Airport Imaginary Surfaces

What is an Airport Imaginary Surface?


Federal Air Regulation Part 77.25, “Civil Airport Imaginary Surfaces”, establishes a complex structure of imaginary surfaces in relation to each runway at civil airports.  The size of each imaginary surface is based on the category of each runway according to the type of instrument approach available or planned for that runway.


According to the provisions set forth in Part 77, an object is an “Obstruction to Air Navigation” if it is of greater height than any imaginary surface established under the regulation.


Imaginary surfaces exist primarily to prevent existing or proposed manmade objects, objects of natural growth or terrain from extending upward into navigable airspace.


There are basically five imaginary surfaces which the Federal Aviation Administration (FAA) has applied to public use airports for the purpose of determining obstructions to air navigation. 


These imaginary surfaces either slope out and up from all sides and ends of runways or are a horizontal plane or a sloping plain above public use airports.


There imaginary surfaces are defined as:


  • Primary Surface:  Aligned (longitudinally) with each runway and extends 200 ft. from each runway end
  • Horizontal Surface:  Horizontal plane 150 ft. above the established airport elevation.  Constructed by swinging arcs around the end of the primary surface
  • Conical Surface:  20:1 slope surface extending beyond the horizontal surface
  • Transitional Surface:   Constructed to join approach and horizontal or approach and transitional surfaces
  • Approach Surface:  Longitudinally centered with the runway and extends beyond the primary surface


Instrument approach and departure procedures also have imaginary surfaces (Terminal Instrument Procedures – TERPS) that specify the minimum measure of obstacle clearance. 



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