FAR 77.25 Civil Airport Imaginary Surfaces

Airport imaginary surfaces explained

The following civil airport imaginary surfaces are established with relation to the airport and to each runway. The size of each such imaginary surface is based on the category of each runway according to the type of approach available or planned for that runway. The slope and dimensions of the approach surface applied to each end of a runway are determined by the most precise approach existing or planned for that runway end.

Chart of surface dimensions

FAR Part 77.25(a)

77.25(a) Horizontal surface. A horizontal plane 150 feet above the established airport elevation, the perimeter of which is constructed by swinging arcs of specified radii from the center of each end of the primary surface of each runway of each airport and connecting the adjacent arcs by lines tangent to those arcs. The radius of each arc is:

1. 5,000 feet for all runways designated as utility or visual;
2. 10,000 feet for all other runways. The radius of the arc specified for each end of a runway will have the same arithmetical value. That value will be the highest determined for either end of the runway. When a 5,000-foot arc is encompassed by tangents connecting two adjacent 10,000-foot arcs, the 5.000-toot arc shall be disregarded on the construction of the perimeter of the horizontal surface.

FAR Part 77.25(b)

77.25(b) Conical surface. A surface extending outward and upward from the periphery of the horizontal surface at a slope of 20 to I for a horizontal distance of 4,000 feet.

FAR Part 77.25(c)

77.25(c) Primary surface. A surface longitudinally centered on a runway. When the runway has a specially prepared hard surface, the primary surface extends 200 feet beyond each end of that runway; but when the runway has no specially prepared hard surface, or planned hard surface, the primary surface ends at each end of that runway. The elevation of any point on the primary surface is the same as the elevation of the nearest point on the runway centerline. The width of a primary surface is:

1. 250 feet for utility runways having only visual approaches.
2. 500 feet for utility runways having non-precision instrument approaches.
3. For other than utility runways the width is:
   (i) 500 feet for visual runways having only visual approaches.
   (ii) 500 feet for non-precision instrument runways having visibility minimums greater than three-fourths statute mile.
   (iii)1,000 feet for a non-precision instrument runway having non-precision instrument approach with visibility minimums as low as three-fourths of a statute mile, and for precision instrument runways.

The width of the primary surface of a runway will be that width prescribed in this section for the most precise approach existing or planned for either end of that runway.

FAR Part 77.25(d)

77.25(d) Approach surface. A surface longitudinally centered on the extended runway centerline and extending outward and upward from each end of the primary surface. An approach surface is applied to each end of each runway based upon the type of approach available or planned for that runway end.

1. The inner edge of the approach surface is the same width as the primary surface and it expands uniformly to a width of:
   (i) 1,250 feet for that end of a utility runway with only visual approaches;
   (ii) 1,500 feet for that end of a runway other than a utility runway with only visual approaches;
   (iii) 2,000 feet for that end of a utility runway with a non-precision instrument approach;
   (iv) 3,500 feet for that end of a non-precision instrument runway other than utility, having visibility minimums greater than three-fourths of a statute mile;
   (v) 4,000 feet for that end of a non-precision instrument runway, other than utility, having a non-precision instrument approach with visibility minimums as low as three-fourths statute mile; and
   (vi) 16,000 feet for precision instrument runways.
2. The approach surface extends for a horizontal distance of:
   (i) 5,000 feet at a slope of 20/1 for all utility and visual runways;
   (ii) 10,000 feet at a slope of 34/1 for all non-precision instrument runways other than utility; and,
   (iii) 10,000 feet at a slope of 50/1 with an additional 40,000 feet at a slope of 40/1 for all precision instrument runways.
3. The outer width of an approach surface to an end of a runway will be that width prescribed in this subsection for the most precise approach existing or planned for that runway end.

FAR Part 77.25(e)

77.25(e) Transitional surface. These surfaces extend outward and upward at right angles to the runway centerline and the runway centerline extended at a slope of 7 to I from the sides of the primary surface and from the sides of the approach surfaces. Transitional surfaces for those portions of the precision approach surface which project through and beyond the limits of the conical surface, extend a distance of 5,000 feet measured horizontally from the edge of the approach surface and at right angles to the runway centerline.