AC 90-108 Explained: Using RNAV in Place of Ground-Based Navaids

Can your GPS legally replace that VOR on your next instrument approach? The answer depends on more variables than most pilots realize. FAA Advisory Circular AC 90-108—active and unchanged since 2011—defines exactly when a suitable RNAV system may substitute for ground-based navaids on conventional routes and procedures. Yet six distinct misconceptions about GPS substitution persist in pilot training literature, and the most dangerous one involves an approach segment where GPS substitution is explicitly prohibited.

AC 90-108 is not a regulatory summary you read once and forget. It is the foundation for a practical decision framework that every instrument-rated pilot should internalize—one that governs when RNAV substitution is legal, where the hard limits are, and what equipment qualifications apply. This article builds that framework for you.

What Is AC 90-108 and Why Does It Matter?

The full title of AC 90-108 is “Use of Suitable Area Navigation (RNAV) System on Conventional Routes and Procedures.” It is an FAA advisory circular that provides operational and airworthiness guidance for using RNAV on conventional, non-RNAV routes and procedures within the U.S. National Airspace System. The current version is Change 1, dated March 3, 2011—over 14 years of stable, active FAA RNAV guidance that has not required revision.

The core concept is precise: AC 90-108 allows a suitable RNAV system to serve as a substitute or alternate means for certain legacy navaid functions. It does not grant blanket permission to ignore ground-based navaids. The distinction matters because the allowance depends on the specific procedure, the phase of flight, equipment capability, and any limitations stated on the chart or in NOTAMs.

What makes AC 90-108 worth studying carefully is its specificity. RNAV substitution is a controlled operational tool—not a universal override. Understanding the boundaries protects you from compliance violations and, more critically, from misapplying GPS in situations where conventional raw data is required.

What RNAV Substitutions Does AC 90-108 Actually Allow?

The answer is structured around specific navaid functions—not broad categories. You are authorized to use a suitable RNAV system to replace defined ground-based navaid functions on conventional instrument procedures, but only within the approved use cases below.

Approved Substitution Use Cases

• Fix identification: You may use RNAV/GPS to determine your position relative to a fix defined by a VOR, DME, or NDB—using GPS in lieu of VOR, GPS in lieu of ADF, or DME substitution to identify crossing fixes, stepdown fixes, or intersection positions on conventional routes.
• Route navigation: You may use RNAV to navigate to or from a VOR or NDB when flying conventional airways or published routes, provided the RNAV system is suitable for the route’s accuracy requirements.
• Holding: You may hold over a fix defined by a VOR, NDB, or DME intersection using GPS-derived position instead of raw conventional navaid data.
• DME arcs: You may use RNAV to fly a DME arc on conventional instrument approach procedures.
• Distance information: You may use GPS-derived distance in place of a DME readout. If you are flying an ILS approach and need DME for a stepdown fix, GPS-derived distance may identify that fix.

In every case, the charted procedure remains a conventional procedure. It does not become an “RNAV approach” because you used GPS to identify a fix or track a route segment. Pilot in command responsibility includes verifying that each specific substitution is permissible for the procedure being flown.

The Localizer Final Approach Restriction

GPS/RNAV substitution is NOT approved for lateral guidance in the final approach segment of any localizer-based approach. This is the most misunderstood rule in AC 90-108, and it carries the greatest safety risk.

The localizer provides precision lateral course guidance calibrated to the runway environment. Its angular sensitivity narrows as you approach the runway—GPS lateral guidance does not replicate this behavior. If you are flying an ILS or LOC approach, you must track the localizer on final. Your GPS moving map does not satisfy this requirement.

However, you may still use RNAV to identify fixes or substitute for DME/ADF functions on those same approaches. The prohibition is specific to lateral course guidance in the final approach segment—nothing more, nothing less.

This restriction is consistent with both FAA AIM guidance and EASA GM1 NCO.IDE.H.195(a), which contains a parallel prohibition and similar approved use cases—reflecting international alignment on this safety boundary.

Equipment Requirements: Does Your GPS Qualify?

