En route and terminal RNP coming for U.S.

 - October 18, 2006, 8:02 AM

The FAA has implemented performance- based navigation in the form of standard instrument departures (SIDs), en route Q and T routes, standard arrival routes (Stars) and RNP special and Rnav approaches.

Over the next five years, according to Jeff Williams, manager of the FAA Air Traffic Organization Rnav/RNP group, the agency plans to move toward en route and terminal implementation of RNP. By 2010 route spacing standards may be based on RNP, and the agency is considering a mandate for Rnav in airspace above FL350. The mandate could be extended to FL290 and above and applied to major airports during the subsequent five years, which may also see reduced lateral aircraft-to-aircraft separation standards.

There are already Rnav IFR terminal transit routes (TTRs) at the 35 airports covered by the FAA’s operational evolution plan (OEP). The OEP airports are those where delays tend to promulgate disruption throughout the National Airspace System, and the TTRs are designed to allow general aviation aircraft access to the terminal airspace without impeding airline operations.

By 2025, Williams expects RNP/Rnav to be universal in all domains, with satellite navigation as the primary basis for the navigation infrastructure. A network of DMEs is expected to provide DME/DME Rnav as back-up, he added, “since that is what is on the aircraft today.”

B-Rnav has been mandatory in ECAC states since the beginning of 1998, three decades after Rnav equipment was developed. The equipment has been approved for use since the 1970s. Roland Rawlings, navigation domain manager in Eurocontrol’s airspace/flow management and navigation division, recalled that flight management systems were sold on the basis that they would provide a 0.5-percent fuel saving but provided no operational benefit because air navigation service providers were focused on maximizing capacity rather than providing a special service to a limited subset of aircraft.

Although it was based on existing aircraft equipment, the B-Rnav mandate had major benefits, Rawlings said. They include a 20- to 30-percent capacity increase through more direct routes, bypass routes for aircraft overflying high-density terminal areas, fuel savings through a reduction in flight distances and a reduction in aircraft dependence on ground navigation facilities.

However, more needs to be done because air traffic is expected to double by 2020. And this time, Rawlings said, the agencies must avoid the previous problem of “wonderful equipment on the aircraft and the ground unable to use it.” So compatible air and ground applications need to be developed in line with operational requirements and avionics and ground equipment capabilities.

The introduction of Rnav SIDs and Stars at Hartsfield-Jackson Atlanta International has demonstrated the efficiencies of Rnav operations, reducing the number of two-way communications with the Tracon by as much as 40 percent on departure and 50 percent on arrival.

Mitre’s Suzanne Porter also listed fuel burn benefits from continuous departure climbs, noting a dramatic reduction in the number of aircraft required to level off at 10,000 feet. The reason for the reduction is that the trials have convinced controllers in the en route facility that airplanes will arrive accurately at the handover point so they are more prepared to let them continue climbing. Flights departing to the west and looping back to the east also save as much as 0.7 nm per flight. Rnav has even provided a substantial reduction in taxi times as departure controllers have been able to reduce separation times.

John McGraw, manager of the FAA’s flight technologies and procedures division, said Rnav approaches have been in use since 1969 and more than 2,600 VOR/ DME Rnav, GPS overlay of conventional procedures or Rnav (GPS) approach procedures are currently published. The next step is RNP approaches.

Special and Public Approaches

Alaska Airlines developed one of the pioneering applications of RNP to lowering approach minimums in the mid-1990s, using the Gastineau Channel as an approach route to Juneau. Steve Fulton, who was involved in that work, subsequently left the airline to form Naverus, a company that works with operators to develop RNP procedures and with regulators to get those procedures approved. The company has been involved in improving access to several challenging airports.

At Wenatchee, to the east of Seattle, for example, the mountainous terrain meant that the original approach procedure using a new ILS had a decision height of 1,200 feet, providing only marginal value to the community, which is home to a corporate operator of Learjets and Challengers and has regional airline service. Using Terps criteria, Naverus helped to get the decision height down to 300 feet using a procedure due for publication this fall.

