The many facets of RNP

March 4, 2010
Defining required navigation performance (RNP) is fairly straightforward: it is the level of navigation performance necessary for operating an aircraft within a defined airspace. RNP is based on area navigation (RNAV), which allows aircraft to fly directly to a destination instead of zigzagging from beacon to beacon.

By David Jensen

Defining required navigation performance (RNP) is fairly straightforward: it is the level of navigation performance necessary for operating an aircraft within a defined airspace. RNP is based on area navigation (RNAV), which allows aircraft to fly directly to a destination instead of zigzagging from beacon to beacon.

However, a deeper look at this growing method of efficiently guiding aircraft through the sky reveals complexities, in large part because it involves virtually all aspects of flight: equipage, crew training, maintenance, and operational procedures.

Even the terminology can be confusing. There is RNP APCH (approach), denoting the simplest RNP procedures for an instrument approach. An RNAV procedure with vertical guidance, RNP APCH would be appropriate for airports with limited ground-based navigation aids and on a relatively obstacle-free, benevolent terrain. It is distinct from RNP AR (authorization required), the highest form of performance-based navigation. RNP AR allows for lower decision heights, flight paths into and out of airports with surrounding mountains or congested airspace, and more advanced capabilities such as curved approaches and departures.

Adding to the confusion is the fact that RNP AR is procedurally identical to RNP with Special Aircraft and Aircrew Authorization Required (SAAAR). In fact, the term RNP AR is steadily replacing RNP SAAAR, a long-time FAA designation.

Both RNP APCH and AR, as well as RNAV, come under the umbrella of performance-based navigation (PBN), an ICAO term referring to a framework for global standardization of area navigation. Within that framework, RNP provides specifications, such as measurable levels of navigation performance and the requirement that aircraft include onboard monitoring and alerting of its navigation equipment.

In 2006, ICAO adopted FAA's procedures for designing precision approaches based on RNP as a worldwide standard.

RNP demands new thinking in navigation. "It's performance-based, not rule-based," explains Dottie Hall, global sales and marketing leader with Naverus, an RNP consulting and service company recently acquired by GE Aviation Systems. Modern digital avionics and satellite navigation have lifted the restrictive use of stationary ground navaids and allowed the flexibility to design flight paths that are more direct, efficient and environmentally friendly.

"RNP is interpretive, not restrictive," Hall adds. It, therefore, is dynamic, allowing for continual improvement, often referred to as procedural "optimization." Hall gives an example: "We designed an RNP flight path for [Chile's] LAN Airlines into Cusco, Peru, but in terms of how the carrier operates, we've jointly found ways of improving it."

"That's not uncommon," she adds. Changes in the surrounding airspace and new avionics technology also can prompt an optimization effort.

The benefits

Optimization is just one of a list of benefits from RNP. Others include:

• Safety – RNP reduces the risk of controlled flight into terrain (CFIT), because the onboard navigation allows three-dimensional approach operations with course guidance to the runway. There have been no reported accidents associated with the use of RNP/RNAV procedures. Conversely, 60 percent of the CFIT accidents have occurred on precision approaches using conventional navaids.

• Capacity – RNP's containment integrity allows more closely spaced procedures, parallel offset routes through terminal airspace, additional ingress/egress points around busy terminal areas, and reduced or eliminated conflict in adjacent airport flows.

• Efficiency – Enhanced reliability, repeatability and predictability of RNP operations lead to increased air traffic throughput and smoother traffic flow. It also delivers operational efficiency; for example, the 16 RNAV departures and three RNAV arrivals at Hartsfield Jackson Atlanta International Airport have cut fuel costs by an estimated $34 million annually. RNP also allows continuous descent arrivals and operator-preferred trajectories at all altitudes.

• Environment – In addition to efficiency, the repeatability of approaches and departures reduces the size of an aircraft's noise footprint. And by decreasing fuel burn with RNP, emissions are reduced too.

• Access – by applying optimized tracks, obstacle and environmental constraints can be better accommodated. RNP AR approaches often have lower minimums and improve backup procedures during instrument landing outages.

A recent tragedy exemplifies the latter RNP benefit. "In Haiti, you couldn't get relief in because the ground [navaid] systems were out," says Robert Holleran, chief technical pilot at Jeppesen, referring to the deadly earthquake occurring Jan. 12. "But with GPS, you could still get relief into the country, making RNP approaches."

RNP requires no dedicated equipment in the cockpit – there is no RNP radio or RNP sensor, per se. Neither does RNP require dedicated tools in an air traffic control (ATC) center or tower.

However, the FAA has been testing since 2008 the Relative Position Indicator (RPI) tool at its William J. Hughes Technical Center in New Jersey. The RPI tool is designed to assist terminal radar approach controllers in sequencing multiple RNAV flows into a single merge point through speed adjustments rather than using the traditional radar vectoring.

The tool predicts and sequences mergers farther out from the airport and assigns numbers to arrival flights in the order in which they merge. An image on the controller's screen conveys the relative positions of the converging aircraft, and a symbol on the screen indicates where the aircraft should be positioned to fit properly into the merge sequence.

The FAA expects to move the RPI tool, designed to improve incoming traffic flow, to a demonstration site – to be determined by Terminal Operations – later this year.

Still Evolving

RNP is evolving, and as it matures, more benefits will be unearthed. Some in the industry believe RNP is in a transitional period, ready to mature rapidly. Supporting that belief is the fact that the FAA and EASA both promote RNP, viewing it as a critical component of the NextGen and SESAR (Single European Sky ATM Research) airspace designs, respectively.

