Garmin GTN Avionics and RF Legs

The release of updated operating software for Garmin GTN-series avionics brings new capabilities to many typical general aviation pilots who fly under IFR. One of the new features is the ability to fly curved radius-to-fix (RF) legs on some instrument approaches.

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Until recently, RF legs were published only on so-called RNP procedures with authorization required (AR) restrictions (for more information, see AIM 5−4−18: RNP AR Instrument Approach Procedures). But FAA has started publishing some approaches with RF legs (like the example above) that are not designated as RNP AR procedures. And, with some limitations, pilots who fly aircraft equipped with GTN-series avionics should be able to fly the RF legs used as transitions/feeder routes on those approaches. (Note that so far, these approaches don’t require RF capability–conventional transitions/feeder routes and/or radar vectors are also available.)

For more information about RF legs, see RNP Procedures and Typical Part 91 Pilots and Garmin Radius to Fix Leg Project Report here at BruceAir. For additional background on GPS navigation and RNP procedures, see also Updated AC 90-105A.

The revised STC for the GTN series (document 190-01007-A5) notes that:

GPS/SBAS TSO-C146c Class 3 Operation
…The Garmin GNSS navigation system complies with the equipment requirements of AC 90-105 and meets the equipment performance and functional requirements to conduct RNP terminal departure and arrival procedures and RNP approach procedures including procedures with RF legs subject to the limitations herein [emphasis added].

Sections 2.12 RF Legs and 2.13.1 RNP 1.0 RF Leg Types of the STC add the following information:

2.12 RF Legs
This STC does not grant operational approval for RF leg navigation for those operators requiring operational approval. Additional FAA approval may be required for those aircraft intending to use the GTN as a means to provide RNP 1 navigation in accordance with FAA Advisory Circular AC 90-105. [Note that per AC 90-105A, domestic Part 91 operations do not require additional approval–only Part 91 subpart K operations and commercial operations need LOAs or the equivalent FAA approval.]

The following limitations apply to procedures with RF legs:

  • Aircraft is limited to 180 KIAS while on the RF leg
  • RF legs are limited to RNP 1 procedures. RNP AR and RNP <1 are not approved
  • Primary navigation guidance on RF legs must be shown on an EHSI indicator with auto-slew capability turned ON
  • GTN Moving Map, EHSI Map, or Distance to Next Waypoint information must be displayed to the pilot during the RF leg when flying without the aid of the autopilot or flight director.
  • The active waypoint must be displayed in the pilot’s primary field of view…

2.13.1 RNP 1.0 RF Leg Types
AC 90-105 states that procedures with RF legs must be flown using either a flight director or coupled to the autopilot.

This STC has demonstrated acceptable crew workload and Flight Technical Error for hand flown procedures with RF legs when the GTN installation complies with limitation set forth in Section 2.12 of this document. It is recommended to couple the autopilot for RF procedures, if available, but it is not required to do so. See section 4.5 of this manual to determine if this capability is supported in this installation.

At present, only a few non-AR approaches with RF legs meet the criteria in the STC and AC 90-105A. But RF legs could become more common on “standard” procedures to provide paths that offer better noise abatement, reduce airspace conflicts, and improve ATC efficiency, and pilots flying with GTN avionics (or similar navigators offered by other manufacturers) will be able to fly those procedures.

IACO Flight Plan Equipment Codes for Aircraft with IFR GPS

dFAA plans to require that all flight plans (VFR, IFR, domestic, and international) use the ICAO format sometime in the fall of 2017 (for the latest details, see this notice).

For more information about the switch to ICAO flight plans, see a news item from AOPA here.

A detailed explanation of the ICAO flight plan form is available here. Flight Service also has a handy tip card here and more details, including links to videos, here.

In November 2013, FAA updated and simplified some of the requirements for filing ICAO flight plans for domestic use. You can read about those changes here (PDF). Note that the instructions from FAA focus on the printed flight plan form, which few pilots use. Apps such as ForeFlight, Garmin Pilot, WingX, FltPlanGo, go FlyQ take care of many of the details for you. You should review the user guides and other instructions for the apps and web-based tools that you use to file flight plans. ForeFlight has detailed information about ICAO flight plans here.

Confusing Codes

Many pilots are confused by elements of the ICAO flight plan format, especially the multiple aircraft equipment codes that you must include to inform ATC of the gizmos and capability that are installed in your aircraft.

The following guide should help you sort out those ICAO codes if you fly a typical light GA aircraft equipped with at least one WAAS-capable, GPS navigator that is approved to fly RNAV (GPS) approaches. Examples of such avionics include:

Garmin has posted detailed information about the ICAO codes for its avionics, including a handy Microsoft Excel worksheet, here.

