroup$ Whether or not you can fly a Victor airway itself using GPS, there's nothing preventing you from entering in the specific fixes and navaids that make up the airway and flying over those. Two differences: 1) You will be flying be reference to coordinates rather than radio beacons, and 2) you probably won't be allowed the full width of a Federal airway (eight miles wide or more). $\endgroup$
    – randomhead
    Mar 10, 2021 at 0:08

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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 . . .

  1. 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”

  1. 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.

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Yes, it is legal to use a GPS to navigate Airways. As you point out, one simply enters two VORs in the GPS flight plan, and then the GPS will guide you from one VOR to the next.

How do they join the airway? A radar controller gives you headings to fly to get you onto the airway. If you are on an instrument flight plan, and you are flying from a major field, they almost always have radar facilities. They give you an initial heading to fly when you take off, and then as soon as you transition from the tower frequency to the departure frequency, they tell you that you are "radar contact", and from there, they will give you a heading to fly (vectors), or they will tell you to fly directly to a VOR or more likely when you are near the field, to a GPS waypoint. But if you filed a flight plan using VORs and airways, it is likely that they will vector you to a point where you can join the airway, or they will vector you to a VOR.

Must they first navigate to a VOR, or can they actually navigate to a nearby fix on the airway, as is conventional? It just depends on the airspace and the way the waypoints and VORS are set up. You may be vectored to a VOR, you may be vectored to a fix, or you may be vectored to a point where you will cross an Airway, and then be told to join the airway at that point.

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  • $\begingroup$ Thanks for the answer! To clarify: I am familiar with taking off and hearing "cleared direct BOONE" or something similar to join an airway. I just want to know why it's legal to fly to BOONE or some other point based on a VOR radial/DME using a GPS. $\endgroup$
    – asb1230
    Jan 20, 2021 at 1:47
  • $\begingroup$ It is legal because BOONE is easily identifiable using GPS. It is also aGPS waypoint based on Lat/Long coordinates. A GPS database will be filed with these positions. Also, the GPS database will be filled with the Lat/Long coordinates of other GPS waypoints that overlay other airports, radio NavAids, intersections, etc. Those coordinates are extremely accurate. As long as the GPS satellite signals are accurate, you have an accurate means of navigation without actual Nav radios. That is why it is important to verify RAIM before flying IFR using RNAV. $\endgroup$
    – Dean F.
    Jan 20, 2021 at 7:20
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Legal is a hard word to define in aviation. It would be better to ask whether an operation was within regulations.

It depends on your type of operation, kinds of operation, and certificate requirements. If flying under 14 CFR Part 91, the pilot would specifically be concerned with the MEL of the aircraft they are flying, and Part 91.205. This leaves the the operator free to fly IFR with only a panel-mounted GPS to fulfill 91.205(d)(2)

(2) Two-way radio communication and navigation equipment suitable for the route to be flown.

Any other requirements for navigational equipment used in flight specified in FAA AC 90-108 is more in the realm of a very strong recommendation, rather than a regulation.

The AC referenced in your question borrows it’s verbiage from the AIM section 1-2-3. AIM, in its entirety, is non-regulatory.

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