Paragraph 77.21 Military (DOD) Airport Imaginary Surfaces.
(a) Related to airport reference points. These surfaces apply to all military airports.
For the purposes of this section a military airport is any airport operated by an armed
force of the United States.
(1) Inner horizontal surface. A plane is oval in shape at a height of 150 feet above
the established airfield elevation. The plane is constructed by scribing an arc
with a radius of 7,500 feet about the centerline at the end of each runway and
interconnecting these arcs with tangents.
(2) Conical surface. A surface extending from the periphery of the inner horizontal
surface outward and upward at a slope of 20 to 1 for a horizontal distance of
7,000 feet to a height of 500 feet above the established airfield elevation.
(3) Outer horizontal surface. A plane, located 500 feet above the established
airfield elevation, extending outward from the outer periphery of the conical
surface for a horizontal distance of 30,000 feet.
(b) Related to runways. These surfaces apply to all military airports.
(1) Primary surface. A surface located on the ground or water longitudinally
centered on each runway with the same length as the runway. The width
of the primary surface for runways is 2,000 feet. However, at established
bases where substantial construction has taken place in accordance with
a previous lateral clearance criteria, the 2,000-foot width may be reduced
to the former criteria.
(2) Clear zone surface. A surface located on the ground or water at each end
of the primary surface, with a length of 1,000 feet and the same width as
the primary surface.
(3) Approach clearance surface. An inclined plane, symmetrical about the runway
centerline extended, beginning 200 feet beyond each end of the primary surface
at the centerline elevation of the runway end and extending for 50,000 feet. The
slope of the approach clearance surface is 50 to 1 along the runway centerline
extended until it reaches an elevation of 500 feet above the established airport
elevation. It then continues horizontally at this elevation to a point 50,000 feet
from the point of beginning. The width of this surface at the runway end is the
same as the primary surface, it flares uniformly, and the width at 50,000 is
16,000 feet.
(4) Transitional surfaces. These surfaces connect the primary surfaces, the first
200 feet of the clear zone surfaces, and the approach clearance surfaces to
the inner horizontal surface, conical surface, outer horizontal surface or other
transitional surfaces. The slope of the transitional surface is 7 to 1 outward
and upward at right angles to the runway centerline.
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