Substituting GPS for Ground-Based Nav

For a couple of decades now, the FAA has been moving the National Airspace System to a GPS-based concept. VOR approaches and airways are disappearing, and RNAV GPS approaches and airways are proliferating. In many cases, you can’t hardly fly IFR without GPS. For example, where I mostly fly in the Northeast Corridor, the Victor airways that were the core of the airspace structure (especially the Tower Enroute Control, or TEC, system) are nearly completely gone, replaced by a system consisting almost entirely of GPS-based T-routes. Even the underlying VORs (like Salisbury, Maryland’s SBY) have been decommissioned.

However, there are still many cases where you may find yourself using approaches that rely in some part on ground-based navaids like VORs, DMEs, and even NDBs. While fewer people might fly IFR with only GPS, I think most folks dispensed with their ADFs and DMEs when they installed an IFR GPS, and new production planes generally don’t come with them for U.S. ops. This then presents questions about how and when you can use GPS to substitute for those legacy ground navaids.

The AIM Says

AIM 1-2-3c. permits the use of an IFR-approved GPS for the following uses:

1: Determine aircraft position relative to, or distance from a VOR (see NOTE 6 below), TACAN, NDB, compass locator, DME fix; or a named fix defined by a VOR radial, TACAN course, NDB bearing, or compass locator bearing intersecting a VOR or localizer 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.

This means you can use your GPS to fly DME arcs on approaches, and substitute for a second VOR on approaches which say “DUAL VOR REQUIRED” or to use the lower minima on approaches with step-down fixes identified by VOR cross-radials, DME distances, or NDB bearings. If we look at the Springfield, Missouri (KSGF) VOR or TACAN RWY 20 approach, you can without an actual DME use your installed IFR GPS in the plane to fly the initial segment DME arcs and identify the 6.5 DME missed approach point. You can also use your IFR GPS on the HFD LDA RWY 2 approach to identify DANNS for the lower “DUAL VOR” minimums.

On the other hand, the one thing you are not allowed to do is substitute GPS for lateral guidance on the final segment of an instrument approach (FAF to MAP). On a VOR approach that doesn’t have “or GPS” in its title, you must have an operable VOR with a current 91.171 check and the course displayed somewhere you can see it during the final segment. So, on that SGF VOR approach, you must have real VOR data in view from the SGF VOR to the MAP. But after about 25 years of searching, the only approach I’ve found in the United States which uses DME for lateral guidance on the final segment is the VOR or TACAN Rwy 15 approach at Martin State Airport in Baltimore, Maryland (KMTN). This approach appears to be unique in that it is constructed entirely of DME arcs from the IAF to the missed approach hold, and I’ve always believed it to be the result of a bar bet among those who design approaches for the FAA.

Setting up the Panel

One work-around to obtain the stabler guidance you get from a GPS for the final segment lateral guidance requirement on a VOR approach is to have your VOR active on a second display such as a bearing pointer on an electronic HSI or on a second CDI head while your primary lateral display (HSI or CDI) displays the GPS data. Of course, with most GPS equipment, having GPS rather than VLOC selected for display on the device driven by the GPS will produce a “Select VLOC on CDI” message, which you cannot make go away without selecting VLOC. Also, since bearing pointers don’t work on localizer signals, for an LOC approach, you’d need a second CDI to display the LOC data.

For en route and terminal operations, there are no restrictions on using GPS to navigate VOR-based airways or procedures (like SIDs/STARs). You need only load the route in the GPS, even if the underlying navaid is inoperative. Current GPS models including the Garmin GTN 650/750 and Avidyne IFDs make use of the airways in the database, so you need only load the airway entry point, select the airway, and then select the exit point. For older units like the Garmin GNS WAAS, which do not have the airways in their database, you will have to load all defining VORs and each intersection where the airway bends. For example, for V469 from Elkins, West Virginia (EKN) to Lynchburg, Virginia (LYH), you would load EKN, BRUCY, and LYH. The other points along the route (BOIER through LUCLI) need not be loaded.

Got a Current Database?

An important related question is whether your database must be current. In the AIM 1-2-3a.2., the FAA permits the use of outdated databases as long as the user confirms that the data being used have not changed since the database expires. For direct-to en route navigation, this would involve pulling up the waypoint data from the database (the WPT function in Garmins) and comparing it to the lat/long data for that point in the Airport/Facility Directory section of the current Chart Supplement.  For procedures like SIDs, STARs, and approaches, this would involve comparing the database expiration date in the lower left corner of the procedure sheet to the expiration date of the database.

However, that general guidance is superseded by limitations in the system’s Flight Manual Supplement. For example, the King KLN 94 AFMS says for instrument approaches, “The KLN 94 aeronautical database must incorporate the current update cycle.” Thus, if your database is expired, you probably aren’t allowed to do GPS-based approaches. On the other hand, none of the GPS FMSs I’ve seen prohibit the use of outdated databases to fly terminal procedures like SIDs and STARs provided the amendment data on the current chart is earlier than the expiration date of your database. So, if your database expired on Feb. 19, 2026, you could use your GPS to fly the CSHEL EIGHT DEPARTURE at KFMY last amended Jan. 22, 2026, but not if your database expired Dec. 25, 2025. And don’t confuse the current edition effective/expiration dates on the sides of the charts (e.g., “19 MAR 2026 to 16 APR 2026”) with the amendment date at the bottom left corner (e.g., “25JUN23”).

GPS for Equipment Checks

Another question is whether you can use the GPS to perform the 91.171 VOR checks. This is an issue for the many pilots whose aircraft has only a single VOR unit. Since the FAA eliminated all the airborne VOR receiver checkpoints a few years ago, and many of the VORs that provided ground checkpoints have been decommissioned, pilots of such aircraft may find themselves without a published checkpoint or VOT to perform the check of a single VOR installation. It would be nice if you could use your GPS as the second half of a dual crosscheck, but the FAA doesn’t permit that due to large differences in station declination from current magnetic variation. You may have to make your own airborne checkpoint using a sectional chart to find a “prominent ground point” on a blue airway radial IAW 91.171(b)(4). When flying an LDA approach—that’s the LDA to Runway 2 at Hartford Brainard Airport charted below—confirm that you’ve tuned the right frequency and confirm the approach is in service with the ATIS.

More on instrument training and certification: It is now possible to complete instrument training, instrument rating certification, and an IPC with nothing but a single WAAS approach capable GPS in the airplane. (I’ll leave the discussion of the wisdom of actually flying serious IFR with nothing more than that for another day.) Recent policy changes consider the various subtypes of RNAV(GPS) approaches, including LNAV, LP, LNAV/VNAV, and LPV, to be different approaches for the purpose of the 14 CFR 61.55(d)(2)(ii) long IFR cross-country, and for practical tests and IPCs under the ACS. 

Thus, an instrument rating practical test may consist of one LNAV and one LP approach for the two non-precision approaches, and an LPV with a HAT of 300 or less for the precision approach. They would have to be three different approaches (e.g., MJX RNAV(GPS) RWY 6 as LPV with its 250-foot HAT for precision, and N12 RNAV(GPS) RWY 6 as LP plus N12 RNAV(GPS) RWY 24 as LNAV for the two non-precisions). In some cases where the approach you want to use for LNAV has an LP option, you might have to disable the SBS function in your GPS to force an LNAV-only operation. However, you would not be allowed to fly the same approach once as LP and then again with SBS disabled in LNAV mode as the second non-precision.

Look for an upcoming report on budget IFR equipage strategies in AvBrief’s Smart Aviator.