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42 As previously described, it is assumed the aircraft will over- there are major consequences. Considering the criteria set by run in a path that is nearly parallel to the runway axis. The FAA, the risk is considered high and suggests the operation is probability that the aircraft will overrun the RSA under Crash not safe enough under such conditions. However, the risk Scenario 1 is given by a product of three probabilities: the estimated for such conditions at any airport should be con- probability of occurring the event (overrun); the probability sidered for planning and risk mitigation strategies only as it the aircraft will stop beyond 100 ft from the threshold; and represents an "average" risk level for such conditions. the probability the aircraft will remain within the RSA lateral limits during the overrun. Mathematically: Step 5--Characterize Risk Prob{CS 1} = Prob{LDOR) Prob{x > 100ft} Frequency Distribution (1-Prob{| y | > 250 ft) The same procedure described for Step 3 can be used to And Prob{| y | > 250 ft} is that given by: compute the probability of severe consequences for every landing operation for Runway 07 that is part of the NOD sam- P{d > y} = e -0.20174 2500.489009 = 5.0% ple. Each operation has a different risk associated with it. If all Probability for crash scenario 1 is: 8.165 10-5 0.81 these risks are estimated, it is possible to build a histogram (1-0.050) = 6.28 10-5 depicting the distribution of risk, as illustrated in Figure 36. Similarly, the probability for crash scenario 2 is simply given by: Step 6--Determine Percentage of Prob{CS 3) = Prob{LDOR) Prob{| y | > 250 ft Operations with Risk Above TLS In this situation the aircraft only needs to overrun the run- An example of the probability distribution generated by way by a small margin; as long as the transverse deviation is the prototype software developed under this project is shown greater than 250 ft, the aircraft will end up in the water. The in Figure 37. Each bar represents the percentage of operations probability for crash scenario 3 is calculated as follows: having a specific risk level. The line represents the percentage of operations having risk higher then the level selected. In this P{CS 3} = 8.165 10-5 0.050 = 4.083 10-6 example, if a TLS of 1:10000000 is selected, approximately The last step is to compute the total probability the aircraft 9 percent of the operations will be subject to undesirable levels will overrun the runway during landing and fall into the water of risk. This is useful, as it evaluates the percentage of opera- with severe consequences: tions with risk above a selected TLS. The area in dark bars represents such flights. P{LDOR Severe} = P{CS 1} + P{CS 3} = 6.28 10-5 The same process is used to estimate the percentage of + 4.08 10-6 = 6.69 10-5 operations having a risk level above the selected TLS for According to the FAA criteria described in Table B1-2 from LDUSs on Runway 25 and for TOORs on Runway 07, using Attachment 1 to Appendix B, the event is probable and such the models associated with these types of events. The proba- probability is unacceptable, as shown in the FAA Risk Matrix bility distribution of risks then can be characterized for each depicted in the same Attachment (Figure B1-1), even when type of accident in the vicinity of that RSA. Frequency distribution estimated from expected traffic conditions at the airport/threshold and risk models Frequency Region of Region of Normal Normal Operations Operations and High Risk And Low Risk Risk threshold (e.g. 1x10-7) Probability WL > X X is the RSA length or distance to existing obstacle Figure 36. Frequency distribution of risk.