Imaginary Surfaces

3-13.1 Types of Airspace Imaginary Surfaces. Airspace imaginary surfaces for Army and Air Force Class B IFR runways are similar to those at fixed-wing DOD facilities, except that the primary surface and clear zone widths are narrower for Army runways. At fixed-wing DOD facilities, these types of airspace imaginary surfaces may be found:

- Class A VFR runway - Class A IFR runway - Class B IFR runway for Army facilities - Class B IFR runway for Air Force facilities - Class B IFR runway for Navy and Marine Corps facilities

3-13.2 Imaginary Surfaces. The area surrounding a runway that must be kept clear of objects that might damage an aircraft is bounded by imaginary surfaces that are defined in this manual. An object, either man-made or natural, that projects above an imaginary surface is an obstruction. Imaginary surfaces for fixed-wing airfields are shown in figures 3-6 through 3-22 and are defined in the glossary. The applicable dimensions and slopes are provided in Table 3-7. These imaginary surfaces include:

- Primary surface - Approach-departure surface - Inner horizontal surface - Conical surface - Outer horizontal surface - Transitional surface - Graded area

Also including the

- Zone of frangibility - RWY lateral clearance