Most airspace designers are familiar with the comparative (or relative) method because it is the most and frequently used. When safety is evaluated using this method, the safety of the proposed Terminal Airspace design is compared in relation to an existing design (called a Reference Scenario – see Part C, Chapter 2). Use of this method could therefore show an increase/decrease or maintenance of safety of a proposed design which has been compared to a Reference Scenario.
In contrast the absolute method involves evaluating safety against an ‘absolute’ threshold. An example of such an absolute threshold could be: that the risk of collision is not to exceed 5 fatal accidents per 1 000 000 000 flight hours. (This would more commonly be expressed
2 See Part A Chapter 2, First Principle.
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as a requirement to meet a target level of safety (TLS) of 5x10-9). A collision risk analysis using a collision risk model is the usual way in which a determination is made as to whether a TLS is being met – See Part D, Chapter 8.
Notably, safety criteria are set whichever method is used. Differently put, a benchmark is created whether the absolute or comparative method is used. The benchmark, however, is different dependent upon the method used. Whatever the method and/or safety criteria used, a safety evaluation can only be rigorous if qualitative assessment forms the backbone of the evaluation process.
ComparativeMethod
Evaluation
Quantitative
Absolute Method
safety of Proposed Scenario is evaluated against an absolute threshold
Figure 3 - 2: Assessment & Evaluation
(
Qualitative
safety of Proposed Scenario is( Relative to a Reference Scenario
Safety
It should be noted that the safety of an airspace design is not only dependent upon the correct application of design criteria when designing routes, holding areas, and airspace structures designed in accordance with the design rules and procedures contained, inter alia, in ICAO Annex 11 and Doc. 8168 (especially Vol. II). Safety factors are considered before and during this design phase, by, for example –
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developing a feasible airspace design concept (see Part C, Chapter 5 and 6) prior to the application of the PANS-OPS design criteria; and
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ensuring the accuracy of critical aircraft and operational assumptions which are used to form the basis of the PANS-OPS design;
In the ‘greater’ context, the design is also required to satisfy the safety objectives which are included, but not limited to the generic ATC objectives and whether these are met is most often determined by qualitative assessment. Thus whilst Annex 11 and Doc. 8168 provide rules relating to airspace dimensions and obstacle clearance criteria respectively, qualitative assessment criteria are included, but not limited to, PANS-ATM and various ICAO Annexes.
Comment:
How does the designer know when safety should be evaluated using the absolute method? Typically, the
absolute method is to be used when required by ICAO. This usually involves instances when the change
envisaged is radical and untried elsewhere (see Ref.1). For example:
-reduction of the vertical separation minima (RVSM)
-determination of new spacing between parallel ATS routes for which lateral navigation accuracy is specified with a view to applying the separation minima in PANS-ATM Chapter 5, as a basis for route spacing in Terminal Airspace; (see Ref 2)
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