recognized, the high value of the maximum expected frequency of an in-flight mishap
resulting in an aircraft impacting an arbitrary square mile in the continental United States
precluded the development of a useable impact frequency screening methodology.
Methodology for Impact Frequency Evaluation. This section describes the approach for
implementing the impact frequency evaluation, using the four-factor formula as given in
Equation 5-1. The following guidance provides a set of steps for calculating impact
frequency. Steps 1 through 6 are for determining the impact frequency from airport
operations; Steps 7 through 19 are for determining the impact frequency from nonairport
operations (Steps 7-8 for general aviation, Steps 9-12 for commercial aviation,
Steps 13-16 for military aviation; and Steps 17-19 for helicopters); and Steps 20 and 21
are for comparing the results with the guidelines.
An example of the use of these steps is included as Section B.5 of Appendix B.
Impact Frequency from Airport Operations.
Step 1. Identify the flight sources affecting the facility. To do this, identify any airports
that can be located within the boundaries of the aircraft crash location probabilities
(Tables B-2 through B-13). Contact these airports to get an estimate of the annual
number of takeoffs and landings, N, for each aircraft category or subcategory. This
information can usually be provided by the airport on a category basis. If the airport can
only provide total operations and is not able to discriminate between operation activities,
assume that one-half (50 percent) of the operations are takeoffs and one-half (50
percent) are landings. This assumption will result in very conservative numbers because
total operations include activities other than takeoff and landing, such as an aircraft
contacting the tower for a change of vector. Finally, have the airport identify the pattern
side of the runway for military aviation, if applicable.
Step 2. For each flight source, determine the orthonormal distance (Cartesian distance,
both x and y coordinates) from the facility, measured from the facility's closest point, to
the center of each runway at the flight source (for guidance on determining the
orthonormal distance see Appendix B, Section B.3.1, and the example in Section B.5).