FAA moves ahead on RNP and Rnav implementation

 - September 25, 2006, 1:19 PM

The FAA is making progress toward instituting a future Rnav and RNP (required navigation performance) environment across the National Airspace System (NAS), the agency told attendees at its recent annual new technologies workshop in Arlington, Va.

Described as a “performance based” operational concept, the program is aimed at gradually transitioning U.S. airspace into a much more efficient medium, combining increased capacity, reduced delays and improved access to currently congested and/or challenging airports. Performance-based means that new procedures will be keyed to the aircraft’s navigating capabilities and performance–and special crew training in certain cases–rather than the traditional philosophy that mandates the carriage of specific avionics.

In other words, if a future procedure calls for, say, a given level of accuracy, operators can use different methods to achieve it. It isn’t that simple, of course, because the FAA will rule on the acceptability of the proposed method, but it underlines the flexibility of the performance-based concept, which heralds the overarching end state for the future National Airspace System–“Rnav everywhere, RNP where beneficial.”

Jeff Williams, Rnav/RNP group manager in the FAA’s Air Traffic Organization, described the expected evolution of navigation in the NAS between this year and 2025. Divided into near-, mid- and far-term segments, the plan outlines the steady transition of the NAS over the next 20 years, with progressively increasing mandates for Rnav and RNP in en route, terminal and approach airspace. The plan forecasts that by 2025, RNP will be mandatory in busy en route and terminal airspace. The details of the plan are still being ironed out, and will be formalized in a final “roadmap” document this spring.

RNP Procedures

While most everyone is familiar with Rnav, there’s less certainty about RNP. The difference between the two is that RNP emphasizes containment. Nomenclatures such as RNP-0.3, RNP-0.1 and so on indicate the navigation accuracy in nautical miles required to fly a given procedure, while staying within its boundaries for 95 percent of the time. So while pilots could use GPS, IRS, DME/DME, VOR/DME or loran to stay within RNP-0.3 airspace, they would need GPS or IRS/GPS to stay within RNP-0.1.

RNP compliance requires an onboard method–usually within the flight-management computer–of monitoring containment and promptly alerting the pilot when he drifts outside the procedure’s boundaries. It isn’t a sudden alert, however; containment is continuously monitored and displayed on the flight-management system or elsewhere as the actual navigation performance.

If the airplane does veer outside the procedure’s boundary, RNP design criteria double the width of the procedure corridor. So while RNP-0.1, for example, requires the pilot to stay within a tenth of a mile either side of track, an extra tenth is added to each side, widening the total width of the corridor to four-tenths of a mile, which virtually ensures 100-percent obstacle clearance.

Curiously, while containment seems logical when operating to low RNP values, neither ICAO nor Eurocontrol–whose Brnav and Prnav are sometimes considered the equivalent of the FAA’s RNP-5 and RNP-1–call for it in their definitions.

Besides its en route and terminal applications, RNP provides major benefits for airports with approach restrictions or difficult access, such as Palm Springs International or Ronald Reagan Washington National (DCA) Airports. On approach to Runway 13R at Palm Springs, for example, safety is assured through RNP’s tight vertical and lateral control throughout the procedure, producing a snaking RNP-0.15 approach into Palm Springs that winds down from 12,000 feet to the airport at the bottom of a valley.

RNP’s positive flight-path control brings with it lower approach limits, guiding pilots down from Palm Springs’ current nonprecision approach minimum decision altitude (MDA) of 1,850 feet agl to the RNP’s decision altitude of 250 feet.

Reducing Airspace Congestion

At Washington Reagan, the standard LDA approach brings pilots down to an MDA of 720 feet. If the pilot is visual at that point, he must then hand fly the remaining two miles to the runway, following the curving path of the Potomac while carefully avoiding the P-56 prohibited area around the nation’s capitol. With its RNP-0.11 approach to DCA, Alaska Airlines aircraft follow the same track but have positive horizontal and vertical IFR guidance to continue their constant descent down to 47 feet.

Most RNP procedures incorporate turns, which is where previous terminal maneuvers and approaches incur the greatest traffic dispersion. Each turn in an RNP procedure follows the imaginary circumference of a circle of specific radius, sometimes large and sometimes small, depending on local terrain, maximum bank limits and so on. These are called radius-to-fix turns, and the autopilot will take pilots smoothly around them, even during descent.

There’s one additional nice touch to RNP approach procedures. Occasionally, GPS satellites are not in an optimum configuration to provide the highest accuracy, which can preclude operations down to the procedure’s lowest limits. However, the satellites’ orbital positions are predictable, and dispatchers use sophisticated computer programs before departure to determine the destination GPS accuracy at the expected approach time. If it is less than optimum, the dispatchers will advise pilots to raise their approach limits accordingly.

Currently, Alaska Airlines, Continental and Canada’s WestJet are the main users of RNP procedures, although equivalent avionics capability is offered on both the BBJ and the ACJ, and possibly other corporate aircraft. The FAA’s objective, said Williams, is to eventually produce “hundreds” of RNP procedures each year. If, as seems likely, most of these are in response to airline requests, business aviation must make its own needs known to the agency, so it can enjoy similar benefits at non-airline destinations.