AC 90-108 is the FAA advisory circular that tells you when a panel-mounted IFR GPS can stand in for a ground-based navaid on conventional routes and instrument procedures. The current version, Change 1, has been active since March 2011 and applies to operations under Parts 91, 121, 125, 129, and 135.1Federal Aviation Administration. AC 90-108 – Use of Suitable Area Navigation (RNAV) System on Conventional Routes and Procedures – Change 1 Short version: a qualifying RNAV system can replace VORs, DME, NDBs, and similar navaids in most places, but not on the final approach segment of a conventional approach and not for lateral guidance on a localizer course without reference to raw localizer data.
What the Circular Covers
AC 90-108 applies to conventional routes and procedures only. If the approach chart title reads VOR, ILS, LOC, or NDB, this is the document that governs how you may use GPS on it. If the title reads RNAV (GPS) or GPS, a different advisory circular, AC 90-105, applies to that procedure. RNAV routes and RNAV terminal procedures fall under AC 90-100.2Federal Aviation Administration. AC 90-108 – Use of Suitable Area Navigation (RNAV) Systems on Conventional Routes and Procedures
So AC 90-108 does not authorize you to fly GPS approaches. It authorizes you to use a suitable RNAV system while flying procedures that were built around ground-based navaids.
What Counts as a Suitable RNAV System
Not every GPS in the panel qualifies. Change 1 requires an RNAV system whose equipment is authorized under TSO-C129, TSO-C145, or TSO-C146 (any revision), installed in accordance with AC 20-138 or AC 20-130A, and approved for IFR en route and terminal operations. Systems previously qualified for GPS in lieu of ADF or DME also meet this standard.3Federal Aviation Administration. AC 90-108 Change 1 – Use of Suitable Area Navigation (RNAV) Systems on Conventional Routes and Procedures
In practice, TSO-C129 is the basic IFR GPS with RAIM. TSO-C145 and TSO-C146 are the WAAS receivers. A handheld or portable GPS does not meet the installation requirements no matter how good its reception is. The navigation database must also be current: expired coding means the waypoints and procedures in the box may not match the published chart, and the system is no longer suitable.2Federal Aviation Administration. AC 90-108 – Use of Suitable Area Navigation (RNAV) Systems on Conventional Routes and Procedures
Substitute Means vs. Alternate Means
The circular splits GPS use into two categories, and the difference is whether the ground-based navaid you are replacing is available at all.
Substitute Means
You are substituting when the conventional navaid isn’t there to use. Three cases: the ground station is out of service, your aircraft doesn’t have the required receiver (no ADF, no DME), or your installed conventional equipment is inoperative. You could, for example, hold over an NDB that’s off the air, or fly a procedure that calls for DME when your DME has failed, provided your GPS qualifies.2Federal Aviation Administration. AC 90-108 – Use of Suitable Area Navigation (RNAV) Systems on Conventional Routes and Procedures
Alternate Means
You are using GPS as an alternate means when the navaid is working and your conventional receiver is working, and you simply choose to fly the GPS. This is what most IFR pilots do every day: flying a VOR airway with the RNAV system driving the CDI because it is more precise and easier to manage than tuning and monitoring the VOR itself.2Federal Aviation Administration. AC 90-108 – Use of Suitable Area Navigation (RNAV) Systems on Conventional Routes and Procedures
Which Navaids GPS Can Replace
Under either category, a suitable RNAV system can provide position or distance information in place of:2Federal Aviation Administration. AC 90-108 – Use of Suitable Area Navigation (RNAV) Systems on Conventional Routes and Procedures
- VOR
- DME
- TACAN
- VORTAC
- VOR/DME
- NDB
- Compass locator, including locator outer marker and locator middle marker
You can use GPS to identify a VOR radial crossing, determine distance to a DME fix, or fly to an NDB-defined holding pattern without the corresponding receiver tuned.
Where GPS Substitution Stops
Two hard limits.
