Activity 6 - Agree Assumptions, Enablers and Constraints including Fleet Profile

Having the right information at the right time provided by the right CNS/ATM expertise within the team allows for informed decision making which provides the basis for a successful concept development.  Future technical system components require initial consideration to facilitate overall system integration in Activity 14.

For the Airspace Concept to be realised, the technical operating environment needs to be agreed. This requires knowledge, as regards the ground infrastructure and airborne capability, as to which CNS/ATM enablers are already ‘available’, the limitations or constraints which exist and what the future environment will be when the when the Airspace Concept is implemented: the assumptions. Whilst enablers and constraints are usually not difficult to establish, agreeing assumptions can be challenging. Their 'realism' is important because the airspace concept which is designed and the PBN specification(s) used as a basis for that design relies on these assumptions being correct.

ATM/CNS assumptions cover a wide field and need to take account of the expected environment applicable for the time when the new airspace operation is intended to be implemented (e.g. in 20XX). General assumptions include, for example: the predominant runway in use within a particular TMA; the percentage of the operations which take place during LVP; the location of the main traffic flows; (in 20XX, are these likely to be the same as today? If not how will they change?); the ATS Surveillance and Communication to be used in 20XX. (Should any specific ATC System aspects be considered e.g. a maximum of four sectors are possible for the en route airspace because of software limitations in the ATM system.

Several key assumptions (related to future enablers) of importance to PBN are singled out for additional explanation. These relate to the airborne navigation capability and Navaid Infrastructure availability.

Traffic assumptions are of crucial importance to the new Airspace. First, the traffic mix must be known: what proportion is there of jets, twin turboprops, VFR single-engined trainers etc., and what are their ranges of speeds, climb and descent performance. Understanding the fleet mix and aircraft performance is important to any airspace concept development, but in a PBN Implementation context, traffic assumptions related to fleet navigation capability are the most significant. This is because the predominant navigation capability in the fleet provides the main indicator as to which ICAO navigation specifications can be used as the basis for designing the airspace concept to make the PBN Implementation cost effective.

A Cost Benefit Analysis (CBA) is an effective way of determining whether the design of PBN ATS routes (incl. SIDs/STARs and instrument approach procedures) will be cost effective. (The Navaid Infrastructure costs are also integral to a CBA and are discussed in a subsequent bullet). Particularly when an airspace mandate is envisaged, the higher the number of aircraft already qualified for the intended navigation specification, the lower the retrofit costs and benefits can be realised more quickly. This is the primary reason that the PBN IR calls for RNAV 1 SID/STARs as the vast majority of the European fleet are, by now, certified to that standard and with the PBN privileges now part of the pilot's Instrument Rating (IR) the flight crews are approved for the operation.

However, a high fleet equipage with a particular functionality is only helpful if ALL the functionalities associated with the targeted navigation specification are also widely available in the fleet. This means that for PBN implementation to be cost effective, the majority of the fleet should have all the capability required in the navigation specification intended for implementation. Partial qualification for a navigation specification is not possible and mixed mode operations provides workload challenges for ATC and the cockpit alike.

Note, however, that the PBN IR is specific about which navigation specifications are to be used. Only those specifically mentioned are to be used, they are not a baseline minima.

Flight plans and an equipage questionnaire are useful tools for analysing fleet capability. 

Flight plans can provide the State and the Service Provider with awareness of current aircraft certification and crew operational approval.  This is achieved by collating the PBN and equipment capability information provided in Item 10 and 18 of the flight plan.  However, the information provided in the flight plan is only as good as the awareness and understanding of PBN codes by the pilot and flight operations community.  EUROCONTROL provides access to the CNS Dashboard, which is a tool collating European flight plan information. 

In undertaking an equipage questionnaire to make a fleet analysis, it is highly beneficial to determine what planned area navigation system upgrades are planned/expected in the period running up to implementation; these may affect the implementation date and significantly impact the CBA. The certification of a specific PBN capability and maintaining pilot currency in the operation of that capability is costly for the operator. As a result, especially with regional operations, operators will only seek approval sufficient to meet the existing navigation requirements for the airspace. The (new) Airspace Concept may require functionality present in the software but not specified in the existing certification.  While it will cost operators to gain approval and undertake the pilot training for this new functionality, the cost is likely to be significantly less than if the aircraft required retrofitting with new equipment or software as well as having an adverse effect on implementation timescales.

The PBN Manual makes it clear that the ICAO navigation specifications cover certain flight phases. For Terminal operations, for example, there are essentially three available navigation specifications i.e. RNAV 1, RNP 1 and Advanced RNP together with RNP 0.3 for rotorcraft. The PBN Manual also explains that certain RNP specifications can be 'augmented' by additional functionalities such as Radius to Fix (RF). So if the airspace concept is for a complex, high-density airspace where routes are to be placed in close proximity, an RNP specification with some extra functionalities are more likely to provide that extra design capability. So in such a case, the fleet analysis could, from the outset, be probing for fleet equipage related to functionalities associated with either/both the Advanced RNP or RNP 1 functionalities thereby focusing the fleet analysis.

The PBN IR requirements are stated in Activity 1. Choices can be made in the PBN IR as regards the navigation specification and functions which may be used. For example, if repeatable, predictable turns are required, the Radius to Fix Function (RF) can be implemented but only with RNP 1 SIDs/STARs or with the RNP APCH. Furthermore, PBN IR also provides the opportunity to apply RNP 0.3 for rotorcraft operations. 

