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OCR for page 74
74 Guidebook for Managing Small Airports
Airspace and Approaches
The purpose of this section is to provide guidance on issues pertaining to airspace clearing and
obstruction standards including a review of 14 CFR Part 77, Objects Affecting Navigable Airspace;
TERPS; and airspace right-of-way and easements.
14 CFR Part 77, Objects Affecting Navigable Airspace (FAR Part 77)
FAR Part 77 establishes the standards for determining obstructions to navigable airspace and
describes the notification requirements for any construction or alteration potentially affecting
navigable airspace surrounding airports.
Standards
Imaginary Surfaces. The navigable airspace areas governed by FAR Part 77 are referred to as
"imaginary surfaces." The size and shape of imaginary surfaces are dependent on the size of air-
planes that use the airport, the approach visibility minimums, and the runway type (e.g., paved
or turf). Figure 5 illustrates dimensions for some of the imaginary surfaces. The FAA Obstruction
Evaluation/Airport Airspace Analysis page of the FAA website (https://oeaaa.faa.gov/oeaaa/
external/portal.jsp) provides a link to FAR Part 77 dimensional requirements and illustrations for
imaginary surfaces as well as other information relating to the process for reviewing airspace issues.
The five types of imaginary surfaces are
· Primary Surface. A primary surface is 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; 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
250 feet for utility runways having only visual approaches or
500 feet for utility runways having nonprecision instrument approaches.
For other than utility runways, the width is
500 feet for visual runways having only visual approaches;
500 feet for nonprecision instrument runways having visibility minimums greater than
three-fourths of a statute mile; or
1000 feet for precision instrument runways, and for nonprecision instrument runways hav-
ing a nonprecision instrument approach with visibility minimums as low as three-fourths
of a statute mile.
The width of the primary surface of a runway will be that width prescribed for the most pre-
cise approach existing or planned for either end of that runway.
· Transitional Surface. The transitional surface extends outward and upward at right angles to
the runway centerline and extends at a slope of 7 feet horizontally for each 1 foot vertically (7:1)
from the sides of the primary and approach surfaces. The transitional surfaces extend to the
point at which they intercept the horizontal surface at a height of 150 feet above the established
airport elevation. For precision approach surfaces that project through and beyond the limits of
the conical surface, the transitional surface also extends 5,000 feet horizontally from the edge of
the approach surface and at right angles to the runway centerline.
· Horizontal Surface. The horizontal surface is a horizontal plane located 150 feet above the estab-
lished airport elevation that encompasses an area from the transitional surface to the conical sur-
face. The perimeter is constructed by generating arcs from the center of each end of the primary
surface and connecting the adjacent arcs by lines tangent to those arcs. The radius of each arc for
all runway ends designated as utility or visual is 5,000 feet and 10,000 feet for precision and non-
precision runway ends.
OCR for page 75
Airport Planning and Development 75
Figure 5. FAR Part 77 imaginary surfaces.
