ATM Modernization, Business & GA, Commercial

Carriers Advance RNP As Standards Evolve

By George Marsh | January 1, 2006
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The desire to free aviation from dependence on fixed routes defined by ground-based navaids–a concept known as free flight–has produced technologies that permit more direct flights from point to point and a greater number of routings through a given airspace. More efficient use of airspace increases capacity, saves fuel and achieves benefits such as noise reduction by avoiding densely populated areas.

Realizing this, the aviation community started to implement area navigation (RNAV) back in the 1970s. RNAV implies a method of navigation enabling an aircraft to fly on a desired flight path, using any chosen means–traditional navaids, self-contained systems, or a combination of these. The related concept of required navigation performance (RNP) implies RNAV plus other capabilities. Although internationally agreed-upon, public standards for RNP and its relationship to RNAV are still evolving, work continues in this area, and a number of carriers are reaping the benefits.

Several years ago Europe introduced basic area navigation (BRNAV) with track following to a nominal accuracy of +/-5 nautical miles (nm) of the course centerline. BRNAV enabled significant capacity gains, some direct routes, better feeder routes into terminal areas (TMAs), and reduced dependence on terrestrial navaids.

Further improving lateral accuracy enables the benefits to be extended from mainly en-route airspace to terminal areas and airports. Accuracies of 1 nm and better are the realm of precision area navigation (PRNAV), which is now in the early stages of implementation in Europe.

As multiple technologies emerged to deliver RNAV, basic or precise, the authorities shifted their focus from specifying in detail the equipment that aircraft should carry to laying down specific standards of navigational capability. This performance-based approach has been dubbed RNP. It is technology-agnostic, allowing users to select any preferred technology combination to meet a given RNP, including legacy equipment.

Use of the two related terms can be confusing. However, Roland Rawlings, navigation domain manager in Eurocontrol’s Airspace Flow Management and Navigation unit, describes a consensus that has grown out of recent discussions. "Everything is RNAV, but RNP has come to mean something more–RNAV with extras, if you like, where the main extra is containment."

Whatever the technology, RNP ensures that the aircraft is contained within defined boundaries around the planned trajectory, with a high level of assurance. For RNAV performance, 95 percent containment is normally quoted, meaning the aircraft is within its specified "guard rails" for 95 percent of the time. Meeting the containment stipulation requires systems of high integrity and continuity.

The International Civil Aviation Organization (ICAO), Montreal, supports Rawlings’ distinction between RNAV and RNP. It sees room for both terms and is trying to harmonize their use. As Erwin Lassooij, technical officer in the flight safety section of ICAO’s Air Navigation Bureau, recently explained to participants in a Eurocontrol/FAA-sponsored international RNP workshop in Toulouse, France, ICAO would like to see RNAV gradings standardized to levels 1 through 5 (RNAV 1, RNAV 2, etc.), denoting accuracy in nautical miles. This would update both U.S. and European usage.

This approach, endorsed by ICAO’s RNP Operational Requirement Study Group (RORSG), is likely to be embodied in a performance-based navigational manual currently in the works. Due for publication in 2006, the manual will cover concepts, implementation, specifications, standards and procedures. ICAO hopes that all stakeholders will agree to follow its guidance.


While stakeholders will widely welcome the ICAO recommendations as something definitive to work toward, progress is being made in implementing RNP without the recommendations. Airframers, operators and avionics manufacturers alike have invested in RNP technologies as a means of reducing operational costs. Unfortunately, one of the difficulties facing RNP is that the air traffic management (ATM) ground infrastructure lags airborne capability. Both Airbus and Boeing provide RNP 0.3 capability as standard on their latest-generation jets and are now targeting certification of RNP 0.1. Operators acquiring these aircraft naturally wish to exploit this capability, especially in TMAs.