AC 90-108 requires the RNAV system to be “suitable” for the intended substitution. Having a GPS in the panel does not automatically make every substitution legal. Equipment suitability is the pilot’s responsibility to verify before invoking any RNAV substitution.

Your IFR navigation database must be current—an expired database invalidates the substitution. This is a compliance requirement, not a suggestion.

TSO-C145 and TSO-C146 designate WAAS-capable GNSS receivers, which provide the broadest authorization for RNAV substitution and WAAS GPS approach operations. Older TSO-C129A equipment may require RAIM prediction before departure and carries more limited authorization for certain operations. If your aircraft has a TSO-C129A receiver, confirm RAIM availability for the time and location of intended use.

RNAV accuracy standards illustrate why not every GPS unit qualifies for every use case:

• RNAV 2: 2 NM total system error at 95% confidence—typical for en route operations.
• RNAV 1: 1 NM total system error at 95% confidence—required for terminal area procedures.
• RNP APCH: Typically RNP 0.3 NM on final approach with SBAS—achievable with TSO-C145/C146 WAAS-capable avionics.

Required accuracy tightens as you move from en route to terminal to final approach. A suitable RNAV system for en route substitution may not meet the standard for a terminal-area DME arc. Verify equipment capability against the specific phase of flight.

RNAV Substitution vs. RNP Authorization: Know the Difference

RNAV (Area Navigation) is the ability to fly any desired flight path within the NAS without requiring direct overstation passage of ground-based navaids. RNP (Required Navigation Performance) builds on RNAV by adding onboard performance monitoring and alerting—the system notifies the crew when navigation accuracy degrades below required standards.

Not all RNAV use qualifies as RNP. AC 90-108 governs RNAV substitution on conventional procedures. It does not authorize you for an RNP approach or any procedure requiring specific RNP operational authorization. These are different categories within the performance-based navigation (PBN) framework.

Some advanced RNP operations—such as RNP AR approaches with curved flight paths—require specific FAA operational authorization, crew training, and aircraft certification beyond what AC 90-108 provides. Using RNAV substitution to identify a fix on a conventional VOR approach is fundamentally different from flying an RNP AR procedure. Conflating the two creates both compliance and safety risks.

Common Misconceptions About GPS Substitution

Six documented misconceptions about AC 90-108 and GPS substitution persist in pilot training literature. Here they are, corrected:

1. “GPS can replace any navaid at any time.” False. RNAV substitution is limited by procedure type, phase of flight, and explicit guidance limitations. Each substitution must be individually verified as permissible.
2. “Using RNAV substitution means the approach becomes an RNAV approach.” False. A conventional approach remains conventional even when RNAV is used as a substitute for a supporting navaid function. You fly it as charted.
3. “You can use GPS on the localizer final instead of tracking the localizer.” False. GPS substitution is prohibited for lateral guidance on the final approach segment of any localizer-based approach. The localizer must be tracked using conventional raw data.
4. “RNAV substitution and RNP are the same thing.” False. RNAV provides area navigation capability. RNP adds onboard performance monitoring and alerting. Different standards, different authorizations.
5. “If the airplane has a GPS, substitution is always legal.” False. The equipment must be suitable for the task—not merely present. Database currency, TSO certification, and RAIM availability all factor into suitability.
6. “Substitution removes the need to monitor the conventional aid.” False in many cases. When the procedure requires conventional guidance—such as the localizer on final—raw data monitoring is mandatory regardless of GPS availability.

Scenario-based training is the most effective way to close these knowledge gaps. CTS course offerings in IFR operations and RNAV procedures use scenario-based training to build lasting proficiency in exactly these distinctions.

Practical Decision Framework: When Can You Substitute?

Use this checklist during preflight planning or in the cockpit when considering RNAV substitution on a conventional route or instrument approach procedure.