Japan's authority has a three-phase PBN roadmap that includes planned RNP approaches to its key airports. Australia has a nation-wide RNP program. China also is having many RNP flight paths designed. Indeed, countries such as China that have a limited ground navaid infrastructure are embracing RNP.

Efforts are made through the International Civil Aviation Organization (ICAO) to harmonize the performance-based navigation programs around the world. The organization's PBN Study Group has published, and revised, its "ICAO Performance Based Navigation Manual," which contains harmonized aircraft and aircrew requirements for RNP and RNAV. In addition, FAA solicits inputs from international agencies such as EASA and Transport Canada as it develops criteria for National Airspace System (NAS) operations.

RNP's history can be traced back to the late 1970s when aircraft such as the Boeing 757 and 767 came factory equipped with flight management computers (FMCs) that process navigation data along with airframe/engine data and real-time inputs from air data computers and inertial reference systems (IRS). FMCs coupled to autopilots and with inputs from multimode receivers that include GPS have paved the way for high levels of RNP.

The first RNAV-capable Airbus models were the A300-600 and A310, delivered initially in the mid to late 1980s. The A320 family of aircraft, delivered from 1988 onwards, is equipped for RNAV, as are the A330s and A340s. These three models are capable of "RNP-basic," with GPS and a flight management system (FMS), according to an Airbus official. "The precise level of capability [beyond basic] depends on the avionics fit and modification level."

Boeing began making RNP-capable aircraft in 1994, according to David Nakamura, the manufacturer's senior technical fellow in airplane systems. "That's when we added monitoring and alerting to our aircraft," he says. All Boeing models have since included equipage for RNP. Calculating Boeing deliveries since 1994, Nakamura estimates the manufacturer has delivered "about 2,200 aircraft" capable of RNP operations.

Airbus says, "It is not possible to estimate the number of RNP-capable [Airbus] aircraft fielded, as it depends on the aircraft's current equipment fit." Nevertheless, considering the more than 18,000 aircraft in commercial service, there remains a sizeable, potential RNP upgrade market. In fact, many airlines – American, Delta, JetBlue, WestJet, Air France, British Airways, China Southern and China Eastern, to name a few – have established RNP programs.

RNP procedures

Beyond proper aircraft equipage, RNP requires carefully design and tested approach and departure procedures. Steve Fulton of Naverus was instrumental in developing the first commercial RNP AR procedure, in 1996, when he was technical pilot for Alaska Airlines. The flight path down the mountain-flanked Gastineau Channel into Juneau was designed for the carrier's Boeing 737s, equipped with a Smiths (now GE) flight management system (FMS). Fulton later became a cofounder of Naverus.

"It is worth mentioning," adds an Airbus official, "that the first widebody aircraft to make formal demonstrations of the highest level of RNP to authorities was March last year [2009], when a China Southern A330 performed 'high precision navigation' test flights to Lhasa [Tibet]."

Because achieving required navigation performance is multifaceted, the FAA has granted approvals to consulting firms that assist in the gamut of RNP requirements: appropriate crew training, aircraft equipage, maintenance and approved flight paths. Companies such as Naverus and Jeppesen also can provide RNP consultation in countries around the world, securing approvals from non-U.S. certification authorities. QuoVadis' consultation, too, includes the design and testing of RNP procedures.

The certified consulting firms' customers comprise airlines, corporate flight departments, airports and air navigation service providers (ANSPs). "Our customers used to be primarily airlines," says Naverus' Hall. "But more and more, they've become the ANSPs."

She explains that ANSPs are the traditional suppliers of flight paths to operators, and "they can take into consideration the needs of all operators" when they deploy PBN. Naverus and Jeppesen can design both private and public RNP procedures. ANSPs usually have public procedures designed, while airlines often pursue their own private RNP flight paths for a competitive advantage.

Aircraft operators that want to take advantage of public-use procedures in the U.S. must meet the requirements outlined in FAA Order 8260.52.

RNP flight plans can be designed precisely for the performance capabilities of a single aircraft, but most – especially ones for public RNP procedures – are "multi-variant designs." Regardless, for procedures in the United States, guidelines in developing PBN approach and departure procedures are available in Appendix 3 of the FAA's advisory circular AC90-101, guidance for operators on how to safely utilize RNP operations in the NAS.

Throughout the process of designing a flight path, the certified consultant/designer works closely with the FAA. He confers with the local air traffic control tower and approach control, as well as with the customer. "Everyone must agree on the [RNP flight path] plan," says Jeppesen's Holleran.

"Then we make a 'stick drawing' [rough draft] of the procedure, based on various weather conditions and aircraft performance, and we incorporate input from the tower and control center," he adds. An exercise in Holleran's training to become third party, vendor-authorized check pilot for RNP procedures reveals how rigorous flight path design and validation must be.

"You try to 'break' a procedure; that's an FAA term," he explains. "You make sure you can fly the approach or departure in all conditions, such as a tail wind – in other words, stay in the contained area while in the worse-case scenario."

Once the RNP departure or approach is finally designed, it is coded for incorporation in the FMC databases in aircraft that will utilize the procedure. Software approval is achieved through a validation and comparison process. It must meet what is often called "the gold standard," which means it must performs exactly as intended according to the approved flight path design.

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