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Page Title: Methodology for Aircraft Crash Release Frequency Evaluation. - Continued
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In this context, the word "could" should be taken to mean "is it mechanistically
possible, given the level of damage." The possibility that failures occurring away
from the material could cause system failures resulting in process accidents should
also be considered. Therefore, intersystem dependencies and support system
interactions should be explicitly evaluated. If there is any doubt about the answer
to any of the three questions listed in this step, the answer should be assumed to
be "yes." If, for the given crash location, the answer to any of these questions is
"no," the scenario can be designated as a nonrelease scenario and eliminated from
further consideration.
Step 5. If the scenario has not been eliminated (i.e., the analysis has shown that it could
lead to a release), calculate the impact frequency by rerunning the four-factor formula for
the appropriate aircraft subcategory, using facility dimensions specific to the impact
location associated with the scenario. The analyst will need to better define the location
as an area (rather than just a single point) where impact could result in the release
scenario. This requires judgement and consultation with the analyst(s) who conducted the
structural response evaluation. Use this information to develop a set of "scenario facility
dimensions" that represents what the target would look like if it encompassed an area
equal to the target area associated with the release scenario being evaluated. Credit
should be taken for shielding effects from other facilities to further reduce the scenario
facility dimensions (Appendix B, Section B-4). The development of the scenario facility
dimensions should be well justified and documented in detail. Once these dimensions
have been established, run the appropriate four-factor formula to calculate the scenario
release frequency.
Step 6. Repeat Steps 1 through 5 for all of the impact locations that exceed the structural
response guidelines. Adding together the scenario release frequencies from each pass
through Step 5 gives the final release frequency for the evaluation step.
Step 7. Compare the final release frequency value to the guideline. If the guideline is
met, no additional analysis of aircraft impact is required. If the guideline is not met, a more
detailed analysis of the exposure associated with each release scenario needs to be
performed in accordance with Section 7.3 of this standard. For the purpose of that

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