Substituting GPS for Ground-Based Navigation Aids

Questions continue about when and how a pilot may substitute GPS for indications and information from ground-based navigation aids, such as VORs, DME, and NDB, including intersections and fixes defined by ground-based navaids. I provided background on this topic in October 2011. But the latest edition of the Instrument Flying Handbook (described here) consolidates and expands on the guidance offered in the AIM and AC 90-108, and it bears mention here.

(The simple explanation is straightforward. If you have an IFR-approved GPS, and absent restrictions in the AFM supplement for that GPS, you can use it instead of a VOR, DME, or NDB indicator, even when a charted navaid is out of service or noted as “required” on an approach chart. GPS can also substitute for fixes (e.g., step-down-fixes) that are part of a conventional approach, such as an ILS. The ground-based navaids and defined fixes must be in the GPS database. You cannot, however, use GPS for lateral guidance along the final approach course of an IAP that is based on a localizer.)

FAA updated its guidance on this issue with the May 26, 2016 update to the AIM. For more information, see this item here at BruceAir.

Chapter 9 of the IFH (see p. 9-27) includes the following information.

GPS Substitution
IFR En Route and Terminal Operations
GPS systems, certified for IFR en route and terminal operations, may be used as a substitute for ADF and DME receivers when conducting the following operations within the United States NAS.
1. Determining the aircraft position over a DME fix. This includes en route operations at and above 24,000 feet mean sea level (MSL) (FL 240) when using GPS for navigation.

2. Flying a DME arc.
Navigating TO/FROM an NDB/compass locator.

3. Determining the aircraft position over an NDB/compass locator.

4. Determining the aircraft position over a fix defined by an NDB/compass locator bearing crossing a VOR/LOC course.

5. Holding over an NDB/compass locator.

GPS Substitution for ADF or DME
Using GPS as a substitute for ADF or DME is subject to the following restrictions:

1. This equipment must be installed in accordance with appropriate airworthiness installation requirements and operated within the provisions of the applicable POH/AFM or supplement.

2. The required integrity for these operations must be provided by at least en route RAIM or equivalent.

3. WPs, fixes, intersections, and facility locations to be used for these operations must be retrieved from the GPS airborne database. The database must be current. If the required positions cannot be retrieved from the airborne database, the substitution of GPS for ADF and/or DME is not authorized.

4. Procedures must be established for use when RAIM outages are predicted or occur. This may require the flight to rely on other approved equipment or require the aircraft to be equipped with operational NDB and/or DME receivers. Otherwise, the flight must be rerouted, delayed, canceled, or conducted under VFR.

5. The CDI must be set to terminal sensitivity (1 NM) when tracking GPS course guidance in the
terminal area.

6. A non-GPS approach procedure must exist at the alternate airport when one is required. 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. (For an update on filing alternates with an IFR-approved GPS, see New FAA Policy on IFR Alternates with GPS.)

7. Charted requirements for ADF and/or DME can be met using the GPS system, except for use as the principal instrument approach navigation source.

NOTE: The following provides guidance that is not specific to any particular aircraft GPS system. For specific system guidance, refer to the POH/AFM, or supplement, or contact the system manufacturer.

To Determine Aircraft Position Over a DME Fix:

1. Verify aircraft GPS system integrity monitoring is functioning properly and indicates satisfactory integrity.

2. If the fix is identified by a five-letter name that is contained in the GPS airborne database, select either the named fix as the active GPS WP or the facility establishing the DME fix as the active GPS WP. When using a facility as the active WP, the only acceptable facility is the DME facility that is charted as the one used to establish the DME fix. If this facility is not in the airborne database, it is not authorized for use.

3. If the fix is identified by a five-letter name that is not contained in the GPS airborne database, or if the fix is not named, select the facility establishing the DME fix or another named DME fix as the active GPS WP.

4. When selecting the named fix as the active GPS WP, a pilot is over the fix when the GPS system indicates the active WP.

5. If selecting the DME providing facility as the active GPS WP, a pilot is over the fix when the GPS distance from the active WP equals the charted DME value, and the aircraft is established on the appropriate bearing or course.

