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it prevents design ‘weaknesses’ identified in the Reference Scenario being repeated.

Although the process of describing current Terminal Airspace operations is sometimes considered a tiresome exercise, one of its additional advantages is that it provides the opportunity to discover (and correct inconsistencies) related to the existing design. Examples of these discoveries may include –

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published SIDs/STARs – that are no longer used; . out-dated instrument approach procedures;

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publication errors in the AIP;

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abandoned navigation aids.

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In general terms, the Reference Scenario is a description of the current Terminal Airspace operations. As such, the Reference Scenario describes the current layout of routes andinstrument approach procedures as well as holding patterns, airspace structures, ATC sectorisation and how the traffic is managed within the airspace and in relation tosurrounding airspace.
Given that the (main) purpose of the Reference Scenario is to provide a benchmark against which the new/modified design is compared, the assumptions, enablers and constraints which formed the basis of the Reference Scenario should also be identified.
REFERENCEA SCENARIO

1 The relevance of this is that a comparative assessment is the most usual way in which safety is assessed in those instances where ‘absolute’ measurement is not required. (See Part C, Chapter 3).
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Nevertheless, there are cases when the current Terminal Airspace is not used as the Reference Scenario. This occurs when, for example, previously validated modifications to any aspect of the Terminal Airspace (i.e. routes, or holds or structure or sectorisation) are to be implemented in the short-term i.e. before the implementation of the current project.
Not currently in use but to be implemented in the

Figure 2- 1: Reference and ‘Pseudo’ Reference
Figure 2- 1 illustrates the ‘Psuedo’ current using an example of a change to airspace dimensions. The yet-to-be-implemented change (i.e. (b)) would thus be used as a ‘Pseudo’ Reference against which new changes are measured. This ‘Pseudo’ Reference could equally be a based upon a new route or routeing structure, holding patterns or the sectorisation.
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At this stage of the Design Methodology, creation of the Reference Scenario is mainly a paper exercise. Even so, the detail and quality of the information contained in the Reference Scenario should be such that someone unfamiliar with the Terminal Airspace and its operating practices is able to form a comprehensive ‘picture’ of the airspace.
The Reference Scenario is created from various sources. Ideally, all these sources should be used so as to build the most complete picture as to the current or ‘pseudo’ current Terminal Airspace operations.
Below, an abridged list is provided showing selected items needed in the statement of the Reference Scenario. A comprehensive Checklist and the sample questions is provided in Chapter 8, Attachment C-8-1:
　
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