8.2.1 INTRODUCTION
In Part C, Chapter 3, discussions concerning Safety Criteria explained the difference between Absolute and Relative methods of evaluating safet in the context of requirements for ATS authorities to undertake safety assessment as per ICAO Annex 11. In Chapter 3, it was explained that safety can be evaluated using one of two methods viz. the Comparative method (by comparing a Reference System to a Proposed System) or the Absolute method (by comparing a Proposed system against an Absolute threshold. It also explained when it is necessary to evaluate (safety) against an absolute threshold.
8.2.2 HOW IT WORKS
Collision Risk Modelling (CRM) is a useful way of assessing complex interactions in the ATM system and determining whether these interactions are safe. Its use is associated with the evaluation of a proposed system’s risk against an absolute threshold (of maximum tolerable risk).
Viewed diagrammatically, the evaluation of risk against an absolute threshold can be depicted as follows:
Evaluate a system risk against an absolute threshold
Estimate risk of collision and e.g. Maximum tolerable risk of
compare it to the maximum collision not to exceed 5 fatal
tolerable collision risk (right) accidents (or 2.5 collisions) per
109 flight hours (fapfh). This is
normally expressed as a Target
Level of Safety (TLS) e.g. 5x10-9 .
Figure 8 - 1: Risk Evaluation against an absolute threshold
The idea is that if the estimated risk is less than the maximum tolerable risk and can remain so during the anticipated lifetime of a proposed system, then the proposed system can be considered safe. In order to evaluate system risk against a threshold, ICAO has developed a
1 Material in this section has been derived from [1] ICAO Doc. 9689 – Manual on Airspace Planning Methodology for the Determination of Separation Minima and [2] ICAO Doc. 9426, the ATS Planning Manual.
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process which is detailed in Doc. 9689 – Manual on Airspace Planning Methodology for the Determination of Separation Minima. This process envisages the following sequence of steps:
[i] Define proposed system e.g. airspace structure, proposed separation minima, complexity of airspace, airspace classification. COMM/NAV/SUR capability and their error rates, physical parameters of aircraft, aircraft navigation performance etc.
[ii] Set evaluation criteria e.g. Maximum tolerable risk of collision not to exceed 2.5 collisions or 5 fatal accidents per 10-9 flight hours.
[iii] HAZID i.e. Identification of all possible hazards, frequency estimation and consequence modelling (the last two provide overall risk estimates, below at [iv])
[iv] Risk Estimation is the result of frequency estimation and consequence modelling.
[v] Risk Evaluation is the process where by the estimated risk [iv] is compared to the evaluation criteria [ii]*.
Risk evaluation involves the construction of mathematical models, which use detailed information about the system
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