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PBN bingo

Making sense of new acronyms

By Bruce Williams

Aviation doesn’t need more abbreviations, acronyms, and initialisms, but the adoption of performance based navigation (PBN), with all its advantages, has added squares to the aviation lingo bingo card.

Instrument Tip
Instrument Tip

And to make matters worse, some of the most important new terms have different meanings, depending on when and how they’re used.

To make sense of these new and repurposed terms, let’s begin with the main concept: PBN. Essentially, PBN is a set of standards for navigating during each key phase of flight.

That core idea has been around for a long time. For example, VOR-based airways are typically 8 nautical miles wide, 4 nm each side of the centerline. Ground-based navaids are spaced along the airway to ensure that the angular courses they transmit can accurately define each segment of a route. Minimum en route altitudes guarantee that aircraft can reliably receive the line-of-sight signals and stay in a lane that keeps them above obstacles.

Instrument TipThat idea of roads—some relatively wide and straight, others narrower and sometimes curved—has a new name under PBN—navigation specifications, or Nav Specs. Each Nav Spec is associated with a key phase of flight.

The basic IFR Nav Specs are described in Aeronautical Information Manual 1−2−1 and other publications. Each Nav Spec is associated with a Required Navigation Performance (RNP) value. En route, the RNP value is 2; that is, 2 nm either side of the centerline of an airway or route 4 nm wide. The terminal phase encompasses the initial stages of an approach and the legs of a missed approach, and is defined by the RNP 1 standard; that is 1 nm each side of the centerline of a lane 2 nm wide. The approach phase is a path from the final approach fix to the missed approach point, and the basic RNP value is 0.3; that is, 0.3 nm from the centerline to the edge, or 0.6 nm wide in total—except when you are flying to LPV or LP minimums that are based on an angular course that narrows like a localizer as you descend toward the threshold.

PBN is also based on another fundamental concept. Under PBN, you can use any approved system to fly the routes and procedures associated with a particular Nav Spec. The procedures themselves don’t need to be renamed or changed if new navigation hardware meeting the Nav Specs is adopted. Today, for those of us flying typical general aviation aircraft, a suitable RNAV system (described in AIM 1−2−3) that meets the basic PBN Nav Specs is an IFR-approved, panel-mount GPS or GPS enhanced with WAAS.

Most charts published outside the United States use "RNP" for procedures that rely on performance-based navigation. FAA charts for procedures designed according to the same Nav Specs are titled "RNAV (GPS)," with an RNP APCH note.
Most charts published outside the United States use "RNP" for procedures that rely on performance-based navigation. FAA charts for procedures designed according to the same Nav Specs are titled "RNAV (GPS)," with an RNP APCH note.

Now let’s clarify more of the jargon presented in AIM Section 2, Performance-Based Navigation (PBN) and Area Navigation (RNAV). It’s here that we encounter another problem: The FAA sometimes uses the same term in different ways.

For example, according to the AIM, RNAV (area navigation) is “a method of navigation that permits aircraft operation on any desired flight path within the coverage of ground.” Familiar enough.

RNP, as we’ve seen, is “a statement of navigation performance necessary for operation within a defined airspace.” But RNP is also defined as “RNAV with the added requirement for onboard performance monitoring and alerting…the ability to monitor…navigation performance, and to identify for the pilot whether the operational requirement is, or is not, being met during an operation.”

The use of RNAV in procedure titles dates to the beginning of area navigation based on VOR-DME fixes or LORAN.But now it clashes with the concept of PBN.That part of the RNP definition probably doesn’t sound familiar, because our panels don’t typically display the current RNP value as a number associated with a Nav Spec. Instead, our avionics show comparable annunciations such as ENR, TERM, LNAV, LPV, and MAPR. Still, your IFR-approved GPS does, in fact, meet that part of the RNP definition.

Next, let’s move to an even more problematic use of key PBN-related terminology.

By now, you’re familiar with PBN-based procedures. In the U.S., such departures, arrivals, and approaches include RNAV in the title; for example, the GARLK ONE (RNAV) departure at Watsonville, California (WVI), the MADEE FOUR (NAV) arrival at Bellingham, Washington (BLI), and the RNAV (GPS) RWY 13 approach at Aberdeen, South Dakota (ABR).

The use of RNAV in procedure titles dates to the beginning of area navigation based on VOR-DME fixes or LORAN. But now it clashes with the concept of PBN. Today, in the United States, RNAV in a procedure title really means that it is based on the core RNP Nav Specs.

