Step 2. From the list of impact frequencies compiled for the impact frequency evaluation,
delete the impact frequencies corresponding to the aircraft categories/subcategories
identified in Step 1.
Step 3. Sum the impact frequencies for the remaining aircraft categories/subcategories.
The calculated sum is the release frequency screening value.
Step 4. Compare the release frequency screening value to the guideline. If the guideline
is met, the safety risk associated with aircraft impact is below the level of concern and no
further analysis is needed; document the results. If the guideline is exceeded, proceed to
the release frequency evaluation (Section 5.5).
Methodology for Aircraft Crash Release Frequency Evaluation. The release frequency
screening does not take into account the fact that, even if a particular aircraft category or
subcategory can cause damage that could potentially lead to a release, only certain
impact locations will have that effect. By making better use of the structural analysis and
the impact frequency calculations, the analyst can define specific release scenarios and
estimate the frequency associated with those scenarios. This makes it possible to
determine the extent to which the actual release frequency may be lower than the initial
release frequency. This section addresses the evaluation process for making this
For each impact location which is determined in the structural response analysis to exceed
the structural response guideline, a release scenario associated with the level of damage
resulting from the impact should be developed. The intent is to specify the most realistic
conditions that can be justified. The scenario selected should be physically possible and
rational within the physical constraints of the level of damage incurred (including the
occurrence of process accidents as a result of system failures). Once it has been
determined that a release can occur, the overall facility dimensions used to assess the
impact frequency are replaced with a partial facility dimension representing the impact
location (a new effective area) for the specific release scenario. The new effective area is
input into the four-factor formula (Equation 5-1) for the appropriate aircraft subcategory,
resulting in a revised impact frequency specific to the impact location being evaluated.