Overview

Here’s a look at the relevant parts of the ICAO flight plan form as shown on the Leidos FSS website. You can find a video that describes the ICAO flight plan form at Leidos FSS here.

I’ve filled in the information for my Beechcraft A36 Bonanza (ICAO identifier BE36), which is equipped with a GTN750, a Garmin GTX 327 transponder (not Mode S), and the GDL 88 ADS-B transmitter and receiver. This aircraft also has a Bendix/King DME receiver.

For more information about the ICAO identifiers to use for the makes and models of aircraft that you fly, see ICAO Aircraft Type Designators here at BruceAir.

The example is for an IFR trip from KBFI to KGEG in the Pacific Northwest. The route includes the ZOOMR1 STAR into KGEG.

ICAO-FlightPlan-Form-Example

Basic Information

The first few items are the same for all typical IFR general aviation flights:

  • Flight Rule: IFR
  • Flight Type: G (for general aviation)
  • Number of Aircraft: 1 (i.e., not a formation flight)
  • Wake Turbulence Category: L (for light)
  • Aircraft Type: The official ICAO designator for the make and model of aircraft you fly (e.g., BE36, C172, C210, M20P, PA28A, etc.)

Aircraft Equipment

On the familiar FAA domestic flight plan form, equipment suffixes for typical GA pilots are simple, and if you’re flying a GPS-equipped airplane with a Mode C transponder, the basic /G was all you needed.

But the ICAO form captures many more details about the equipment installed in your aircraft, and the fun typically begins with this item.

For a WAAS-equipped aircraft such as we’re discussing, you should enter the following codes in the Aircraft Equipment box:

  • SBDGR

ICAO-FlightPlan-Form-Example-A

As you can see in the illustration from the Leidos FSS web form, these letters represent the following equipment:

  • S: Standard communication and navigation receivers/transmitters (VOR, VHF communications radios, and ILS receiver). If you enter S in this box, you shouldn’t include the letters L (ILS), O (VOR), or V (VHF) here. S includes that equipment.
  • B: LPV approach capability. If you have a WAAS GPS, but your installation isn’t approved for LPV procedures (see the user’s guide and AFM supplement), omit this letter.
  • D: DME. If you don’t have DME, omit the D.
  • G: IFR-approved GPS (the preferred term is now GNSS, Global Navigation Satellite System)
  • R: PBN approved. This letter means that your aircraft meets basic RNP standards. All aircraft with an IFR-approved GPS are PBN approved (see AIM 1-2-1). You must include R, and associated information in the Other Information box, to ensure that the computer will accept a routing that includes RNAV routes, SIDs, STARs, or charted ODPs. See PBN and RNP Confusion, below.
  • Z: Indicates additional information to be added to the Other Information box, described below.
  • If you still have an ADF, include F.

PBN and RNP Confusion

The aviation world uses RNP (required navigation performance) for two related, but different purposes.

In general, RNP is an RNAV specification (e.g., RNAV 5, RNAV 2, and RNAV 1) that indicates that an aircraft is capable of maintaining a course (track) within designated limits 95 percent of the time. For example, RNAV 5 means the aircraft as equipped can reliably maintain a track with 5 nm; RNAV 2 limits are 2 nm, and so forth. If your aircraft is equipped with an IFR-approved GPS authorized to fly RNAV (GPS) approaches, it matches this sense of RNP and PBN.

The basic RNP (RNAV) specifications used in the U.S. (RNP 0.3, RNP 1.0, RNP 2.0, and RNP 1.0) are shown in the following illustration from the Instrument Flying Handbook (FAA H-8083-15B). For more information about RNP and RNAV specifications, see “Required Navigation Performance” on page 9-44 of the IFH.

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Current WAAS-approved GPS receivers for typical GA aircraft, such as those listed earlier, meet the U.S. RNP specifications, as described in AC 90-100A: U.S Terminal and En Route Area Navigation (RNAV) Operations and its associated AC90-100 Compliance Table (a Microsoft Excel worksheet). To confirm your GPS receiver’s capabilities, check the user guides and the AFM supplements for the equipment installed in your aircraft.

The term RNP is also applied as a descriptor for airspace, routes, and procedures (including departures, arrivals, and IAPs). RNP can apply to a unique approach procedure or to a large region of airspace. In this sense, RNP means something similar to Category II and Category III instrument approaches. For example, an approach with RNP in the title (e.g., RNAV (RNP) Z RWY 16R) requires special equipment and detailed crew training/qualification. Such RNAV (RNP) approaches include the note AUTHORIZATION REQUIRED on the chart.

