Chapter 8 at Attachment C.8-4 provides a checklist which designers may find useful for the identification of assumptions, constraints and enablers. Although some of the items on the sheet do not always appear to be directly related to issues of Terminal Airspace design issues, many of them capture the factors which may influence the design plan.
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Figure 4 - 6: Phases for Identifying Assumptions, Constraints & Enablers
As shown in the above diagram, Assumptions, Enablers and Constraints are identified at different stages of the design process. Constraints and Enablers enter the design process during the critical review of the Reference Scenario where the constraints and enablers refer to the Reference Scenario. The Assumptions are identified prior to commencing the conceptual design -–and these are verified at different stages of the process. During the design process i.e. the conceptual design of Routes, Holds, Structures and Sectors, constraints, mitigation and enablers are identified. In some cases, a Cost-Benefit analysis may be required (see para. 4.2.4).
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The importance of correctly identifying assumptions, constraints and enablers cannot be over-stated for it is on these elements that the design concept of the Terminal Airspace rests. Most importantly, these assumptions, constraints and enablers should be realistic.
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Area Navigation as an Enabler.
Whilst communication, surveillance and navigation are all vital elements to be considered in the design of a Terminal Airspace, the importance of navigation into the design equation has increased through the application of area navigation (RNAV) in Terminal Airspace.
In its Definitions, PANS-ATM describes Area Navigation (RNAV) as A method of navigation which permits aircraft operation on any desired flight path within the coverage of station-referenced navigation aids or within the limits of the capability of self-contained aids, or a combination of these. (Attention is drawn to the fact that this definition describes a navigation method and not RNAV system capabilities or certification).
In order to design RNAV Terminal Routes (e.g. SIDs/STARs for use by RNAV-equipped aircraft in Terminal Airspace), coherency is required between
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the availability of a navigation infrastructure that supports the standard of RNAV to be employed. (Navaids can be ground- or space-based or self-contained on-board the aircraft); and
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