application of this standard, each facility needs to obtain (1) the
expected number, N, of helicopter local overflights per year; (2) the
average length, L, in miles, of the flights corresponding to the site-
specific overflights; and (3) the effective area for helicopter in-flight
crashes, using Equation B-4, assuming an impact angle of 60 degrees,
i.e., cot = 0.58 (note skid length is assumed to be 0). For these
calculations, as shown in Equation 5-3, the lateral variations in crash
locations for a helicopter are conservatively assumed to be one-quarter
a mile on the average from the centerline of its flight path.
The analysis for helicopter impact frequency calculations is as follows:
Step 17. Obtain NH, the expected number of local helicopter overflights per year, and LH,
the average length of a flight.
Step 18. Compute the effective area, AH, using Equation B-4.
Step 19. Using the values of the probability of a helicopter crash per flight, PH, in
Table B-1 in Appendix B, compute the helicopter impact frequency, FH.
Calculated Impact Frequency.
Step 20. Sum the calculated impact frequency for airport and nonairport operations for
each aircraft category or subcategory. For example, add up all the general aviation
impact frequencies calculated in Steps 6 and 8. Rank the impact frequencies for all
aircraft categories/subcategories in decreasing order. Sum the impact frequencies over
the aircraft categories/subcategories to get the total impact frequency for the facility of