For more information about RNP approaches, see RNP Procedures and Typical Part 91 Pilots and Garmin Radius to Fix Leg Project Report here at my blog.

Surveillance Equipment (Transponder and ADS-B)

This box on the ICAO form tells ATC what type of transponder and related equipment are installed in your aircraft.

  • For most GA pilots flying IFR, this box will include at least C, for a transponder with altitude-reporting capability.
  • If you have a Mode S transponder, you should select the appropriate letter, E, H, I, L, P, S, or X, based on the information in the user guide and AFM supplement for your transponder.
  • If you have ADS-B equipment installed (not a portable ADS-B receiver such as the Stratus or Dual XGPS170), include U1 or U2. The Garmin GDL 88 in my airplane both transmits and receives ADS-B signals, so I add U2 to this box.

ICAO-FlightPlan-Form-Example-B

Other Information

The final box for designating your RNAV capabilities and additional data is Other Information. You must use prefixes, followed by letters, to include different categories of information.

As described above here and here, it’s important to add a PBN/ group in this box to ensure that the ATC system understands the RNP/RNAV capabilities of your aircraft.

ICAO-FlightPlan-Form-Example-D

If you have a GPS approved for at least IFR en route and terminal operations, add the following letters:

  • C2, which designates RNAV 2 capability based on GPS (GNSS)
  • D2, which designates RNAV 1 capability based on GPS (GNSS)

If you have a Mode S transponder that complies with the ADS-B out requirements, add the following group to this box:

  • SUR/260B

If you have a UAT box such as the Garmin GDL 88 to meet the ADS-B requirements, add the following group to this box:

  • SUR/282B

You should also file the six-digit Mode S Code (base 16 / hex) assigned to your aircraft by inserting a CODE/ group. You can find the hexadecimal code for your aircraft by checking the FAA N-number registry:

  • CODE/xxxxxx

For example, the code for one of the aircraft at the flight school where I instruct is A66E8E. The entry for that aircraft is:

  • CODE/A66E8E

For more information about the filing the appropriate codes related to ADS-B capabilities, see Filing for Advanced Surveillance Broadcast Capability (PDF) at the FAA flight plan website.

You can also add a NAV/ group in this box to indicate your RNAV capability. This group isn’t necessary if you use the appropriate PBN codes described above. But you can use a NAV/ group such as D1E2A1to indicate that you have RNAV 1 capability for departure, RNAV 2 capability for the en route segment, and RNAV 1 capability for arrival.

  • NAV/RNVD1E2A1

These groups and letters mean that you can fly RNAV routes (e.g., T-routes), RNAV SIDs and STARs, and charted ODPs (charted ODPs are often RNAV procedures, usually based on GPS).

For more information about charted ODPs, see:

Don’t worry about the options in the RNP Specifications part of this box. Unless you are authorized to fly RNAV (RNP) procedures (see above), these items don’t apply to you.

Summary

Here’s a quick review of what to put in the equipment-related boxes of the ICAO flight plan form if, like me, you fly an aircraft with one of the common IFR-approved, WAAS-capable GPS receivers:

  • Aircraft Equipment: SBGR
  • Surveillance Equipment: C (for a Mode C transponder).
    (If you have ADS-B in/out capability, such as a Garmin GDL 88, add U2. If you have a Mode S transponder, include the appropriate letter for your model.)
  • Other Information: PBN/C2D2 SUR/260B or SUR/282B and CODE/xxxxxx
    (where xxxxxx is the six-digit hexadecimal code assigned to your aircraft as part of its registration record at the FAA).

ICAO-FlightPlan-Form-Example

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.

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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.

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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.

RNP Procedures and Typical Part 91 Pilots

Some pilots remain confused about GPS-based RNAV approaches, especially procedures with RNP (Required Navigation Performance) in the title and AUTHORIZATION REQUIRED (AR) notes on the chart. For many reasons, those approaches (now called RNP AR procedures) are currently off-limits to a typical IFR pilot flying an airplane equipped with a WAAS-capable, IFR-approved GPS.

The situation is becoming more complicated with the publication of RNAV approaches such as the RNAV (GPS) X RWY 24 at KCRQ (see below). Although that procedure is not labeled RNP AR, one transition (from VISTA) includes an RF (radius-to-fix) leg, a curved path that resembles a DME arc, but which requires specific equipment to fly–see below. Many of us flying with GPS units like the Garmin GNS400/500W  navigators may not see this procedure in the database, but it may appear among the updated charts you receive from Jeppesen or FAA AeroNav Products.

