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43 LDOR Probability Distribution of Risk 50% 45% % Ops @ Risk Level 40% % Ops @ Higher Risk 35% Frequency 30% 25% 20% 15% 10% 5% 0% 3.36E-09 1.41E-08 2.49E-08 3.56E-08 4.63E-08 5.71E-08 6.78E-08 7.86E-08 8.93E-08 1.00E-07 1.11E-07 1.22E-07 1.32E-07 1.43E-07 More Risk Figure 37. Typical frequency distribution of risk from prototype software. Step 7--Total Percentage of These differences are most likely related to the airport's Operations with Risk Above TLS runway use policy. In fact, data on the airport's runway usage patterns revealed that landings on Runway 07 are relatively The three probability distributions (LDOR RWY 07, LDUS rare. It is reasonable to assume this runway is used for land- RWY 25, and TOOR RWY 07) can be combined to provide ings only for exceptional circumstances, such as adverse wind an overall probability distribution that a serious accident will conditions, and this also would explain the discrepancy in occur in the specific RSA. The percentages for each cell risk exposure for landings on Runway 07 and Runway 25. should be weighted relative to the traffic for each type of Using these numbers, the percentage operations with risk operation. A hypothetical example is presented to help un- above the TLS and challenging the RSA being evaluated can derstanding of the process. be estimated as follows: Assuming these are the conditions for the airport being evaluated if the TLS is set to 1 10-6, results for the approach (0.073 0.34 + 0.09 0.01 + 0.3 0.02) % > TLS = = 6.9% end of Runway 25 are summarized on Table 16. (0.073 + 0.09 + 0.3) There are significant differences to risk levels for each op- Based on the annual number of operations at the airport, eration, particularly noting that 34 percent of landings on the analysis also can be used to estimate the annual rate of Runway 07 have risk above the threshold level selected for accidents or the number of years likely to take for a severe this analysis. But there also are notable discrepancies in terms accident to occur. These are useful parameters as they allow of exposure to visibility, ceiling height, and fog between land- comparing different RSAs of the same or different airports to ings on Runway 07 and those on Runway 25. identify and prioritize the RSAs requiring risk management Over 30 percent of landings on Runway 07 take place in actions or improvements. visibility under 2 statute miles, compared to 1.67 percent on Runway 25. In a related measure, almost 40 percent of land- Steps 8 and 9--Repeat the Analysis ings on Runway 07 experienced fog versus less than 7 percent for Other Runway Ends for Runway 25. Moreover, 39 percent of landings on Runway 07 took place in ceiling height under 1000 ft, while the equiv- In repeating the analysis for the remaining runway ends of alent for Runway 25 is only 3.9 percent. the same airport, it is possible to evaluate which RSA poses Table 16. Summary results for hypothetical airport. Operation % of Movements Ops @ High Sample Size Risk Landing on RWY 07 7.3% 34% 487 Takeoff on RWY 07 9.0% 1% 606 Landing on RWY 25 30.0% 2% 2020

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44 Table 17. Summary risk for hypothetical airport. Annual Ops Annual Ops w/ Risk RSA %Ops>TLS Challenging RSA Above TLS Approach End RWY 11 300,000 6% 18,000 Approach End RWY 29 160,000 7% 11,200 Approach End RWY 03 410,000 1% 4,100 Approach End RWY 21 40,000 14% 5,600 higher risk during the year. The assessment may help improve Using another hypothetical example, the following param- runway use so that risk is minimized for the airport. To com- eters can characterize the conditions for an existing airport pare different RSAs, it is preferred to use a parameter that can having two runways: 11/29, and 03/21. provide a direct comparison. One possibility is to transform the In Table 17, the approach end for Runway 11 has a higher percentage of operations above TLS into a volume of annual number of annual operations with risk above the selected operations above TLS by multiplying the percentage by the vol- TLS. A number of risk mitigation measures may be priori- ume of flights challenging the specific runway end. Using such tized to improve safety for such area. Some safety manage- a parameter, it is possible to compare multiple RSAs of the same ment alternatives may include modifying runway use for the airport, or even for different facilities, to determine and priori- airport, installing new NAVAIDS, or improving the RSA, tize which RSAs may require risk mitigation actions. among other procedures.