The failure of air navigation service providers (ANSPs) to match airborne equipage is not surprising in view of the economies of scale gained in equipping thousands of B737s and A320s vs. the tendency of comparatively small number of individual states to develop proprietary ground systems with little standardization between them. Nevertheless, the providers will increasingly feel obliged to support RNP operations. Moving forward in phase will be a far better way to achieve the RNP vision of 3-dimensional (3D) and 4D, trajectory-based navigation–taking into account the vertical spatial dimension and the temporal dimension–than the present fragmented approach.

Global navigation satellite systems (GNSS) are the main enabling technology. Although few experts now foresee space-based systems’ providing sole-source navigation for all flight phases, the use of GPS and Galileo–with ground-based backup such as Loran or DME/DME–is seen as key to RNP delivery. Flight management systems (FMS) also can integrate other navigation sources, including dual or triple inertial reference units, VOR, DME and ILS. Full RNP requires an addition to some FMS, whereby pilots can see when actual navigation performance differs from intended navigation performance, resulting in a flight technical error (FTE). An "unable RNP" alert should result.

Operator Experience

Given the harmonization work still in progress, it is easy to overlook the fact that RNP is already in daily use, even without an international standard describing it. Admittedly, so far, this has tended to be at challenging airports, where access otherwise would be limited by high terrain or frequent bad weather. But authorities are extending RNP procedures to more normal situations, too.

Among airlines benefiting from PRNAV/RNP are Alaska Airlines and sister carrier Horizon Air in the United States, Austrian Airlines and its subsidiary Austrian Arrows (formerly Tyrolean Airways) in Europe, Qantas and Air New Zealand in the Antipodes, and WestJet in Canada. Also pursuing the RNP path are American and Continental Airlines, especially now that the United States, Canada and Mexico have committed to it. (In September the FAA published its first public RNAV/RNP procedure, for runway 19 at Reagan/National Airport, Washington, D.C.)

Alaska’s role in pioneering low-level IFR (instrument flight rules) operations is well known, and FAA’s Capstone project has been extensively reviewed in Avionics Magazine. Alaska has developed RNP approaches to the mountain- and weather-bound airport at Juneau, for example, in conditions that would previously have prevented access. Alaska Airlines and Horizon Air have improved their service reliability by operating these procedures since 1996. RNP makes it easier to design instrument approaches through narrow valleys, such as at Juneau, because the notional RNP rectangular "tubes" in space that the aircraft fly through are of constant cross-sectional dimensions. As a result, RNP provides the same linear path 30 nm away from the runway as it does at the runway threshold. ILS and other guidance beams spread wider the farther they are from the point of origin.

Recently, Alaska Airlines started to earn RNP bounty at Palm Springs, Calif., an airport that suffers from poor weather and is located in a valley between mountains. FAA’s approval for RNP operations there gives the airline a competitive advantage since it can land in visibility conditions that bar other carriers’ access.

Another location providing RNP benefits is Innsbruck Airport in Austria, where RNP 0.3 procedures (with 95 percent containment at 2xRNP) were approved by the Austrian civil aviation authority in May 2005. This has opened up new operational possibilities for Austrian Airlines and its subsidiary Austrian Arrows, improving the reliability of their "ski run" services into the airport. Innsbruck is notorious for its difficult one-way-in/one-way-out approach and departure routes down narrow valleys between 9,000-foot-high mountains.

To take advantage of the new procedures, aircraft must be fitted with dual FMS and positioning systems. Austrian Airlines has upgraded the navigational avionics on its Boeing 737s to fully achieve RNP 0.3. The carrier also had to provide special pilot training. But the four years of hard work to reach this point have paid off, as the benefits are now reaching fruition. According to spokesman and navigation specialist, Robert Ahornegger, the benefits include:

  • Better controlled and stabilized approaches,

  • Improved missed approach procedures,

  • Lower approach minimums,

  • Reduced crew workload on fully coupled approaches (including autothrottle),

  • Reduced fuel burn,

  • Fewer instances of close approaches to terrain, and

  • Improved service reliability.

Dependence on terrestrial approach aids is avoided, and the decision height for runway 26 is being reduced to 2,200 feet from 3,500 feet, Ahornegger says.

