
 DOESTD301496
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 inflight
crashes, using Equation B4, 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 53, the lateral variations in crash
locations for a helicopter are conservatively assumed to be onequarter
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 B4.
Table B1 in Appendix B, compute the helicopter impact frequency, FH.
2
FH
NH
PH
AH
(53)
LH
5.3.3
Calculated Impact Frequency.
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
interest.
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