14 CFR Part 77 imaginary surfaces establish standards for determining obstructions in navigable airspace. These imaginary surfaces run out from the runway in a manner that reflects where aircraft are likely to fly while also accommodating unforeseen aircraft maneuvers. The height above the ground of the imaginary surface is lowest near the runway and increases at a distance from the runway. Away from airports, airspace begins at 200 feet above ground level.
Large structures, like communication towers and wind turbines, often exceed 200 feet in height and therefore are required to submit FAA Form 7460, Notice of Proposed Construction or Alteration. Structures shorter than 200 feet but located within 20,000 feet of a runway may also penetrate navigable airspace.
The FAA requires any proposed structure that meets certain criteria must be assessed against Part 77. Documents must be filed with the FAA for new structures so a determination can be made if the structure will be a hazard to air navigation.
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