To get there via the narrow valley used for the approach, Fulton said, the requirements are good navigation performance, good path prediction, a good display system to enable the pilot to monitor the path and a good guidance system. And while there may not be a lot of airports with the same characteristics as Wenatchee there are a lot of areas–such as the San Francisco Bay–where the constraints are not terrain but a complex web of approach and departure paths to a cluster of airports.

Another recent project was the development of approach procedures to Queens-town, New Zealand, for local carrier Air New Zealand. Airbus gained RNP 0.15 certification earlier this year and is working with Naverus and the airline to gain certification for landing with minimum decision heights down to 270 feet: without the RNP procedures, aircraft bound for Queenstown cannot descend below 3,000 feet unless VMC prevails.

Don Pate, FAA flight procedure standards branch manager, said the U.S. standard for RNP approach procedure with special aircraft and aircrew authorization required (SAAAR) was developed on the basis of the way Category II and III ILS operations were approved, with specified aircraft and aircrew requirements. Published in Order 8260.52, it aims to facilitate the development of public rather than operator-specific procedures: “Juneau was a special procedure and was very resource-intensive to develop,” he said. “So we want to standardize the process.”

The RNP SAAAR procedures use an airspace containment measure of two times RNP, based on navigation, equipment and aircrew performance, with RNP values of 0.1 to 0.3 nm. “The navigation system error is usually small,” Pate commented. “If there is any error it is usually from aircrew performance.”

While ATC keeps airplanes separated from each other, the task of flight standard procedures is to keep them separated from the ground, he said. The final obstacle clearance separation is based on the barometric vertical navigation (Vnav) error budget. Other attributes include fixed-radius turns with 2 times RNP lateral containment and a missed approach obstacle clearance separation gradient between 200 and 425 fpm.

Missed-approach RNP values can be between 1 and 0.1 nm–“only use the lower values if you need them,” Pate advised–and fixed-radius turns are permitted on the final approach segment between the final approach fix and the decision height. “Fixed-radius turns allow you to fly in restricted areas,” he observed.

The first U.S. published RNP SAAAR approach procedure, for Runway 19 at Ronald Reagan Washington National Airport (DCA), uses the fixed-radius turn capability to follow the Potomac River and is designated Rnav (RNP). “ATC just gives an Rnav clearance, but the title shows that the aircraft needs RNP,” Pate explained. “Special aircraft and aircrew authorization is required, just as it is for Category II or III approaches.”

Meanwhile, Order 8260.52 has been presented to the ICAO Obstacle Clearance Panel, which accepted it and plans to reformat and republish it by July as a stand-alone ICAO RNP procedure design manual. The plan is then to revise it in the light of implementation experience and publish it in ICAO’s PANS OPS by late 2007.

Mitre’s Porter said the introduction of RNP SAAAR approaches promised to enhance capacity at busy airports by, for example, permitting parallel approaches to runways less than 4,300 feet apart, with a visual transition from an offset track where the runway separation was between 750 and 4,300 feet. Simultaneous approaches to converging runways should also be possible, along with deconfliction of traffic using adjacent airports such as those in New York and Chicago.

The capacity gains would be accompanied by other benefits, including reduced delays, improved safety through vertically guided procedures, improved access for general aviation operations and reduced reliance on conventional navaids and procedures.

Perhaps most important, RNP procedures promise to enhance safety. At Palm Springs, for example, a nonprecision approach into a valley surrounded by mountainous terrain is due to be replaced by a public RNP SAAAR approach next month. Already implemented as a special procedure for Alaska Airlines, it reduces the distance involved by approximately 30 nm, cuts the VOR or GPS B minimums of 2,300 feet and three miles to 68 feet and one mile, and provides a guided, stabilized 3-D path to the runway.