To Fly a DME Arc:
1. Verify aircraft GPS system integrity monitoring is functioning properly and indicates satisfactory integrity.

2. Select from the airborne database the facility providing the DME arc as the active GPS WP. The only acceptable facility is the DME facility on which the arc is based. If this facility is not in your airborne database, you are not authorized to perform this operation.

3. Maintain position on the arc by reference to the GPS distance instead of a DME readout.

To Navigate TO or FROM an NDB/Compass Locator:
1. Verify aircraft GPS system integrity monitoring is functioning properly and indicates satisfactory integrity.

2. Select the NDB/compass locator facility from the airborne database as the active WP. If the chart depicts the compass locator collocated with a fix of the same name, use of that fix as the active WP in place of the compass locator facility is authorized.

3. Select and navigate on the appropriate course to or from the active WP.

To Determine Aircraft Position Over an NDB/Compass Locator:
1. Verify aircraft GPS system integrity monitoring is functioning properly and indicates satisfactory integrity.

2. Select the NDB/compass locator facility from the airborne database. When using an NDB/compass locator, the facility must be charted and be in the airborne database. If the facility is not in the airborne database, pilots are not authorized to use a facility WP for this operation.

3. A pilot is over the NDB/compass locator when the GPS system indicates arrival at the active WP.

To Determine Aircraft Position Over a Fix Made up of an NDB/Compass Locator Bearing Crossing a VOR/LOC Course:
1. Verify aircraft GPS system integrity monitoring is functioning properly and indicates satisfactory integrity.

2. A fix made up by a crossing NDB/compass locator bearing is identified by a five-letter fix name. Pilots may select either the named fix or the NDB/compass locator facility providing the crossing bearing to establish the fix as the active GPS WP. When using an NDB/compass locator, that facility must be charted and be in the airborne database. If the facility is not in the airborne database, pilots are not authorized to use a facility WP for this operation.

3. When selecting the named fix as the active GPS WP, pilot is over the fix when the GPS system indicates the pilot is at the WP.

4. When selecting the NDB/compass locator facility as the active GPS WP, pilots are over the fix when the GPS bearing to the active WP is the same as the charted NDB/compass locator bearing for the fix flying the prescribed track from the non-GPS navigation source.

To Hold Over an NDB/Compass Locator:
1. Verify aircraft GPS system integrity monitoring is functioning properly and indicates satisfactory integrity.

2. Select the NDB/compass locator facility from the airborne database as the active WP. When using a facility as the active WP, the only acceptable facility is the NDB/compass locator facility which is charted. If this facility is not in the airborne database, its use is not authorized.

3. Select non-sequencing (e.g., “HOLD” or “OBS”) mode and the appropriate course in accordance with the POH/AFM or supplement.

4. Hold using the GPS system in accordance with the POH/AFM or supplement.

Interview about Stall/Spin/Upset Training at Hangar 49

The Hangar 49 podcast for January 26, 2013 includes an interview with me about stall/spin/upset training. My part of the program begins at about the 26-minute mark, but if you like things aeronautical, you’ll find several other interesting segments in the same program.

Garmin Radius to Fix Leg Project Report

The accuracy provided by GPS (especially with WAAS augmentation) has vastly expanded the number and quality of instrument approaches available to properly equipped aircraft, including procedures that provide guidance comparable to the ILS. For example, as of January 10, 2013, in the U.S., there were 3,052 approaches with LPV (localizer performance with vertical guidance) minimums, more than double the number of category 1 ILS approaches (the current inventory of instrument flight procedures in the U.S. is available here).

At present, however, approaches that take full advantage of the capabilities of satellite-based navigation remain in a special “authorization required” category. Flying these RNAV (RNP) procedures requires additional crew training and approved avionics, such as flight management computers, autopilots, and cockpit displays, as described in AC 90-101A and AIM 5-4-18. At present, RNAV (RNP) (required navigation performance) procedures, like Category II ILS approaches, are available to authorized airline crews and pilots flying business jets equipped with the appropriate avionics, but not to typical instrument-rated pilots, even those flying aircraft with WAAS-capable IFR GPS navigators such as the Garmin GNS430W/530W and newer GTN750/650 series boxes.