Note that A-RNP, could bring benefits for airports with parallel runways looking to utilise RNP approaches to support simultaneous parallel approach operations as the performance in the initial and intermediate phases of the approach is the same as the performance requirement of the RNP APCH in the Final Approach Segment ie +/- 0.3 NM.  Note however, that Advanced RNP is not authorised for use by the PBN IR.

The Navaid Infrastructure is comprised of all navigation aids permitted by PBN, be they ground or space based. Navaids provide positioning information which is received by the appropriate on-board sensor providing input to the navigation computer. The flight crew in combination with the Flight Management System (FMS)/navigation computer enables path steering to be maintained along a route within a required level of accuracy.

Ground-Based (or terrestrial NAVAIDS) permitted for use with navigation specifications include DME, and to a more limited extent VOR. NDB and LORAN are not a PBN positioning sources.

Space-Based NAVAIDS are synonymous with GNSS (including augmentation systems). Existing operational GNSS constellations include GPS (USA), GLONASS (Russia) with the following navigation constellations evolving but not yet declared as Full Operational Capability (FOC)l: Galileo (EU), Beidou III (China) and QZSS (Japan). External augmentation systems include wide-area and local area augmentations (termed Space Based Augmentation System or Ground Based Augmentation System, SBAS and GBAS, respectively). Wide-area augmentations are included in PBN; operational GNSS augmentations in use today include EGNOS (Europe) and WAAS (US), Gagan (India) and MSAS (Japan); SDCM the Russian satellite-based augmentation system, monitoring both GLONASS and GPS is still under development.

One of the original aims of PBN is to permit aircraft to use any available sensor (e.g., navigation aid and/or aircraft integration with IRU, inertial reference unit). However, in practice this freedom of choice is increasingly limited by the performance capabilities of the NAVAIDs to meet the accuracy required to meet a particular navigation specification.  Therefore, as the required performance becomes more demanding then only a specified set of sensor combinations will be suitable to achieve the performance requirements stipulated in a specific navigation specification. Therefore, on the NAVAID infrastructure side, the Service Provider must ensure the sensor or sensors permitted in the State's airspace to meet the desired performance have the matching NAVAIDs and that these navigation facilities are available and provide coverage over the desired airspace volume.

Each navigation specification stipulates which positioning sensor may be used for a particular navigation application, as can be seen from the table on the next page. The table shows that the only navigation specification with full sensor flexibility is RNAV5. The flexibility gets reduced the more demanding the navigation specification becomes. The table also shows that only GNSS is able to meet the requirements of any navigation specification. Because GNSS is available globally, it is essential to make GNSS available for aviation use. The steps required to do this are described in detail in the ICAO GNSS Manual (ICAO Doc 9849). However, as GNSS can easily be interferred with it is logical that implementers consider mitigations for GNSS signal loss; this should form an important element of the implementation safety case as it should be on the hardard identification (HAZID) list.  more information on loss of the GNSS signal can be found here.

Consequently, matching up the local fleet avionics capability with a particular navigation specification requires that infrastructure is available to support all potential airspace users. Specifically, Air Navigation Service Providers should provide VOR/DME infrastructure for RNAV5, and DME/DME infrastructure for RNAV5, RNAV1 and potentially also RNP specifications. However, if it would be cost prohibitive or impractical (terrain limitations etc.) to provide a specific type of infrastructure coverage, then this limitation of sensor choice will need to be declared in the AIP, with the consequence that airspace users who do not have the required sensor combination can not use those routes or procedures. Aligning airspace requirements with aircraft PBN equipage and available NAVAID infrastructure is the interactive process implied by the PBN triangle. Normally it is the navigation aid engineering department which performs the assessment of available infrastructure, in cooperation with procedure designers and flight inspection services. If facility changes are required to enable a certain application, such as the installation of a new DME or the relocation of an existing facility, sufficient lead time is required. Consequently, this interaction should take place as early as possible to determine the initial feasibility of the infrastructure to meet airspace requirements. More information on the infrastructure assessment process can be found here. The input that is needed for this activity from airspace planners is which type of coverage is needed in which geographic area (horizontal and vertical dimensions). In setting those requirements, it should be remembered that providing terrestrial navaids coverage is increasingly difficult at lower altitudes.

The traffic sample for the new Airspace Concept is as important as the knowledge of the fleet and its navigation performance composition. This is because RNP and RNAV route placement (be they ATS Routes, SIDs/STARs or Instrument Approach  Procedures) is decided to ensure maximum flight efficiency, maximum capacity and minimum environmental impact. In a terminal area, for example, RNP or RNAV SIDs and STARs/Approaches provide the link between the major en route ATS routes with the active runway (hence the importance of knowing the primary and secondary runway in use). 

A traffic sample for a new Airspace Concept is usually a future traffic sample i.e. one where certain assumptions are made about the fleet mix, the timing of flights, and the evolution of demand with respect to both volume and traffic pattern. Various models are used to determine air traffic forecasts, e.g. the econometric model, and it is not surprising to note that the success of an airspace design can stand or fall on its traffic assumptions. Despite ATC’s intimate knowledge of existing air traffic movements, the future traffic sample for 20XX must be thoroughly analysed (in very futuristic cases, it may even be necessary to create a traffic sample). Invariably, certain characteristics will be identified in the traffic sample e.g. seasonal, weekly or daily variations in demand (see diagram at left); changes to peak hours and relationship between arrival and departure flows (see diagram).

Once the Implementation team has agreed the anticipated future environment then these assumptions and agreed future enablers should continue to be the stable basis for the project. That said, continuous revision of validation of these assumptions and enablers should be ensured particularly where projects continue over several years.

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