( 4 ) 23 feet for a railroad .
( 5 ) For a waterway or any other traverse way not previously mentioned , an amount equal to the height of the highest mobile object that would normally traverse it .
Apart from the ( CFR ) Title 14 Part 77 , the FAA has developed imaginary surfaces around runways to determine how structures and facilities are evaluated as to whether they pose a vertical obstruction relative to the surrounding airspace . The levels of imaginary surfaces build upon one another and are designed to eliminate obstructions to air navigation and operations , either natural or man‐made . The dimension or size of an imaginary surface depends on the runway classification . Figure 5‐17 illustrates all the imaginary surfaces of a runway and the heights and ratios that buildings and structures are evaluated for vertical obstructions .
Figure 5‐17 Example DoD Imaginary Surfaces Cross‐Section
The following are definitions of imaginary surfa
Primary Surface . This surface defines the limits requirements in the immediate vicinity of the la surface comprises surfaces of the runway , runw safety zones and extends 200 feet beyond the d end of the runway . The width of the primary su at Keesler AFB is 2,000 feet , or 1,000 feet on ea centerline .
Clear Zone Surface . This surface defines the lim clearance requirements beginning at the runwa threshold . The length and width ( for a single ru surface at Keesler AFB is 3,000 feet by 3,000 fee
Approach‐Departure Clearance Surface . This su the runway centerline extended , begins as an in 200 feet at the end of the primary surface of th runway end , and extends for 50,000 feet . The s approach‐departure clearance surface is 50:1 a ( glide angle ) centerline until it reaches an eleva established airfield elevation . It then continues to a point 50,000 feet from the start of the glide surface at the runway end is 2,000 feet ; it flares 50,000 feet is 16,000 feet .
Inner Horizontal Surface . This surface is a plane 150 feet above the established airfield elevation scribing an arc with a radius of 7,500 feet above the runway and interconnecting these arcs with
Background Report