Required Navigation Performance

Required Navigation Performance

Bold textRequired Navigation Performance (RNP) is defined by ICAO as "a statement of the navigation performance necessary for operation within a defined airspace". Part of a broader concept called "Performance-based Navigation," RNP is a method of implementing routes and flight paths that differs from previous methods in that not only does it have an associated performance specification that an aircraft must meet before the path can be flown but must also monitor the achieved performance and provide an alert in the event that this fails to meet the specification. It is the monitoring and alerting facility that distinguishes RNP from RNAV (aRea NAVigation) from which it developed. RNP equipped aircraft can safely operate routes with less separation than previously required which is significant because it increases the number of aircraft that can safely use a particular airspace and therefore accommodate the increasing demand for air traffic capacity.

Under the ICAO definitions, GNSS is the primary navigation system to support currently defined RNP standards. Before the introduction of GNSS, aircraft navigated using ground-based navaids or Inertial Navigation Systems (INS), and the degree to which they follow an assigned track was based on the accuracy of those systems. These assumptions on the accuracy and precision to which an aircraft could fix its position in three dimensions in turn drove the size of the protected airspace which needed to be maintained around each aircraft. On many densely traveled routes, such as those over the northern Atlantic Ocean North Atlantic Tracks (NAT) connecting the United States and Europe, aircraft were packed as tightly as the separation rules allowed during peak times thus limiting the number of flights which could travel that route per day.

RNP changes those assumptions. Under RNP, the nature of the navigational aids is not specified, rather the volume of airspace around the aircraft is, and this volume may be smaller (in some cases much smaller) than that of conventional navigation. In practice, the RNP aircraft is assumed to be navigating using a combination of ground-based navaids (radio navigation), GPS, and inertial guidance systems, which give far greater precision than previously possible. This allows air traffic control to reduce the spacing between aircraft without compromising safety. Certain blocks of airspace are being designated with RNP standards; only aircraft meeting the designated RNP level for that airspace will be allowed to operate in that area.

The performance required to fly an RNP route is generally specified in nautical miles, e.g. RNP 4 which implies that the total system error will be no greater than 4 NM for 95% of the time. The RNP specification requires that if the error exceeds or is likely to exceed twice the specified value (i.e. 8NM for RNP 4) then an alert must be generated. Since the deviation is likely to exceed the alerting deviation before the error can be rectified, route spacing must be sufficient to ensure that two aircraft deviating to the alert level toward one another will remain safely separated. RNP 4 thus supports 30 NM lateral or longitudinal spacing.

Benefits of RNP In The Terminal Area

Operators who implement the methods and standards of RNP may gain benefits in operating efficiency, improved access to airports, and safety.
*Compared to routes that rely on ground-based navigation aids, RNP approaches in the terminal airspace offer potentially shorter routes, resulting in reduced fuel consumption. This represents an economic benefit for operators, and reduced emissions of CO2 and NOx for the environment.Fact|date=May 2008
*Approach procedures based on RNP often provide improved minima compared to the existing non-precision approaches, in many cases comparable to (but not better than) existing ILS approaches. This allows aircraft to complete landings in a broader range of weather conditions.
*RNP approaches typically incorporate a Continuous Descent Approach (CDA), which is safer, more fuel-efficient and quieter than approaches that require the aircraft to “dive and drive” on a stepped descent through the final approach.
*RNP, together with other technologies such as ADS-B, also offers a means to support greater capacity and less airspace congestion in crowded environments. It features prominently in the FAA’s roadmap for its Next Generation Air Transportation System (NextGen), intended to modernize US airspace through 2025.

