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 .
Figure 3‐7 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 . Keesler AFB runway is classified as a Class B runway .
• The Centerline at the Threshold is the begi portion of the approach‐departure surface Runway 03 is 23 feel MSL . The centerline e Runway 21 is 17 feet MSL .
• The Approach‐Departure Clearance Surfac runway centerline and begins as an incline beyond the displaced threshold , and exten runway . The slope of the approach‐depart along the extended runway centerline unti 500 feet above the established airfield elev horizontally at this elevation to a point 25,0 glide angle . The width of the surface is 2,0 and flares uniformly to a width of 16,000 fe runway .
Figure 3-7 . Imaginary Surface Cross Section
The definition of imaginary surfaces per the DoD criteria are described below and illustrated on Figure 3‐8 .
• The Runway End is the defining location to begin or end the primary surface . This is generally the normal threshold for a runway . Keesler AFB has displaced thresholds . These displaced thresholds are defined as the end of the runway .
• The Established Elevation of the Airfield is the highest point on any usable landing surface . The established airfield elevation ( EAE ) at Keesler AFB is 33 feet above MSL .
Background Report