The Problem that PBN Solved

PBN Concept

The PBN Concept was developed over the period 2004 to 2007 and aimed to resolve the limitations of the RNP Concept.  This was the fact that the RNP Concept described a required navigation performance but did not specify how this would be achieved.  Therefore, different States and regions airspace concepts called for the same performance requirements but demanded differeing aircraft capabilities or operational approval. 

The PBN Concept, which replaced the RNP Concept (and inherited the same ICAO Document Number - 9613), was developed to address the 'how' and to provide a globally coordinated set of Navigation Specifications for the development of common aircraft certification and flight crew operation approval documentation.

Whilst PBN has broadly resolved the uncertainties that the RNP Concept created, and now provides a set of globally recognised Navigation Specifications, there are still challenges that PBN faces and these are considered below:

In 1983, the predicted traffic growth caused concern within ICAO as it was recognised that the current navigation techniques and systems, which had been used for the decades, would not be able to cope with the increased traffic demand.

Therefore, ICAO set up the Future Air Navigation Systems (FANS) committee with a task of studying, identifying and assessing new concepts and technologies to meet the expected demand over a 25 year period.  The FANS committee expressed concerns that the Minimum Navigation Performance Specification (MNPS), which had been evolved in the 1970's for oceanic operations and required specific equipment such as Long Range Navigation Systems - Inertial Navigation Systems and Omega, would become impractical with the variety of GNSS expected to evolve and the emerging continental and terminal expectations using GNSS.  Therefore, the FANS committee introduced the concept of Required Navigation Performance Capability (RNPC).  RNPC defined as a parameter describing lateral deviations from an assigned or selected track, as well as along track positioning accuracy, to produce an appropriate containment value.  Containment value (or containment distance) was defined as the distance from the intended position within which flights would be found for at least 95% of the total flying time.

ICAO subsequently recognised that the aircraft capability and the measured performance was distinctly different and that airspace planning is dependent on measured performance to determine the aircraft 'containment' so that separation minima can be applied with confidence. Thererefore, RNPC was Required Navigation performance (RNP).   Subsequently, ICAO developed the RNP Concept which was defined as a statement of the navigation performance necessary for operation within a defined airspace or on a defined route.

This whole concept revolved around accuracy, which meant that certification standards developed to achieve that accuracy resulted in multiple different standards in use world-wide and no interoperability. This was unnecessary and costly. Interoperability was needed which is why the ICAO navigation specifications are published in the ICAO PBN manual and used as the basis for certification and operational approval.

Although the PBN Concept resolved the issue of interoperability created by the RNP concept, it has not managed to resolve all of the interoperability issues. In fact, the problems can be stated as being 80% resolved leaving roughly a 20% lack of global standardisation. This problem stems, partially, from the enduring mix of ‘old’ and ‘new’ generations of aircraft, that can all be ‘qualified’ to the minimum standard which is not adequately progressive for the growth of air traffic.  A tendency exists, when mandating PBN, to mandate to the ‘lowest common denominator’.  This can be seen in the European region where a PBN regulation PBN IR 1048/2018, mandated RNAV 1 SIDs/STARs in the EU by 2030. RNAV 1 is a specification based on a mid-1990s aircraft generation (which was an upgrade to the 1970s generation before it).  At the same time, there are a great number of very modern aircraft in the fleet, that have all the latest functionalities – that ATC has difficulty exploiting because of the challenges associated with handling mixed-equipage in an airspace.

Probably due to the complexity and sheer volume, ICAO documentation still bears the trace of non-PBN RNAV even though this is not ICAO strategy. The organisation is explicit that all use of area navigation must fall under the PBN umbrella, meaning that aircraft and air crew must be appropriately qualified to use published PBN applications.

The explicit requirement for certification of aircraft has always been evident to the equipment manufacturers. Making certification visibly explicit to the other PBN stakeholders created a considerable awareness as to the cost involved in certifying aircraft for the relevant PBN qualification.  In some cases, it is the manufacturer whose costs are high, and in others, it is the certifying authority. As a consequence, there has been an increasing tendency to use advanced PBN functions (e.g. RNP authorisation required with the Radius to Fix) without requiring the crew or the aircraft to be qualified to the RNP AR standard. Discussions have revealed that this is primarily down to a cost. In some States, RNP AR certification and specific operational approval is prohibitively expensive.

Minimum Navigation Performance Specification (MNPS) was evolved in the 1970's to support operations in oceanic airspace.