Garmin released system software 6.11 for the GTN series on March 1, 2016. That update includes the ability to fly RF legs on approaches that are not classified as Authorization Required procedures.

The presence of RF legs no longer automatically classifies an approach as an AR procedure. For more information, see AC 90-105A and the updated Pilot’s Guide and other documentation related to the March 1, 2016 update of the system software for the Garmin GTN series navigators.

Garmin, working with the FAA and Hughes Aerospace Corporation, has recently completed the first part of a study that may persuade the FAA to change the requirements and make some RNP procedures available to most pilots flying aircraft equipped with WAAS-capable avionics. You can download the complete Garmin Radius to Fix Leg Project Report (PDF) published January 15, 2013, here.

A key feature of RNP procedures is the radius-to-fix (RF) leg, a curved flight path that resembles the familiar DME arc. The Instrument Procedures Handbook describes RF legs this way:

Constant radius turns around a fix are called “radius-to-fix legs,” or RF legs. These turns, which are encoded into the navigation database, allow the aircraft to avoid critical areas of terrain or conflicting airspace while preserving positional accuracy by maintaining precise, positive course guidance along the curved track. The introduction of RF legs into the design of terminal RNAV procedures results in improved use of airspace and allows procedures to be developed to and from runways that are otherwise limited to traditional linear flight paths or, in some cases, not served by an IFR procedure at all. (5-23)

Figure A-13 from Appendix A of the IPH shows a hypothetical RF leg.

Unlike DME arcs, RF legs are defined by points in space, not distances from a ground-based transmitter. They can be strung together into sinuous paths, that, as noted above, provide lower minimums while avoiding obstacles, airspace conflicts, and noise-sensitive areas. The plan view from the RNAV (RNP) Z RWY 13R approach at Boeing Field in Seattle (KBFI) shows such RF legs.

You can find examples of RF legs in non-RNP approaches at Carlsbad, CA (KCRQ) and Ketchikan, AK (PAKT) [thanks to John D. Collins for those references]. Because of the restrictions placed on flying RF legs, however, those procedures are not currently available to typical IFR pilots. RF legs are also features of some RNAV instrument departure and arrival procedures (SIDs and STARs).

FAA includes RF legs in procedures assuming that an aircraft can follow the curved path with great precision, hence the detailed requirements spelled out in AC 90-101A, AC 90-105A, and AIM 5-4-18. Chief among those requirements are a flight director and/or a roll-steering autopilot, stipulations that rule out many, if not most, light general aviation aircraft.

The new Garmin study demonstrates, however, that:

Instrument-rated general aviation pilots are able to hand fly RF legs and meet the 0.5 nm 95% FTE [standard flight technical error]  target and RF leg altitude restrictions without the aid of a flight director or autopilot in Part 23 Category A and B aircraft that are either minimally equipped or technically advanced…All pilots demonstrated acceptable proficiency on both straight legs and RF legs. The increase in RF leg FTE over straight leg FTE can be expected to be about the same magnitude from a minimally equipped aircraft to a technically advanced aircraft.

In other words, pilots were able to remain well within the boundaries specified for an RF leg, whether flying a Cherokee equipped with just a Garmin 430W and basic instrumentation or a speedier Cessna 400 outfitted with Garmin’s latest G2000 integrated glass cockpit and autopilot. The pilots achieved this performance while hand-flying challenging procedures that incorporated multiple RF legs specifically designed to stress-test both the avionics and their flying skills. You can see diagrams of these special procedures in the Garmin report.

Based on the findings (described in great detail in the Garmin document), Garmin concludes that:

Garmin recommends FAA revise its installation and operational guidance for RF legs to make clear that applicants may obtain airworthiness approval for installations without flight director/autopilot. To preclude the need to demonstrate adequate FTE margin for aircraft flying RF legs at greater than 200 knots without flight director/autopilot, Garmin recommends FAA revise its installation and operational guidance for RF legs to allow applicants to utilize an Aircraft Flight Manual limitation that restricts flying RF legs to 200 knots or less.

