The Safety Analysis and Risk Mitigation Process  Section 3  49 3.5_SMSM_201904  Originally published April 2019  Uncontrolled copy when downloaded The values in  Table 3.5 are derived from an analysis of historical ATC data mapped to the  established engineering standard (the current version of Advisory Circular 25.1309-1,  System  Design Analysis ) and can be applied to both ATC and Flight Procedures. The ratios binding  each expected occurrence rate range were determined through calculations made using ten  years of aviation data. In each calculation, the numerator was the number of occurrences of a  given severity level occurring during a ten-year period, as obtained from various relevant  databases. The denominator was the number of ATO operations (or flight hours) in that  ten-year period, as obtained through the Operations Network database or the National  Transportation Safety Board database. The value was adjusted to reflect a forecasted air traffic  increase. A cut-off point of 10 -14  was established to define the boundaries of credible events for  the purposes of calculating likelihood. Figure 3.6 depicts the likelihood continuum and the  expected occurrence rate ranges.  For some NAS changes, the necessary data are not available. There may not be a similar  enough change/procedure/situation in the NAS to provide similar data from which to estimate a  rate of occurrence. In situations where modeling is not feasible, pure subject matter expertise is  the only input available, providing a qualitative approach to determining likelihood. This  approach is only recommended when all avenues of data collection have been exhausted or  when the change proponent is attempting to implement a new operation for which no data exist. For a majority of changes to the NAS, SMEs can collect and analyze data from a similar NAS  change to determine the number of expected occurrences of an effect.  Table 3.5: Likelihood of the Effect Standards – ATO Operations and NAS Equipment  3.5.4.3.4  Determining Likelihood When No Data Are Available  Figure 3.6: Likelihood Continuum  Operations: Expected Occurrence Rate (per operation / flight hour / operational hour 3 )  Quantitative (ATC / Flight Procedures / Systems Engineering)  Frequent  A  (Probability) ≥ 1 per 1000 Probable  B  1 per 1000 > (Probability) ≥ 1 per 100,000 Remote  C  1 per 100,000 > (Probability)  ≥ 1 per 10,000,000 Extremely Remote  D  1 per 10,000,000 > (Probability) ≥ 1 per 1,000,000,000 Extremely Improbable  E  1 per 1,000,000,000 > (Probability) ≥ 1 per 10 14 3. It is important to note that the close correlation between flight hours and operations is entirely coincidental; average flight time is roughly two hours, and each flight has about two Tower and two TRACON operations. The two numbers are not interchangeable.