The concept of MNPS was that all flights within the airspace achieve the highest standards of horizontal and vertical navigation performance and accuracy. Formal monitoring programmes were undertaken to quantify the achieved performances and to compare them with standards required to ensure that established Target Levels of Safety (TLS) were met.  Collision Risk Modelling was used to estimate risk and target maxima were set for these estimates in terms of potential collisions per flight hour and these were known as ‘Target Levels of Safety’ (TLS).

MNPS defined a set of navigation requirements for operations requiring that aircraft carry navigational equipment which permitted an enhanced degree of accuracy, especially as regards tracking. Equipment redundancy (two independent long-range navigation systems) could be legislated to provide for the failure case.For the North Atlantic, which introduced MNPS airspace in 1977, and where, for the most part, in the NAT Direct Controller Pilot Communications (DCPC) and Radar Surveillance were unavailable. Therefore, aircraft separation assurance and hence safety would be ensured by demanding the highest standards of horizontal and vertical navigation performance/accuracy and of operating discipline.

To operate in MNPS airspace required specific approval(s) was required.  Therefore, aircraft operating within MNPS Airspace were required to meet a Minimum Navigation Performance Specification (MNPS) in the horizontal plane through the mandatory carriage and proper use of a specified level of navigation equipment that has been approved by the State of Registry, or State of the Operator, for the purpose.

Approval for MNPS operations required the checking by the State of Registry or State of the Operator, of various aspects affecting navigation performance. These aspects included: the navigation equipment used, together with its installation and maintenance procedures; plus the crew navigation procedures employed and the crew training requirements.

In the NAT, MNPS has now been replaced by the High Level Airspace (HLA) requiring as a minimum, RNAV 10 capability.

More information can be found on the ICAO/EUR website.

In late 1983, the ICAO Council established the Special Committee on Future Air Navigation Systems (FANS) and charged it with developing the operational concepts for the future of air traffic management (ATM). The FANS committee delivered their report in 1988 and this provided the basis for the aviation industry's future strategy for ATM primarily using digital CNS using satellites and data links.

The FANS committee recognised that the mandatory carriage of certain equipment could constrain the optimum application of the latest avionic equipment.  The committee expressed concerns that the Minimum Navigation Performance Specification (MNPS).  MNPS required specific equipment such as Long Range Navigation Systems and the committee felt that Inertial Navigation Systems and Omega, would become impractical with the variety of GNSS expected to evolve.  Furthermore, with the emerging continental and terminal applications expected to use satellite navigation, the evolution of GNSS could make it difficult for ICAO to justify one specific space-based system over another.  Therefore, to resolve these issues the FANS committee developed the concept of Required Navigation Performance Capability (RNPC).

RNPC defined as a parameter describing lateral deviations from an assigned or selected track, as well as along track positioning accuracy, to produce an appropriate containment value.  Containment value (or containment distance) was defined as the distance from the intended position within which flights would be found for at least 95% of the total flying time.

ICAO had established the Review of General Concept of Separation Panel (RGCSP) at the beginning of the 1970s. The Panel progressively refined airspace design goals through the setting of a Target Level of Safety (TLS) as the measure of tolerability for airspace design.  A fatal accident is defined as one in which there are one or more fatalities. In considering the trans-Atlantic airways, a goal of no collision in less than 150 years was originally required. This goal has been progressively refined to a point where the probability of failure must be less than, or equal to, 5 x 10-9 fatal accidents/flight hour (fa/fh) per navigation dimension.

When the FANS committee defined RNPC it was in terms of lateral and along track position fixing accuracy. The RNPC concept was approved by the ICAO Council and was assigned to the RGCSP for further elaboration. The RGCSP subsequently recognised that the aircraft capability and the measured performance was distinctly different and that airspace planning is dependent on measured performance to determine the aircraft 'containment' so that separation minima can be applied with confidence. Therefore, in 1990 the RGCSP noting that measured performance was more relevant for airspace planning than the designed-in capability, changed RNPC to the Required Navigation Performance (RNP) concept. The RNP Concept was defined as a statement of the navigation performance necessary for operation within a defined airspace or on a defined route.

The ICAO Manual on Required Navigation Performances (RNP), Doc 9613, as developed by the RGCSP and accepted by the ICAO ANC, described the application of RNP mainly for en-route purposes. 4 RNP types were defined, which could be applied to airspace's and/or routes, and which set accuracy requirements to airborne navigation equipment and requirements for the navigation infrastructure. In this context the navigation performance accuracy was understood as the combination of the navigation sensor error, airborne receiver error, display error and flight technical error.  For example, to enter RNP4 airspace an accuracy of 4 NM (95%) was required. RNP was described as only one (fundamental) factor in the determination of safe separation standards. Other factors such as ATC intervention capability and traffic exposure (traffic density) were also considered.  However, in the early period no integrity, availability and continuity of service requirements were associated to the various RNP types that the RGCSP developed.