ICAO recognizes that conflict, and members of the International Civil Aviation Organization recently adopted a new naming standard. Today, most charts published outside the United States label procedures that rely on PBN as RNP departures, arrivals, and approaches. Here’s an example (above right) of a typical RNP approach in the United Kingdom that uses the new naming scheme.

The title of the approach is RNP RWY 27, meaning you can use any approved PBN technology to fly it. Again, for most GA pilots, today that is an IFR-approved GPS. But in the future, you could fly that same RNP approach using an exotic navigation system based on signals received from pulsars, as long as that equipment meets the RNP Nav Specs.

Compare that to a typical U.S. GPS-based approach, the RNAV (GPS) Runway 20 at Bremerton, Washington (above left).

RNAV in that title declares that the procedure depends on your ability to navigate point-to-point; (GPS) tells you satellite navigation is required, just as ILS, LOC, or DME in the title of a conventional approach specifies the primary equipment necessary to fly those procedures.

The RNP APCH note on the chart confirms that the procedure was designed according to the basic RNP APCH Nav Spec. That is, 1 nm for the intermediate segments, the missed approach path, and the holds, and, along the final approach segment, 0.3 nm, or a localizer-like angular course for LPV or LP minimums.

The FAA is sticking to that old convention because, as the agency explained in 2016:

“‘RNAV’ will be retained in the U.S. to maintain operational safety and avoid costs related to retitling several thousand PBN IAPs…The FAA does plan to implement other coordinated changes…to enhance usability and align with ICAO guidance (InFO 16020).”

That policy means that FAA charts labeled RNAV (GPS) now must include an RNP APCH note. That’s a shorthand way to tell you that a procedure was designed according to the RNP APCH Nav Spec, and it’s usually a redundant reminder that you need an IFR-approved GPS to fly it.

But you may also see RNP APCH notes on conventional procedures, such as the ILS or LOC Runway 21 at Stevens Point, Wisconsin (STE). That approach title is familiar. A closer look, however, reveals the RNP APCH–GPS note near the top of the chart.

That annotation is necessary because this approach includes PBN elements—the terminal arrival areas (TAA) and a missed approach and hold—all anchored by RNAV fixes. In other words, flying this seemingly conventional ILS requires both a LOC/GS receiver and GPS equipment that meets the RNP-APCH Nav Spec. Including RNP in an approach title presents yet another puzzle to pilots using procedures published by the FAA. Consider the RNAV (RNP) Y Runway 17R approach at Reno, Nevada (RNO).

In the United States, the letters RNP in a procedure title mean that special authorization is required, a fact confirmed by the boldface Authorization Required note at the bottom of the chart. These approaches are like an ILS with CAT II or CAT III minimums. In the U.S., you can’t fly an approach with RNP in the title unless your aircraft has specific equipment on board, and you must also complete special training and meet additional IFR currency requirements. Under the new ICAO chart-naming scheme, charts for such procedures include (AR), for Authorization Required, in the title.

The ILS at Reno adds a further curious element—arcs that lead to the final approach course. Those RF—radius-to-fix—legs look like familiar DME arcs, but they are a PBN feature. Until recently, RF legs were included only in AR procedures. Today, however, some procedures that include RF legs that meet the RNP 1 standard are available to aircraft with newer GPS equipment—with some limitations, such as a maximum airspeed of 180 KIAS and using an electronic HSI that auto-slews the course as you fly the curved path. Check the pilot guide and AFM supplement for your avionics to see if you can fly such RF legs.

I’ll end this discussion on a hopeful note. The FAA is working on a new advisory circular, AC 90-119, that consolidates and updates guidance on PBN, RNAV, RNP, and related matters in one document. It should be published sometime in 2023.

If the final version resembles the draft released for comment in 2021, the new AC will clarify many issues and update FAA guidance, such as when you can use suitable RNAV systems while flying conventional procedures. The AC also should edge FAA policy closer to ICAO standards; for example, by supplanting the definitions of precision and non-precision approaches with 2D (lateral guidance only) and 3D (lateral and vertical guidance) labels. Procedures flown to a decision altitude, like an approach to LPV minimums, should at last officially become “precision approaches.”

But those are topics for a discussion when the new AC is released—and perhaps when the FAA simplifies PBN bingo by adopting the ICAO naming standards.

Bruce Williams is a flight instructor in Seattle, Washington.

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