Use of GPS in lieu of DME/ADF
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Until recently, pilots operating with GPS receivers certified for instrument flight rules (IFR) were not permitted to use GPS for any approach operations where no GPS overlay or stand-alone approach existed. Furthermore, GPS was not approved for use in any capacity for localizer-type approaches; e.g., ILS, localizer, SDF, or LDA. This forced those equipped with IFR GPS receivers to maintain and replace DME and ADF receivers in order to utilize all approaches in the National Airspace System. GPS has proven itself a reliable and highly accurate navigation system over the past four years, which warranted a change in these policies.
AOPA, working together with the FAA Flight Standards Division, has reached agreement on FAA policy changes that permit IFR-certified GPS receivers to be used in lieu of DME and ADF for most IFR operations (see “Limitations”). This policy fundamentally permits DME and ADF avionics to be removed with little compromise in operational capability. AOPA will continue to work with the FAA to implement GPS approaches at airports served only by an NDB approach.
This fact sheet is intended to clarify and provide guidance on how to take advantage of this new policy.
The new policy
Effective July 16, 1998, pilots may substitute IFR-certified GPS receivers for DME and ADF avionics for all operations except NDB approaches without a GPS overlay. GPS can be used in lieu of DME and ADF on all localizer-type approaches as well as VOR/DME approaches, including when charted NDB or DME transmitters are temporarily out of service. It also clarifies that IFR GPS satisfies the requirement for DME at and above Flight Level 240 specified in FAR 91.205(e). This approval represents a major step toward removing the need to retain DME or ADF in our cockpits for any reason.
Note: Air carrier operators should consult their operations specifications and their principal operations inspector for approval.
There are still three instances in which DME or ADF are still required.
- NDB approaches that do not have an associated GPS overlay approach must still be flown using an ADF.
- A non-GPS approach procedure must exist at the alternate airport when one is required to be filed by regulation. If the non-GPS approaches on which the pilot must rely require DME or ADF, the aircraft must be equipped with DME or ADF avionics as appropriate. GPS substitution for DME/ADF is not permitted in this case.
- DME transmitters associated with a localizer may not be retrievable from your GPS until the manufacturer incorporates them in the database. Pilots are not authorized to manually enter coordinates.
AOPA is working with the FAA and the manufacturers to have these restrictions removed and will keep you informed.
Note: Pilots should exercise caution when selecting the appropriate DME and NDB/LOM locations to avoid erroneous distance information.
As with most operational capabilities, there are minimum requirements that must be met in order to take advantage of their benefits in a safe manner. When using GPS in lieu of DME and ADF for the purposes stated above:
- The receiver must be certified for IFR operations and be installed and approved in accordance with FAA guidelines.
- When using GPS course guidance; i.e., navigating to/from an NDB or locator outer marker (LOM), the course deviation indicator (CDI) must be set to 1 nm or terminal sensitivity.
- The required integrity for these operations must be provided by at least en-route receiver autonomous integrity monitoring (RAIM), or an equivalent method. Although currently not available, Wide Area Augmentation System (WAAS) receivers will also be eligible for the substitution.
- Pilots must conduct GPS operations within the guidelines contained in the aircraft flight manual supplement or approved operations manual.
- The DME, ADF, or intersection names or identifiers must be retrieved from the database, and pilots must ensure that data is current in accordance with the aircraft flight manual supplement or approved operations manual.
GPS substitution for DME
- If a DME is co-located with a VOR, tacan, or NDB, the DME location should be in the database by searching for the identifier of that navaid (see Figure A).
- DME transmitters that are part of an ILS, localizer, SDF, or LDA approach may not be in your GPS database. The FAA, database producers, and manufacturers are all working to add these locations to the GPS database. In the interim, pilots may use other GPS navigation waypoints to identify stepdown points; i.e., a FAF, named stepdown waypoint.
Caution: Pilots should be extremely careful to ensure that correct distance measurements are used when utilizing this interim method. It is strongly recommended that pilots review distances for stepdown points during preflight preparation. (See Figure B; a GPS receiver set to the FAF, AASON, would be counting up to 8.1 at the missed approach point, where as a DME would have counted down to 0.9 at this same missed approach point.)
- DME arcs associated with instrument approaches may be flown using GPS distance provided the DME transmitter, on which the arc is based, is identified in the GPS database (see Figure A).
- The aircraft must be equipped with a DME receiver if DME is required to fly the approach procedure(s) at the alternate airport.
- Aircraft utilizing IFR GPS in lieu of DME operating at or above FL240 are not required to be equipped with DME.
GPS substitution for NDB
- All LOMs and NDBs with three-letter identifiers should be available from the GPS database. Again, an NDB approach without a GPS overlay cannot be flown using GPS. It must be flown using an ADF (see Figure C).
- Most LOMs are assigned a two-letter identifier and a five-letter name. Some GPS manufacturers have designed their receivers to use the five-letter name, not the two-letter ID. Five-letter names are retrieved through the intersection waypoint page. Other manufacturers use the two-letter ID retrievable through the NDB waypoint page (see Figure D).
- Pilots must be equipped with an ADF receiver if ADF is required to fly the approach procedure(s) at the required alternate airport.
- Consider your instrument panel scan visibility of the GPS output information. (Neither IFR enroute or approach receivers require a separate CDI by the TSO 129A specifications. However, both probably do require a separate conventional CDI display, or connection to an existing VOR-CDI, to meet Advisory Circular 20-138 installation requirements for display within the pilot’s field of vision/scan.)
- Some GPS receivers can drive an ADF bearing pointer; this would make a handy GPS/RMI-like presentation for improved situational awareness. Note: Labeling or switching functions would be required.
- Verify how any switching of GPS vs. ILS/VOR input to an external CDI is to work.
Caveat: The FAA offers the following—GPS is a supplemental navigation system, in part due to signal availability. There will be times when your system will not receive enough satellites with proper geometry to provide accurate positioning or sufficient integrity. Procedures should be established by the pilot in the event that GPS outages occur. In these situations, the pilot should rely on other approved equipment, radar vectors, delayed departure, rerouting, or discontinuance of IFR operations.
Further guidance will be provided in updated editions of the Aeronautical Information Manual (AIM) and Advisory Circular AC 90-94 titled “Guidelines for using global positioning system equipment for IFR en route and terminal operations and for non-precision instrument approaches in the U.S. National Airspace System.”
AOPA members may direct questions to the AOPA aviation services department at 800/872-2672.
December 13, 1999