Like the Future Air Navigation System (FANS), the Performance-Based Navigation (PBN) system is a concept of how we ensure our aircraft are where we say they are, when we say they are. The designation is mostly consistent, but not completely.
— James Albright
Lateral Navigation Errors, from ICAO Doc 9613, figure ii-a-2-1
The PBN concept specifies that aircraft RNAV and RNP system performance requirements be defined in terms of the accuracy, integrity, continuity and functionality, which are needed for the proposed operations in the context of a particular airspace concept. The PBN concept represents a shift from sensor-based to PBN. Performance requirements are identified in navigation specifications, which also identify the choice of navigation sensors and equipment that may be used to meet the performance requirements. These navigation specifications are defined at a sufficient level of detail to facilitate global harmonization by providing specific implementation guidance for States and operators.
Source: ICAO Doc 9613, p. I-(iii)
This isn't to say PBN systems cannot use a particular type of sensor, only that the sensor isn't the defining characteristic of the specification. An acceptable solution could be GPS-based or even DME/DME-based. But approval for that solution for the specification depends on more than just what sensor is used.
Under PBN, generic navigation requirements are defined based on operational requirements. Operators then evaluate options in respect of available technology and navigation services, which could allow the requirements to be met. An operator thereby has the opportunity to select a more cost-effective option, rather than a solution being imposed as part of the operational requirements. Technology can evolve over time without requiring the operation itself to be reviewed, as long as the expected performance is provided by the RNAV or RNP system.
Source: ICAO Doc 9613, p. I-(iii)
The navigation specification
The navigation specification is used by a State as a basis for the development of their material for airworthiness and operational approval. A navigation specification details the performance required of the RNAV system in terms of accuracy, integrity, availability and continuity; which navigation functionalities the RNAV system must have; which navigation sensors must be integrated into the RNAV system; and which requirements are placed on the flight crew.
Source: ICAO Doc 9613, ¶1.2.
On-board performance monitoring and alerting is the main element that determines if the navigation system complies with the necessary safety level associated to an RNP application; it relates to both lateral and longitudinal navigation performance; and it allows the aircrew to detect that the navigation system is not achieving, or cannot guarantee with 10–5 integrity, the navigation performance required for the operation.
Source: ICAO Doc 9613, ¶126.96.36.199
The 10-5 integrity is the 0.9999 probability concept. The monitoring and alerting is the key component of a PBN specification.
Both RNAV and RNP specifications include requirements for certain navigation functionalities. At the basic level, these functional requirements may include:
- continuous indication of aircraft position relative to track to be displayed to the pilot flying on a navigation display situated in his primary field of view;
- display of distance and bearing to the active (To) waypoint;
- display of ground speed or time to the active (To) waypoint;
- navigation data storage function; and
- appropriate failure indication of the RNAV system, including the sensors.
Source: ICAO Doc 9613, ¶188.8.131.52
- For oceanic, remote, en-route and terminal operations, an RNP specification is designated as RNP X, e.g. RNP 4. An RNAV specification is designated as RNAV X, e.g. RNAV 1. If two navigation specifications share the same value for X, they may be distinguished by use of a prefix.
- For both RNP and RNAV designations, the expression “X” (where stated) refers to the lateral navigation accuracy in nautical miles, which is expected to be achieved at least 95 per cent of the flight time by the population of aircraft operating within the airspace, route or procedure.
- Approach navigation specifications cover all segments of the instrument approach. RNP specifications are designated using RNP as a prefix and an abbreviated textual suffix, e.g. RNP APCH or RNP AR APCH. There are no RNAV approach specifications.
- Because functional and performance requirements are defined for each navigation specification, an aircraft approved for an RNP specification is not automatically approved for all RNAV specifications. Similarly, an aircraft approved for an RNP or RNAV specification having a stringent accuracy requirement (e.g. RNP 0.3 specification) is not automatically approved for a navigation specification having a less stringent accuracy requirement (e.g. RNP 4).
Source: ICAO Doc 9613, ¶1.2.5
This gets messy, here are two examples:
- If your airplane is approved for RNP 0.3 and RNP 10, you might think RNP 4 is automatically included. But it isn't. RNP 0.3 does not have the same communications requirements of RNP 4, specifically CPDLC and ADS-C. RNP 10 isn't a PBN specification at all. So you cannot infer you are RNP 4 qualified.
- If you have an RNP 4 qualification you are good to go for RNP 10. In fact, if you had an LOA that said RNP 10 and got approval for RNP 4, the RNP 10 disappears.
- Be careful about RNP 4. Some U.S. FSDOs are granting RNP 4 LOAs based on the mistaken notion that aircraft with P-RNAV approval are more accurate than RNP 4. That might be true and it will work in the Caribbean. But it won't work in the rest of the world where RNP 4 also requires CPDLC and ADS-C.
- The existing RNP 10 designation is inconsistent with PBN RNP and RNAV specifications. RNP 10 does not include requirements for on-board performance monitoring and alerting. For purposes of consistency with the PBN concept, RNP 10 is referred to as RNAV 10 in this manual. Renaming current RNP 10 routes, operational approvals, etc., to an RNAV 10 designation would be an extensive and expensive task, which is not cost-effective. Consequently, any existing or new operational approvals will continue to be designated RNP 10, and any charting annotations will be depicted as RNP 10.
- In the past, the United States and member States of the European Civil Aviation Conference (ECAC) used regional RNAV specifications with different designators. The ECAC applications (P-RNAV and B-RNAV) will continue to be used only within those States. Over time, ECAC RNAV applications will migrate towards the international navigation specifications of RNAV 1 and RNAV 5. The United States migrated from the USRNAV Types A and B to the RNAV 1 specification in March 2007.
Source: ICAO Doc 9613, ¶1.2.5
- The airworthiness approval process assures that each item of the RNAV equipment installed is of a type and design appropriate to its intended function and that the installation functions properly under foreseeable operating conditions. Additionally, the airworthiness approval process identifies any installation limitations that need to be considered for operational approval. Such limitations and other information relevant to the approval of the RNAV system installation are documented in the AFM, or AFM Supplement, as applicable. Information may also be repeated and expanded upon in other documents such as pilot operating handbooks or flight crew operating manuals. The airworthiness approval process is well established among States of the Operators and this process refers to the intended function of the navigation specification to be applied.
- Approval of RNAV systems for RNAV-X operations. The RNAV system installed should be compliant with a set of basic performance requirements as described in the navigation specification, which defines accuracy, integrity and continuity criteria. It should also be compliant with a set of specific functional requirements, have a navigation database, and support each specific path terminator as required by the navigation specification.
- Approval of RNP systems for RNP operations. Aircraft must be equipped with an RNP system able to support the desired navigation application, including the on-board performance monitoring and alerting function. It should also be compliant with a set of specific functional requirements, have a navigation database, and should support each specific path terminator as required by the navigation specification.
Source: ICAO Doc 9613, ¶3.4.2
You do not get approved for PBN, rather you get approved for each specification. Details here:
ICAO Doc 9613 - Performance Based Navigation (PBN) Manual, International Civil Aviation Organization, Fourth Edition, 2013