Will single approach type catch on at FAA?

 - April 23, 2008, 7:07 AM

The FAA is considering a Boeing proposal that eventually could replace 18 different kinds of instrument approaches with a single, ILS-like procedure. Based on required navigation performance (RNP) and area navigation concepts, the new RNP Rnav approaches would use the aircraft FMS to create procedures that eliminate stepped or non-precision approach paths. In their place, longer stabilized glideslope approaches using ILS-like cockpit guidance and procedures would emerge.

But even the most ardent supporters of the plan agree RNP Rnav will not be easy to implement, nor is it a concept that will become a reality soon. To be useable, RNP must be implemented in an approved–and inevitably expensive–fashion. The bottom line is that, contrary to popular belief, RNP is not for everyone, and will not be so for many years.

The big concern for corporate operators should be the costs associated with RNP certification, which by all accounts will not be cheap.

With RVSM already making inroads, RNP is receiving more attention as a technique to squeeze further capacity from the available airspace. RNP certification–which, like RVSM, culminates with official authorization in the airplane flight manual–is both demanding and costly, reportedly starting at around $500,000 for an individual aircraft. While much of this is in avionics that may already be installed, there are, again like RVSM, substantial costs involved in the overall system certification and in demonstrating that the total installation meets RNP standards.

RNP flight is predicated on a flight management system’s Rnav capability, and many–but by no means all–of today’s higher-end FMSs meet RNP criteria. However, because RNP is performance-based, any navigation system can be used, including IRS, GPS, AHRS, WAAS, DME/DME, Loran and even VOR/DME, as long as it meets the required RNP criteria for the mission. Additionally, the system must provide the crew with a continuing indication of actual navigation performance (ANP) along with alerts should the aircraft exceed its RNP.

And again, it must be stressed that simply having capable equipment on board does not automatically make the airplane RNP Rnav qualified.

The benefit of RNP is that appropriately equipped aircraft can fly preferred routes currently unavailable due to obstacle clearance, traffic separation or other reasons. Yet coupled with this is the fact that, as with RVSM, RNP will create another demarcation line between the haves and have nots of the aviation user community.

RNP is basically a statement of the navigation performance necessary for operation within a defined airspace, and is usually expressed in terms like RNP-0.1, RNP-5 and so on. RNP-0.1 means that the aircraft’s navigation system will keep the aircraft within +0.1 nm along and across a designated RNP route for 95 percent of the time. Likewise, RNP-5 means a tolerance ±5 nm for 95 percent of the time, and so on.

The longstanding notion that “one size fits all” in controlled airspace seems to be slowly disappearing as advanced-technology airplanes offer much need capacity solutions not available to their low-tech brethren. At a recent RTCA forum, Kenneth Higgins of Boeing put it succinctly: “RNP is the key to the future of CNS/ATM.”

While that’s true, and is being demonstrated every day in the northwest by Alaska Airlines and Horizon Air–the U.S. pioneers of RNP–the comment of Delta captain Bill Watts at the same meeting should also be borne in mind. He said, “RNP and other new technology investments must have a payback, which includes no more common airspace access.” This is a delicate issue for the FAA and, while acknowledging the benefits of RNP, the agency’s Steve Brown stressed the need to develop strategies and tactics to maintain system access for unequipped commuter and general aviation airspace users.

But again it must be stressed that simply having these units on board and fully operational does not automatically make the airplane RNP/Rnav qualified.

All current production commercial jets, and many regional turboprops, offer various levels of RNP capability as factory options; as do higher-end corporate aircraft. Boeing for example, offers RNP-0.1 for its 737NG line (including the BBJ), while Bombardier offers RNP-5 and RNP-10 in the Challenger 604.

Why not opt for the highest accuracy level in your new airplane and thereby qualify automatically for all other levels? That’s a different strokes question because if your RNP-0.1 certification is based on DME, you won’t qualify for RNP-10 if you then want to fly over the ocean. Therefore, RNP certification must also take your area of operations into account.

So far, while there are hundreds of Rnav across the country, there are no public RNP routes in U.S. airspace, although several have been approved as “specials” for specific operators flying specific aircraft, and these are likely to increase. Each route has an individually assigned RNP value, with its across-track width normally being twice the RNP value, to contain up to 99.999 percent of an aircraft’s navigation errors. For example, an RNP-0.3 route will extend to 0.6 nm either side of the center line.

But it is not always quite this simple. In oceanic airspace the parallel tracks of its RNP-10 routes should place their center lines 40 nm apart–four times RNP. In fact they are set at 50 miles apart to allow a 10-nm buffer for weather avoidance and any other unplanned deviations while out of ground radar coverage. Similarly, as oceanic RNP-4 is gradually introduced, the theoretical 16-nm spacing between parallel tracks will instead be set at 30 nm to maintain a 14 nm buffer.

In 1998, Eurocontrol introduced mandatory public RNP-5 routes in its upper airspace, designated basic Rnav (B-Rnav), and is commencing trials in Sweden of the more demanding precision Rnav (P-Rnav), which will require aircraft to have RNP-1 capability. And while there is no forecast date for public RNP routes in the U.S., several speakers at the RTCA forum highlighted their importance to the FAA’s overall NAS modernization effort.

Pointing out that the large majority of his organization’s members were already Rnav-equipped, NBAA senior v-p of operations Bob Blouin stressed that widespread industry training was also a critical component in the transition to RNP. NBAA had, he said, already anticipated this in its Automated Flight Deck Training Guidelines publication.

Yet even while all this activity is going on, U.S. and international operations and procedures experts are still refining the full definition of RNP to expand it from a simple accuracy statement to one that includes the probabilities of navigation system availability, integrity and continuity during a given flight operation. At ICAO this work comes under its Separation and Airspace Safety Panel (SASP), while in the U.S., the overall subject is being studied by an FAA/industry group called the Terminal Area Operations Aviation Rulemaking Committee (TAOARC). TAOARC is currently reviewing major FAA documents and revising them, where necessary, to ensure that earlier terminology does not inhibit the introduction of RNP into the NAS, with the additional expectation that user operational tests in selected terminal areas could commence as early as 2004.

Unquestionably, RNP has the potential to enhance capacity, efficiency and overall flight safety. At the RTCA forum, Higgins of Boeing showed how RNP techniques could be gradually applied, over the next several years, to the current wide range of different approach types and procedures to end up with a common ILS look-alike approach, with high-accuracy lateral and vertical guidance down to very low limits. Not only would safety be greatly improved, but training costs could also be dramatically reduced.