Travis AFB Sustainability Study Report Final Background Report | Page 74
Imaginary Surfaces
Federal Aviation Regulations, Part 77 specifies a series of imaginary height
restriction surfaces surrounding an airfield. The imaginary surfaces of an
active runway are used to define the required airspace that must remain
free of vertical obstructions near aviation operations to optimize safe flight
operations. Figure 3‐6 shows the slope of the surfaces that help guide
military and community planners in land use planning around an airfield.
Structures should not exceed these heights to protect the navigable airspace
associated with the airfield, the safety of pilots and people, and the land
uses on the ground. This is especially important in the clear zone and the
approach‐departure surfaces.
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 them. Class A runways are for smaller or lighter
aircraft, while Class B runways are the category for the majority of military
aircraft. The two Travis AFB runways are Class B runways and the relative
imaginary surfaces of the runways are shown on Figure 3‐7. For a complete
technical explanation of the imaginary and transitional surfaces for
Travis AFB, see Chapter 5, Compatibility Assessment (Section 5.23 Vertical
Obstructions).
The Department of Defense Unified Facilities Criteria 3‐260‐01, Airfield and
Heliport Planning and Design, establishes airspace imaginary surfaces
associated with assault landing zones located at Travis AFB.
Figure 3‐6. Example Views of the Different Imaginary Surface Layers
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Travis AFB Profile