Imaginary Surfaces
The DOD has identified certain imaginary surfaces around military use
runways to determine how structures and facilities are evaluated and identify
if they pose a vertical obstruction relative to the airspace around a runway.
The imaginary surfaces of an active runway are used to define the required
airspace that must remain free of vertical obstructions in the vicinity of
aviation operations to ensure safe flight approaches, departures, and
patterns. The various imaginary surfaces build upon one another and are
designed to guide the height of structures so there are no vertical
obstructions to air navigation and operations, either natural or man‐made.
runway and extends 3,000 feet outward along the runway centerline.
This is the area where an aircraft accident is most likely to occur.
Figure 3‐10 illustrates a three‐dimensional cross‐section of the imaginary
surfaces. This figure shows the slope of the potential heights of structures
that should be followed to prevent the obstruction of navigable airspace.
The extent or size of an imaginary surface depends on the type of runway.
Military runways are categorized as either Class A or Class B based on the
type of aircraft that use the runways. Class A runways are for smaller or
lighter aircraft. Class B runways are the category for the majority of military
aircraft. Seymour Johnson AFB runway is classified as a Class B runway.
Figure 3-10.
Imaginary Surfaces Class B Runway: Seymour Johnson AFB
The imaginary surfaces relative to Seymour Johnson AFB’s Class B runways
are described below and illustrated on Figure 3‐11.
The Primary Surface defines the limits of the obstruction clearance
requirements in the immediate vicinity of the landing area. It
comprises surfaces of the runway, runway shoulders, and lateral safety
zones and extends 200 feet beyond each end of the runway. The width
is 1,500 feet fixed on the runway centerline.
The Clear Zone defines the limits of the obstruction clearance
requirements in the vicinity contiguous to the end of the primary
surface. The CZ is located immediately adjacent to the end of the
Background Report
Imaginary Surfaces Cross Section
The Approach‐Departure Clearance Surface is symmetrical about the
runway centerline and begins as an inclined plane (glide angle) 200
feet beyond each end of the primary surface of the centerline
elevation of the runway end, and extends for 50,000 feet from the
runway. 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 continues horizontally at this elevation to a point 50,000
feet from the start of the glide angle. The width of the surface is 2,000
feet at the runway end and flares uniformly to a width of 16,000 feet
at 50,000 feet from the runway.
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