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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.