Not on the Final Approach Segment
You cannot use the RNAV system to replace the navaid that provides lateral guidance on the final approach segment. On a VOR approach, the VOR flies final. On an NDB approach, the ADF flies final. GPS can take you to the final approach fix and guide the missed approach, but lateral guidance on final belongs to the ground-based navaid.3Federal Aviation Administration. AC 90-108 Change 1 – Use of Suitable Area Navigation (RNAV) Systems on Conventional Routes and Procedures
Not for Lateral Nav on a Localizer Course Without Raw Data
You may not use GPS for lateral navigation on any localizer-based course, including a back-course, without also referencing the raw localizer data. On an ILS or LOC procedure, you must transition to the localizer signal itself. RNAV can fly published RNAV transitions to the ILS final approach course and can fly RNAV missed approach segments, and the system must let you become established on the ILS final course with minimal overshoot.3Federal Aviation Administration. AC 90-108 Change 1 – Use of Suitable Area Navigation (RNAV) Systems on Conventional Routes and Procedures
Alternate Airport Planning
If your flight plan requires an alternate and you are flying a non-WAAS GPS, the alternate must have at least one available instrument approach that doesn’t depend on GPS in any way. You also cannot plan to substitute GPS for a broken navaid at the alternate. If the alternate’s only usable ILS has a missed approach built around a VOR that happens to be out of service, that airport doesn’t work as your alternate.2Federal Aviation Administration. AC 90-108 – Use of Suitable Area Navigation (RNAV) Systems on Conventional Routes and Procedures
This restriction does not apply to RNAV systems using TSO-C145 or TSO-C146 WAAS equipment.2Federal Aviation Administration. AC 90-108 – Use of Suitable Area Navigation (RNAV) Systems on Conventional Routes and Procedures It is one of the most practical reasons to move from a basic TSO-C129 receiver to a WAAS unit.
Checking Integrity Before You Go
A qualifying GPS must have a way to detect an unreliable position solution. On a non-WAAS TSO-C129 receiver, that function is RAIM, which cross-checks satellite signals and needs a minimum number of them to work. Before departing IFR on a non-WAAS GPS, you must check that RAIM will be available along the route and at the destination during your expected time of arrival. The FAA’s prediction tool at sapt.faa.gov models the constellation against reported outages.4Volpe National Transportation Systems Center. Monitoring GPS Performance to Ensure Safer Flights If the prediction shows an outage for your route or destination, delay the flight, change the route, or plan a different means of navigation.3Federal Aviation Administration. AC 90-108 Change 1 – Use of Suitable Area Navigation (RNAV) Systems on Conventional Routes and Procedures
WAAS receivers (TSO-C145 and TSO-C146) get correction and integrity data from a ground network in real time and do not require a separate RAIM prediction before flight when the system is functioning normally.
If RAIM fails or you receive an integrity alert in flight, stop using GPS for navigation and revert to a conventional navaid or ask ATC for radar vectors.
You Still Need a Backup
AC 90-108 does not let GPS be your sole navigation source. The aircraft must carry at least one other independent navigation system capable of getting you to an airport and completing an instrument approach if the GPS quits.3Federal Aviation Administration. AC 90-108 Change 1 – Use of Suitable Area Navigation (RNAV) Systems on Conventional Routes and Procedures For most general aviation aircraft, that means a working VOR receiver. The philosophy is consistent throughout the circular: GPS here is a substitute or alternate, layered on top of the conventional navigation system, never replacing it entirely.
Who the Circular Applies To
AC 90-108 is not just a Part 91 document. It lists applicability across several operating parts:2Federal Aviation Administration. AC 90-108 – Use of Suitable Area Navigation (RNAV) Systems on Conventional Routes and Procedures
- Part 91, general operating and flight rules
- Part 121, domestic, flag, and supplemental air carrier operations
- Part 125, operations of large airplanes with 20 or more passenger seats or 6,000 pounds or more payload capacity
- Part 129, foreign air carriers and foreign operators of U.S.-registered aircraft
- Part 135, commuter and on-demand operations
Carriers under Parts 121 and 135 need their operations specifications to authorize RNAV use as described in the circular. Part 91 operators don’t have that opspec layer, but the equipment and procedural rules apply in full.