established reference point of an airport, excluding heliports, with its
longest runway more than 3,200 feet in actual length is considered a
vertical obstruction. This height increases in the proportion of 100 feet
for each additional nautical mile of distance from the airport up to a
maximum of 499 feet.
Currently, there is no issue with aviation operations between Maxwell AFB
and MGM, and it should be noted that the overlap of the MGM imaginary
surfaces protects the south‐southwest area of Maxwell AFB due to the
restrictions imposed by the MGM imaginary surfaces.
Figure 3‐13.
Cross‐Section of Imaginary Surfaces
Figure 3‐12 provides an illustration of this measure of vertical obstruction.
Note that this is in addition to, not a replacement of, imaginary surfaces
discussed later in this section.
Imaginary Surfaces
Federal Aviation Regulation, Part 77 specifies a series of imaginary height
surfaces surrounding military and civilian airports. 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 operations. Figure 3‐13 shows a cross‐section of the slope of the
surfaces that help guide military and community planners in land use planning
around an airfield. Structures on the ground 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 utilize them. Class A runways are for smaller or lighter aircraft.
Class B runways are the category for the majority of military aircraft.
Maxwell AFB runway is classified as a Class B runway and its relative imaginary
surfaces are shown on Figure 3‐14.
Proximity of MGM to Maxwell AFB creates an overlap of the imaginary
surfaces associated with each individual airfield which are also illustrated on
Figure 3‐15. MGM is located within Maxwell AFB’s outer horizontal surface
which restricts height to 500 feet above the established airport elevation.
Page 3‐26
Background Report
June 2017