Furthermore, the study showed that a moving map, while a great benefit to situational awareness, isn’t necessary to fly RF legs accurately:

As this project has shown, FTE is decreased when a moving map is available and is thus consistent with the MLS curved path study conclusion that led to the FAA installation and operational guidance that “an aircraft must have an electronic map display depicting … RF legs.” However, this project has also clearly shown that a moving map is not required to maintain acceptable RF leg FTE, even during complex procedures and missed approaches.

The executive summary of the Garmin report outlines additional conclusions and recommendations that address specific issues related to flying RF legs in typical light GA aircraft.

The FAA isn’t saying when or if it will adopt the recommendations in Garmin’s report. So far, the agency has said only:

This demonstration project has shown early success and will continue with more flight testing and data collection.

But the well-designed study and detailed analysis suggest that many more pilots may in future be able to take advantage of some advanced RNP procedures, if the FAA agrees with the recommendations and avionics manufacturers and database providers include RF legs in future updates to the WAAS units now common in light GA aircraft.

Garmin released system software 6.11 for the GTN series on March 1, 2016. That update includes the ability to fly RF legs on approaches that are not classified as Authorization Required procedures. For more information, see also AC 90-105A.

Interview about PC-Based Simulation

The friendly folks at the Hangar 49 podcast (produced in the Pacific Northwest) have posted their latest installment (mp3), which includes an interview with me about using PC-based flight simulations to complement flight training.

This is one of several interviews and webinars that I’ve done recently on this topic. You can watch the webinar, hosted by EAA, here. Another interview is available as a podcast at PilotSafetyRadio.

For more information about my two books about PC-based flight simulation, visit my website.

Database Currency for IFR Operations

Most instrument-rated pilots now fly with GPS-based navigation equipment (according to AOPA, at least 78 percent of its members rely on GPS as their primary navigation tool).

To use an IFR-approved GPS when operating IFR, the unit’s database must be current or you must verify the accuracy of the data.

For more details, see note 4 in AIM 1-2-3: Use of Suitable Area Navigation (RNAV) Systems on Conventional Procedures and Routes and AC 90-108.

Keeping a typical GPS unit up-to-date usually involves downloading fresh data to a card every 28 days.

Of course, the effective dates of databases don’t always fall conveniently between trips, and FAA has outlined procedures to help pilots ensure that the data in GPS avionics matches the key information on current charts, especially instrument approach plates.

You must also review either the AFM or the AFM Supplement for the avionics installed in your aircraft. For example, the latest AFM Supplement (190-01007-A2_08) for the Garmin GTN series (750, 650, etc.) notes:

2.8 Navigation Database
GPS/SBAS based IFR enroute, oceanic, and terminal navigation is prohibited unless the flight crew verifies and uses a valid, compatible, and current navigation database or verifies each waypoint for accuracy by reference to current approved data.

“GPS”, “or GPS”, and “RNAV (GPS)” instrument approaches using the Garmin navigation system are prohibited unless the flight crew verifies and uses the current navigation database. GPS based instrument approaches must be flown in accordance with an approved instrument approach procedure that is loaded from the navigation database…

If the navigation database cycle will change during flight, the flight crew must ensure the accuracy of navigation data, including suitability of navigation facilities used to define the routes and procedures for flight. If an amended chart affecting navigation data is published for the procedure, the database must not be used to conduct the procedure.

Regarding instrument approaches, the key information for matching the database to the chart is the procedure amendment reference date, not necessarily the date printed at the top of the chart. On charts published by the FAA, the procedure amendment reference date appears in the lower-left corner, next to the amendment number.

The best description of the procedure amendment reference date and how to use it is in Jeppesen Briefing Bulletin JEP 09-C (PDF)–even if you use charts published by FAA Aeronautical Information Services. The Jeppesen briefing bulletin includes a simple flow chart that helps you use the procedure reference date.

FAA published a safety alert (PDF) in 2009 that explains the difference between chart dates and procedure amendment dates.

PilotSafetyRadio Podcast

Yesterday I was interviewed for the PilotSafetyRadio podcast. The host, Mark Grady, was a traveling presenter for the AOPA ASI for many years. We talked about using flight simulations, stall/spin awareness, and flying technically advanced aircraft.

You can listen to the conversation online here.