It was recognised that accuracy requirements were not sufficient for the description of the entire system performance.  Therefore, the joint work of EUROCAE (WG13) and RTCA (SC181) would provide the definition of integrity, availability and continuity of service requirements for oceanic, continental and terminal phases of flight.

The RNP concept revolved around accuracy, which meant that certification standards were developed to achieve that accuracy.  This resulted in multiple different standards to meet the same performance and leading to a lack of global interoperability; this was unnecessary and costly. Interoperability was needed and is why the ICAO introduced Performance-based Navigation (PBN).  Today, navigation specifications are published in the PBN manual and is designed to be the basis for certification and operational approval worldwide.

Based on Recommendation 1/1 of the fourth meeting of the Global Navigation Satellite System Panel (GNSSP/4), the Air Navigation Commission created the RNP Special Operational Requirements Study Group (RNPSORSG) in June 2003 to act as a focal point for addressing several issues related to RNP/RNAV. The RNPSORSG recognised that while the RNP Concept identified a performance requirement for airspaces and/or ATS routes, it did not explain how this would be achieved.  This led to different States and/or regions to introduce operations with the same performance requirement but with differing certification and/or operational approval requirements developed for that specific operation.  The problem with this concept was that there were costs for the airspace users to be qualified for multiple airspaces.  Moreover, there was the real danger that the flight crew and/or the aircraft were not qualified for that specific operations when they believed they were.

Therefore, to stop further proliferation of localised area navigation applications the RNPSORSG developed the concept of performance-based navigation (PBN); the concept was delivered in April 2007.  The Study Group started by reviewing the certification standards for all area applications already in operation in 2003.  The RNPSORSG used these standards as the basis for developing a set of navigation specifications that could be applied world-wide.  These navigation specifications detailed not only the required performance the aircraft had to deliver but also informed all the stakeholders how to achieve that performance.

ICAO took the decision that as PBN replaced RNP, then the manual on PBN would replace and supersede the RNP manual.  Therefore, when the first edition of PBN manual was officially published in 2008 it was ICAO Doc 9613, Edition 3.  The RNPSORSG was stood down following the delivery of the PBN manual.  In its place, the PBN Study Group (PBNSG) was set up to oversee future updates of the manual.

There has been one evolution of the PBN manual and the PBNSG introduced three new navigation specifications; the fourth edition of Doc 9613 was published in 2013.  Since 2017, the PBNSG has been working on a second evolution of the PBN manual.  This update will see the introduction of RNP Authorisation Required (AR) departures, changes to A-RNP as well as ongoing work on vertical navigation.

ICAO published the first edition of PBN manual in 2008 as ICAO Doc 9613, Edition 3.

When this edition of the manual was published, there were eight navigation specifications published in Volume II.  Each of these specifications already existed as a navigation application in some part of the world.  In Europe, we had mandated Basic RNAV (B-RNAV) in April 1998 for en route operations and, although not mandated at the European level, we had developed Precision RNAV (P-RNAV) for terminal operations.  The RNAV 5 navigation specification is B-RNAV; the European certification standard was the basis of this nav spec.  P-RNAV required a lateral performance of +/- 1 NM which is the performance requirement of RNAV 1; however, the P-RNAV certification allowed for limited use of VOR, whereas the RNAV 1 specification only allows use of GPS and DME/DME.  Therefore, European aircraft operators (AOs) who had certified their aircraft against the P-RNAV standard (Temporary Guidance Leaflet (TGL) 10 Revision 1) had to confirm that their aircraft met the RNAV 1 standard.  For this specification, the RNPSORSG had developed a set of equivalency tables to assist AOs understanding whether their aircraft met RNAV 1 or not.

The PBNSG, who replaced the RNPSORSG, updated the manual in 2013.  Three new specifications (RNP 2, RNP 0.3 and A-RNP) together with other functionality additions were introduced.  Currently, the PBNSG is finalising the next edition of the manual.  It is expected that this fifth edition of Doc 9613 will see the introduction of RNP Authorisation Required (AR) departures, changes to A-RNP as scaleability is not operating consistently across the different aircraft and ongoing work on vertical navigation.

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