ATM Modernization, Business & GA, Commercial

Perspectives: Updating the RNP Concept

By Erwin Lassooij | March 1, 2006
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The International Civil Aviation Organization (ICAO) is revising its concept of required navigation performance (RNP) to meet the demands of aircraft operators. The intent is to implement performance-based navigation in order to improve efficiency, using existing aircraft capabilities, and increase airspace capacity through reduced separation minima. ICAO expects that the revised concept will harmonize currently available area navigation (RNAV)- and RNP-designated, performance-based navigation applications, particularly in the terminal area, where a divergence in implementations exists.

The guidance will appear in a new ICAO Manual that aims to eliminate confusion regarding terminology and implementation. ICAO also is preparing new International Standards and Recommended Practices for air traffic control and aircraft operations. This will ensure that regulatory authorities base their national regulations on one common international framework, assuring harmonization and minimizing impact on aircraft equipage and safety oversight. This material will provide states a "one-stop shop" guide for implementing performance-based navigation now and in the future.

The current ICAO concept was widely acknowledged and well received. However, the industry perceived it as not detailed enough to be of practical use, especially in terminal airspace. Industry then developed RNP/RNAV, which provided a more comprehensive technical concept for the performance, design, development, implementation and qualification of aircraft navigation systems.

Recognizing the Problem

As aircraft system capabilities evolved, it became readily apparent that ICAO’s original guidance did not meet current industry demands, which resulted in industry’s going in slightly different directions, causing confusion in the community. Different types of RNP and/or RNAV were implemented, based on the needs in different regions–e.g., RNP 5 and Precision RNAV (P-RNAV). These implementations met the requirements in these regions. However the potential for continuing proliferation raised concerns in the international community. Aircraft operators were facing the increasing burden of regulatory compliance necessary to operate in the various regions, each having a different approval requirement. Potential safety risks also were identified, as operators and flight crews attempted to comply with all the pertinent regulations in an environment where the rules changed from region to region.

In order to solve the problem, ICAO established a new group, the Required Navigation Performance Special Operational Requirements Study Group, to address the issues and make recommendations on how to proceed.

The ICAO study group concluded that it was feasible to develop a globally harmonized concept that would meet most of the current operational requirements and yet be flexible enough to meet future requirements. The group recognized that industry developments in onboard, self-contained performance monitoring and alerting requirements were valuable, and even of critical need in some cases, especially in the final approach phase. The group also recognized that these capabilities would not necessarily be required to satisfy the operational requirements in all types of airspace, or in every application within a given airspace, and would not be cost-beneficial. It therefore concluded that a concept focused on performance-based navigation, taking into account elements of both existing concepts, would best answer the need.

An essential element of the performance-based navigation concept is the recognition that a clear distinction must be made, in the designation of operations, between those aircraft that require onboard performance monitoring and alerting and those that do not. It was agreed that operations that do not require onboard performance monitoring and alerting should be designated RNAV-X, while operations that require these capabilities would be designated as RNP-X, where "X" refers to the lateral navigation accuracy in nautical miles that is expected to be achieved during at least 95 percent of flight time.

The specifications associated with each designation meet current operational requirements, while allowing global harmonization, leading to greater efficiency and lower costs for aircraft operators and enhancing safety. Furthermore, the specifications are fully compatible with existing implementations. For example, RNAV-1-approved aircraft can fly in P-RNAV airspace. The concept also needs to be flexible enough to accommodate potential future requirements for 4D navigation.

Erwin Lassooij serves as technical officer, operations/airworthiness in ICAO’s Flight Safety Section.

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