Implementing RNP

Operators seeking to implement RNP currently face three basic hurdles: aircraft eligibility, operational approval to conduct RNP operations, and publication of airport-specific RNP approach procedures.
*Aircraft eligibility: The aircraft and its avionics must be certified by the FAA, EASA or other relevant regulatory agency that it is capable of meeting the requirements of performance-based navigation. Specific combinations of flight management systems, navigation sensors, flight guidance systems, cockpit displays and other equipment are required, together with the monitoring and alerting capabilities that distinguish performance-based navigation from other satellite-based navigation methods such as RNAV. Aircraft certification typically specifies an accuracy level, such as RNP 0.10 or RNP 0.30, at which the aircraft can navigate.
*Operational approval: The operator must gain operational approval from its regulator to conduct RNP operations. In addition to having an eligible fleet, the operator must incorporate changes to aircrew procedures, dispatch, maintenance, and other areas of operations, similar to the requirements necessary to conduct ILS CAT II/III operations. The operator must also meet stricter requirements for monitoring the integrity of navigation databases and flight-specific availability of levels of RNP performance based on satellite conditions.
*Airport-specific approach procedures: The operator must have access to RNP approach procedures that are designed for the specific runway end they intend to use. These procedures may be tailored to their specific aircraft and operations, or be public procedures available to all approved operators at that airport.

There is a fourth component to RNP operations which has to be considered. Integration of RNP operations in the terminal environment in busy terminal airspace. Currently a trial is being conducted at Brisbane Airport (Australia) integrating RNP operations into terminal airspace. The trial has been running for a year (since 18 January 2007). It has been successful to date with limited participants. The trial continues.

RNP Implementation Worldwide

Implementation of RNP in the terminal airspace began in 1996 with a program by Alaska Airlines to address challenges it faced in Juneau, Alaska. In recent years, operators in Australia, Canada, China, New Zealand and the US began RNP operations, while both ICAO and the FAA made significant progress on evolving the procedure design criteria and other requirements for RNP. Key milestones in RNP implementation worldwide include:

*Australia/New Zealand: Qantas and Air New Zealand launched efforts in the region with RNP procedures at Queenstown, New Zealand. RNP-AR operations are being progressively deployed at 15 airports in Australia. To date terminal RNP procedures are conducted at Cairns, Townsville, Brisbane, Gold Coast, Alice Springs, Ayers Rock, Sydney (limited basis), Canberra and Hobart. January 2008 will also see operations commencing at Mount Isa, Broome, Kalgoorlie and Karratha. Melbourne, Adelaide, Darwin and Port Hadland will follow later in 2008. To date, in excess of 20,000RNP procedures have been flown.Jetstar will commence RNP operations at Brisbane and Gold Coast in March 2008.
*Canada: WestJet has deployed RNP procedures at 22 airports in their Canadian network
*China: Air China conducts RNP operations at multiple airports including Lhasa, Linzhi and Jiuzhaigou. The Linzhi airport, opened in 2006, is notable for allowing only RNP-based operations, since reliable navigation using ground-based navigation aids is not feasible there due to surrounding terrain.
*US: Alaska Airlines conducts RNP operations at 12 US airports in Alaska and the continental US, and credits the technology with 1300 “saved” operations in 2006 due to improved access in marginal weather. Alaska’s sister airline Horizon Air gained approval to conduct RNP operations at specific airports in late 2006. In 2007, Hawaiian Airlines fleet of B767-300 aircraft are already upgraded with RNP enable Flight Management Systems (Honeywell Pegasus) and is seeking certification approvals. Southwest Airlines announced plans to upgrade its entire fleet to RNP capability, and commissioned development of RNP procedures for all airports it serves. American Airlines, Continental Airlines and Delta Airlines have announced similar plans for equipment upgrades and RNP operational approval.

FAA/ICAO Framework

After the initial program with Alaska Airlines in the late 1990s, the FAA evolved its RNP program with development of refined RNP procedure design criteria and other guidelines, culminating with publication of FAA Order 8260.52 and Advisory Circular AC 90-101. In parallel, working groups within ICAO established criteria for RNP-AR (RNP – Authorisation Required) which was published in draft in early 2007. Efforts are underway to harmonize the FAA and ICAO standards.

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