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FAR 77.21

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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