5.23. Vertical Obstructions
Vertical obstructions are created by buildings, trees, structures, or other
features that may encroach into the navigable airspace or line‐of‐sight of
radar signal transmission pathways used by the military. These obstructions
can be a safety hazard to both the public and military personnel and
potentially impact military readiness.
Vertical obstructions can compromise the value of low‐level flight training by
limiting the areas where such training can occur. These obstructions can
include a range of items from man‐made, such as telephone poles, utility
transmission towers, and radio antennas, to natural, such as tall trees and
land features. Vertical obstructions can also interfere with radar
transmissions, compromising the integrity of data transmission between the
transmitter and receiver. Though most critical near the transmitter, the
geographic area impacting the transmissions, or radar viewshed, can be
broad depending on the distance between the transmitter and receivers.
Key Terms
Imaginary Surfaces. The term imaginary surface refers to the areas
surrounding a heliport or airfield that must be kept clear of objects that
might pose a safety threat to aviation activities. A man‐made or natural
object that projects above an imaginary surface is an obstruction. See
Technical Background for more information.
Technical Background
In relation to flight operations from an airport (military or civilian), vertical
obstructions are addressed through compliance with Federal Regulation
Title 14 Part 77, which establishes standards and notification requirements
for objects affecting navigable airspace. Commonly referred to as Part 77
June 2017
compliance, this regulation provides details to evaluate the potential for a
vertical obstruction based on the elevation of the airfield, the height and
resulting elevation of the new structure or facility, and the location of the
structure or facility in relation to the airfield in question.
To determine when structures or facilities should be evaluated for vertical
obstruction, Part 77 states the following requirements for notifying the FAA:
§77.9 ‐ Any person/organization who intends to sponsor any of
the following construction or alterations must notify the
Administrator of the FAA:
– Any construction or alteration exceeding 200 feet above
ground level.
Any construction or alteration:
– within 20,000 feet of a public use or military airport which
exceeds a 100:1 surface from any point on the runway of each
airport with at least one runway more than 3,200 feet.
– within 10,000 feet of a public use or military airport which
exceeds a 50:1 surface from any point on the runway of each
airport with its longest runway no more than 3,200 feet.
– within 5,000 feet of a public use heliport which exceeds a
25:1 surface.
Any highway, railroad, or other traverse way whose prescribed
adjusted height would exceed the above noted standards.
Background Report
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