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 Garmin GTN Avionics and RF Legs here at BruceAir.

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For a basic explanation of why you need specific authorization to fly RNP AR procedures, see the Instrument Procedures Handbook:

In the United States, operators who seek to take advantage of [RNP AR] approach procedures must meet the special RNP requirements outlined in FAA [AC 90-101A], Approval Guidance for RNP Procedures with Authorization Required (AR). Currently, most new transport category airplanes receive an airworthiness approval for RNP operations. However, differences can exist in the level of precision that each system is qualified to meet. Each individual operator is responsible for obtaining the necessary approval and authorization to use these instrument flight procedures with navigation databases. (4-30)

AC 90-101A explains in part:

a. RNP AR approaches include unique capabilities that require special aircraft and aircrew authorization similar to Category (CAT) II/III instrument landing system (ILS) operations. All RNP AR approaches have reduced lateral obstacle evaluation areas and vertical obstacle clearance surfaces predicated on the aircraft and aircrew performance requirements of this AC. In addition, selected procedures may require the capability to fly an RF leg and/or a missed approach, which requires RNP less than 1.0. Appendix 2 of this AC identifies specific aircraft requirements that apply to these capabilities.

A more detailed description of an RNP procedure from AC 90-101A, helps you understand why a typical WAAS-capable piston flown by a single pilot doesn’t qualify:

2. Characteristics of RNP AR Approaches.a. RNP Value. Each published line of minima has an associated RNP value. For example, Figure 2 shows both RNP 0.3 and RNP 0.15 lines of minima. Each operator’s RNP AR authorization documents a minimum RNP value, and this value may vary depending on aircraft configuration or operational procedures (e.g., use of flight director (FD) with or without autopilot). RNP AR approaches will have an RNP value of RNP 0.3 or less.

b. Procedures with Radius to Fix (RF) Legs. Some RNP AR approaches include RF legs. The instrument approach charts will indicate requirements for RF legs in the notes section or at the applicable initial approach fix (IAF). Figures 1 and 2 provide examples of procedures with an RF leg segment (e.g., between SKYKO and CATMI).

[Note that the presence of RF legs alone no longer automatically classifies an approach as an RNP 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. More details are also available at Garmin GTN Avionics and RF Legs here at BruceAir].

c. Missed Approaches Requiring Less Than RNP 1.0. At certain locations, the airspace or obstacle environment may require RNP capability of less than 1.0 during a missed approach. Operation on these approaches typically requires redundant equipment. This requirement ensures that no single point of failure can cause loss of RNP capability. Figure 2 provides an example of a missed approach requiring RNP less than 1.0. The notes section of the chart indicates this requirement.

d. Non-Standard Speeds or Climb Gradients. Normally, RNP AR approach procedure design relies on standard approach speeds and climb gradients including on the Missed Approach Segment (MAS). The approach procedure will indicate any exceptions to these standards, and the operator must ensure it can comply with any published restrictions before conducting these approach operations. Figure 2 provides an example of a non-standard climb gradient and speed restrictions.

AC 90-101A also describes the training necessary for crews authorized to fly RNP AR procedures and the equipment requirements (beyond a WAAS-capable GPS receiver) necessary to fly those approaches.

With all that in mind, it’s important to understand that if your airplane is equipped with, say, a GNS530W or one of the new GTN boxes (with an appropriate AFM supplement), you can fly RNAV SIDs and STARs based on RNP-1 criteria. And you can, of course, fly en route segments that require RNP-2 accuracy. No special authorization is required at these RNP levels. For an explanation of RNP levels, see “Required Navigation Performance” in the Instrument Procedures Handbook (2-34).

AC 90-105A, published in March 2016, revises some of the terminology associated with RNAV and RNP procedures, in part to bring FAA conventions into agreement with ICAO standards associated with performance based navigation (PBN). The AC describes Required Navigation Performance Approach (RNP APCH) procedures, which you should not confuse with RNP AR procedures. RNP APCH is essentially equivalent to RNAV (GPS). If you fly with an IFR-approved GPS, your aircraft meets the requirements to fly basic RNP APCH procedures as described in the AC. But you can’t fly RNP AR procedures unless you also meet the requirements of AC 90-101A.

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For more information about operations approved for different IFR-approved GPS units, see AC 90-100A, “U.S. Terminal and En Route Area Navigation (RNAV) Operations,” and the associated Excel compliance worksheet. A good summary of AC 90-100A is available here.