"RNAV/RNP is an important tool in eliminating step-downs and offset approaches and for supporting IFR operations in an obstacle-rich environment," he adds. "It has proved its worth at Innsbruck, and we are now looking at upgrading to RNP 0.2 or 0.15." Every approach, he says, becomes a precision approach, while the integrity and continuity processes within RNP safeguard approach and possible go-round margins, even if all GPS satellite signals should fail.

RNP Down Under

A further exemplary case is that of Queens-town in New Zealand. The civil aviation authorities of Australia and New Zealand already have approved Qantas Airways to operate RNP procedures there. And, as we write, Air New Zealand is about to follow suit. Final approval to operate its RNAV/RNP-capable A320s into Queenstown will lead, Air New Zealand officials expect, to reductions in decision height minimums from about 3,000 feet above ground level to as little as 270 feet. The carrier also will be able to fly curved final segments, saving both time and fuel. Working with Airbus and U.S.-based RNP procedures developer Naverus, Air New Zealand claims to be the first A320 operator to have productively exploited the potential offered by the type’s built-in combination of advanced FMS and GPS navigation system.

Without the use of PRNAV and RNP, Queenstown can only be approached in visual meteorological conditions (VMC) via a VOR procedure. A series of A320 demonstration flights to the airport during June 2005 included RNP arrivals at both runways, together with missed approaches and departures with and without a simulated engine failure. Pilots were able to monitor any deviation from the required flight path, using their primary flight displays. Like Austrian Airlines, Air New Zealand plans to gain experience with RNP 0.3 and then transition to RNP 0.15. The carrier also intends to certify for RNP six B737s equipped with the requisite dual FMS.

A sign of RNP’s extension beyond difficult airports is Canada’s WestJet’s approval from Transport Canada to fly RNP procedures throughout the country. Naverus has embarked on the design of more than 90 RNP procedures for airports at 24 cities served by the carrier’s 35 B737NG aircraft. These procedures should, says WestJet, improve access to any of the airports in poor visibility, while also enhancing safety and operational efficiency. The initial and flagship location is Kelowna, British Columbia, where WestJet reckons to save some $1.5 million a year on just one approach. Kelowna also is notable for pioneering operations to high-precision RNP 0.1.

Tim Morgan, WestJet senior vice president and co-chief operating officer, describes the implementation as exciting. "Naverus is unlocking the advanced navigational systems in our 737NG fleet with its expertise, and Transport Canada has responded with the right combination of scrutiny, knowledge and willingness to make advances."

One reason for protracted development periods in some cases has been the scarcity of regulations in place. ICAO’s recognition of this and its subsequent actions will, it is hoped, speed up the approval process. Another stumbling block has been doubts about the integrity of data used in designing PRNAV procedures. This situation is being addressed, too. FAA and the European Aviation Safety Agency (EASA) have collaborated in auditing the production processes of a number of database suppliers. EASA, for instance, has granted initial approval to Lufthansa Systems so that 110 airline customers of its Lido FMS database can use new PRNAV procedures. Rival supplier EAG also has been approved. EASA approval means that suppliers’ databases comply with the ED-76 European standard, whose U.S. equivalent is the RTCA’s DO-200A. U.S. supplier Jeppesen has undergone a similar audit process. Approval of these data suppliers means that operators no longer have to rely on their own resources to show that the data they use is accurate and safe.

Need to Harmonize

The really big issue for RNP, however, remains harmonization. Air traffic controllers add their joint voice to the general cry for convergence of RNP requirements, standards, procedures and practices. Jeremy Davidson, a senior consultant with DW International, points out that controllers want sufficient commonality to avoid having to handle aircraft with different levels of capability differently. They don’t want to have to handle BRNAV-capable, PRNAV-capable and RNP 0.3 or 0.1-capable airplanes differently. In other words RNP mixed-mode operations would be anathema, he says.

Controllers also would like common FMS functionality, so that, for example, they do not have to differentiate between FMS that provide tight-turn capability and others that do not. On the ground, controllers would welcome RNP-oriented ATC tools, especially to track integrity monitoring and provide sufficient "blunder" warning to ensure safety. RNP Special Aircrew and Aircraft Approach Required (SAAAR) procedures into dense airspace will have to be common to many aircraft, argues Davidson, or controllers will not accept them.