1. Identify the navaid function you need to replace. Be specific: VOR fix identification? DME arc? ADF-defined hold? The answer determines whether substitution is an option at all.
2. Confirm it is an approved substitution use case. Cross-reference against AC 90-108 and AIM guidance. If the function is not on the approved list, stop here.
3. Verify your RNAV equipment is suitable and your database is current. Check TSO certification, RAIM availability (if TSO-C129A), and database expiration date. Equipment must be suitable—not merely installed.
4. Check the approach plate or chart for notes, restrictions, or requirements. Some charted procedures include notes that prohibit substitution or require conventional raw-data monitoring for specific segments.
5. Check NOTAMs for relevant restrictions. Temporary restrictions or navaid outages may affect the legality or safety of your planned substitution.
6. Confirm the substitution does NOT involve lateral guidance on a localizer final segment. This is the single most critical check. If you are flying a LOC or ILS approach, GPS does not replace the localizer on final.
7. Maintain awareness that the procedure remains conventional. Fly it as charted. Use charted altitudes, courses, and missed approach procedures. RNAV substitution supports—it does not replace—the charted procedure.

Frequently Asked Questions About AC 90-108 and RNAV Substitution

What does AC 90-108 allow pilots to do with GPS on conventional routes?

AC 90-108 permits pilots to use a suitable RNAV system as a substitute for specific ground-based navaid functions—including VOR fix identification, DME arc navigation, holding over a defined fix, and GPS-derived distance in lieu of a DME readout—on conventional instrument routes and procedures, subject to equipment suitability and procedure-specific limitations.

Can I use GPS instead of a VOR on an IFR approach under AC 90-108?

Yes, with conditions. Under AC 90-108, GPS may substitute for a VOR to identify fixes, navigate along a conventional route, or fly a DME arc. However, GPS may not replace the localizer for lateral guidance on the final approach segment of any ILS or LOC approach. Each substitution must be individually verified as approved for the specific function and phase of flight.

Why is GPS not approved for lateral guidance on the localizer final segment?

The localizer delivers angular course guidance that narrows progressively as the aircraft approaches the runway threshold. GPS lateral guidance does not replicate this behavior. AC 90-108 explicitly prohibits RNAV substitution for localizer lateral guidance on final to preserve the precision and runway-specific alignment that the localizer system provides.

What RNAV equipment is required to use GPS as a navaid substitute under FAA rules?

The RNAV system must be “suitable” under AC 90-108—meaning it must hold an appropriate TSO certification (TSO-C145/C146 for WAAS capability, or TSO-C129A with RAIM verification), maintain a current IFR navigation database, and meet the accuracy standard for the specific phase of flight (RNAV 2 for en route, RNAV 1 for terminal). Equipment presence alone does not establish suitability.

What is the difference between RNAV substitution and RNP authorization?

RNAV substitution under AC 90-108 allows a GPS-capable system to replace specific ground-based navaid functions on conventional procedures. RNP (Required Navigation Performance) is a separate framework that adds mandatory onboard performance monitoring and alerting capability. RNP operations—especially RNP AR approaches—require distinct FAA authorization, crew training, and aircraft certification beyond what AC 90-108 provides.

Staying Current as the NAS Evolves

The National Airspace System continues transitioning toward RNAV/RNP-centric operations under the broader performance-based navigation (PBN) framework. Ground-based navaids are being decommissioned in some areas. Legacy navaid substitution knowledge remains essential—both as a backup skill when GPS service is degraded and as a compliance requirement when flying conventional procedures.

AC 90-108 Change 1 has been active since 2011. That stability marks it as foundational, evergreen FAA RNAV guidance—not a temporary policy awaiting replacement. Internationally, EASA GM1 NCO.IDE.H.195(a) provides parallel substitution concepts, making this knowledge relevant for pilots who operate across regulatory boundaries or train in mixed-regulatory environments.

The operational imperative is clear: structured, scenario-based training is the most effective way to build and maintain competence in AC 90-108 RNAV substitution decision-making. Knowing what you may substitute, what you may not, and what equipment qualifies is not academic—it directly affects every IFR flight.

Build Your RNAV Substitution Knowledge with Structured Training

CTS offers Part 135 and IS-BAO/Part 91 training programs that cover IFR operations in depth, including RNAV procedures, conventional navigation, and regulatory compliance. These programs reinforce the kind of decision-making framework outlined in this article—through scenario-based training designed for working pilots and operators. If you want to formalize your AC 90-108 RNAV substitution knowledge and sharpen your instrument proficiency, explore what CTS has to offer.