Effective Area Calculations.
The effective area represents the ground surface area surrounding a facility such that if an
unobstructed aircraft were to crash within the area, it would impact the facility, either by direct
fly-in or skid into the facility. The effective area depends on the length, width, and height of
the facility, as well as on the aircraft's wingspan, flight path angle, heading angle relative to
the heading of the facility, and the length of its skid. The effective area consists of two parts,
the fly-in area and the skid area. The former represents the area corresponding to a direct
fly-in impact and consists of two parts, the footprint area and the shadow area. The footprint
is the facility area that an aircraft would hit on its descent even if the facility height were zero.
The shadow area is the facility area that an aircraft hits on its descent, but which it would
have missed if the facility height were zero.
For this standard, the facility is represented by a bounding rectangle, and the heading of the
crashing aircraft with respect to the facility is assumed to be perpendicular to the diagonal of
the bounding rectangle, as shown in Figure B-3. These assumptions provide a conservative
approximation to the true effective area.
The formula for calculating the skid and fly-in areas for an aircraft crashing into a rectangular
building is given in Equations B-3 to B-5. Details are provided in Reference 1. Table B-16
provides typical wingspans for commercial, general aviation, and military aircraft. Table B-17
provides values for the mean of the cotangent of the impact angle. Table B-18 provides
mean skid distances for each aircraft category. Values of wingspans for selected aircraft,
values of marginal cumulative distribution functions for the impact angle, and aircraft
subcategory skid distances are provided in Reference 2.