Harmony and mandates may seem mutually exclusive. But should certain standards of RNP equipage need to become mandatory, then U.S., European and other authorities should at least march in step. Eurocontrol believes that a full-up RNP environment, with 4D trajectories followed from gate to gate with high precision, may not be realized for another 20 years, but that an initial 1-nm PRNAV standard could be achieved by 2008. Major steps like this may ultimately require mandates, so the lack of equipage of a few does not to prevent progress for the many that are equipped. Naturally, this prospect worries operators who do not–and may not wish to fly aircraft that are RNP capable. One airline spokesman, preferring not to be named, expressed it, as follows: "Mandates are coming thick and fast. We have had 8.33-KHz VHF spacing and RVSM [reduced vertical separation minimum], and we can expect Mode S and ADS-B. Do we need more? Should we not identify the most productive [RNAV/RNP] solution and go for that, rather than all the options? We also have to bear in mind that navigation will not be alone and that communication and surveillance will give rise to mandates, too. The last thing we want is CNS mandate overlaps, resulting in extra costs."

With a world fleet of 20,000 transport aircraft in service (including those of the CIS), a figure likely to be doubled by 2020, retrofitting could be an enormous undertaking. At the least, forward fit and retrofit requirements should be harmonized–or the airlines may just fail to buy into RNP.

Defining RNAV

RNAV is an acronym for area navigation, so called because aircraft routes are spread over a given area rather than being fixed with respect to ground-based navaids that have to be over-flown. These more direct and efficient routes can be flown using satellite-based and other self-contained systems, traditional navaids, or a combination of both. Onboard RNAV systems can be stand-alone or included as part of the flight management system (FMS).

There are two broad bands of RNAV capability. Basic RNAV (BRNAV)–referred to as RNAV Type A in the United States–is used for en route navigation and can be supported by relatively unsophisticated navigational equipment. Precision RNAV (PRNAV)– known as RNAV Type B in the United States–is a much more precise navigation standard suitable for use in terminal areas. Traffic separations must allow for the navigational accuracies of the aircraft involved.

Defining RNP

Because multiple technical solutions can provide a given level of navigational performance, aviation authorities now prefer to define the navigational capability an aircraft needs before it can be allowed to use certain airspace. Specifying a required navigational performance (RNP) level avoids having to refer to or specify the aircraft’s equipment. Thus RNP is equipment-agnostic: a given performance requirement can be met with any mix of suitable equipment, according to operator preference.

Levels of RNP are expressed numerically. Thus RNP 5 describes an aircraft’s ability to deviate no more than 5 nm laterally from its intended track and is suitable as an en route standard, while RNP 0.3 to RNP 0.1 imply tracking to between three-tenths and one-tenth of a nautical mile, a precision capability suitable for use in terminal areas.

RNAV and RNP terminologies are not simply interchangeable, however, because, by common consent (now likely to be supported by ICAO), RNP has come to mean more—i.e., RNAV plus something. A large part of that "something" is onboard navigational containment, meaning a near certainty that the technical equipment can never let the aircraft stray outside of certain notional "guard rails" around its intended track. This is largely a statistical construct based on the integrity and availability levels of the navigational equipment plus its calculated continuity of service. A requirement for a monitoring system able to alert the flight crew to any degradation in navigational performance is likely to be associated with integrity.

The other part of the "plus factor" is a number of extra functions, the exact mix of which has yet to be decided. Typically these would be embedded within the FMS and might include fixed-radius transitions, radius-to-fix legs, required time of arrival, and parallel offset. Eventually, as the RNP concept expands to include navigation in the vertical and temporal domains, as well as the two lateral dimensions, extra functionality could include vertical containment and 4D control.

In time, RNAV/RNP is likely to supersede the present ground-based navaid infrastructure, increasing airspace capacity, improving operating economics, and enhancing air traffic management of all flight phases, gate to gate.

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