Clear Zone Surface. This surface defines the limits of the obstruction
clearance requirements in the vicinity contiguous to the end of the primary
surface. The length and width (for a single runway) of a Clear Zone surface at
Seymour Johnson AFB is 3,000 feet by 3,000 feet.
Apart from the Part 77, the FAA has developed imaginary surfaces around
runways to determine how structures and facilities are evaluated as to
whether they pose a vertical obstruction relative to the surrounding airspace.
The levels of imaginary surfaces build upon one another and are designed to
eliminate obstructions to air navigation and operations, either natural or
man‐made. The dimension or size of an imaginary surface depends on the
runway classification. Figure 5.23‐1 illustrates all the imaginary surfaces of a
runway and the heights and ratios that buildings and structures are evaluated
for vertical obstructions.
Approach‐Departure Clearance Surface. This surface is symmetrical about the
runway centerline extended, begins as an inclined plane (glide angle) 200 at
the end of the primary surface of the centerline elevation of the runway end,
and extends for 50,000 feet. The slope of the approach‐departure clearance
surface is 50:1 along the extended runway (glide angle) centerline until it
reaches an elevation of 500 feet above the established airfield elevation. It
then continues horizontally at this elevation to a point 50,000 feet from the
start of the glide angle. The width of this surface at the runway end is
2,000 feet; it flares uniformly, and the width at 50,000 feet is 16,000 feet.
Inner Horizontal Surface. This surface is a plane, oval in shape at a height of
150 feet above the established airfield elevation. It is constructed by scribing
an arc with a radius of 7,500 feet above the centerline at the end of the
runway and interconnecting these arcs with tangents.
Conical Surface. This is an inclined surface extending outward and upward
from the outer periphery of the inner horizontal surface for a horizontal
distance of 7,000 feet to a height of 500 feet above the established airfield
elevation. The slope of the conical surface is 20:1.
Figure 5.23-1
Example Imaginary Surfaces Cross-Section
Outer Horizontal Surface. This surface is a plane located 500 feet above the
established airfield elevation. It extends for a horizontal distance of
30,000 feet from the outer periphery of the conical surface.
As defined in the Seymour Johnson AFB AICUZ, the following provides a
description of each of the imaginary surfaces.
Primary Surface. This surface defines the limits of the obstruction clearance
requirements in the immediate vicinity of the landing area. The primary
surface comprises surfaces of the runway, runway shoulders, and lateral
safety zones and extends 200 feet beyond the runway end. The width of the
primary surface for the type of runway at Seymour Johnson AFB is 2,000 feet,
or 1,000 feet on each side of the runway centerline.
Background Report
Transitional Surfaces. These surfaces connect the primary surfaces, Clear
Zone surfaces, and approach‐departure clearance surfaces to the outer
horizontal surface, conical surface, other horizontal surface, or other
transitional surfaces. The slope of the transitional surface is 7:1 outward and
upward at right angles to the runway centerline. To determine the elevation
Page 5.23‐3