659 

Federal Aviation Administration, DOT 

§ 77.25 

a slope of 7 to 1 from the sides of the 
primary surface and from the sides of 
the approach surfaces. Transitional 
surfaces for those portions of the preci-
sion approach surface which project 
through and beyond the limits of the 
conical surface, extend a distance of 
5,000 feet measured horizontally from 
the edge of the approach surface and at 
right angles to the runway centerline. 

§ 77.21

Department of Defense (DOD) 

airport imaginary surfaces. 

(a) 

Related to airport reference points. 

These surfaces apply to all military 
airports. For the purposes of this sec-
tion, a military airport is any airport 
operated by the DOD. 

(1) 

Inner horizontal surface. A plane 

that is oval in shape at a height of 150 
feet above the established airfield ele-
vation. The plane is constructed by 
scribing an arc with a radius of 7,500 
feet about the centerline at the end of 
each runway and interconnecting these 
arcs with tangents. 

(2) 

Conical surface. A surface extend-

ing from the periphery of the inner 
horizontal surface outward and upward 
at a slope of 20 to 1 for a horizontal dis-
tance of 7,000 feet to a height of 500 feet 
above the established airfield ele-
vation. 

(3) 

Outer horizontal surface. A plane, 

located 500 feet above the established 
airfield elevation, extending outward 
from the outer periphery of the conical 
surface for a horizontal distance of 
30,000 feet. 

(b) 

Related to runways. These surfaces 

apply to all military airports. 

(1) 

Primary surface. A surface located 

on the ground or water longitudinally 
centered on each runway with the same 
length as the runway. The width of the 
primary surface for runways is 2,000 
feet. However, at established bases 
where substantial construction has 
taken place in accordance with a pre-
vious lateral clearance criteria, the 
2,000-foot width may be reduced to the 
former criteria. 

(2) 

Clear zone surface. A surface lo-

cated on the ground or water at each 
end of the primary surface, with a 
length of 1,000 feet and the same width 
as the primary surface. 

(3) 

Approach clearance surface. An in-

clined plane, symmetrical about the 

runway centerline extended, beginning 
200 feet beyond each end of the primary 
surface at the centerline elevation of 
the runway end and extending for 50,000 
feet. The slope of the approach clear-
ance surface is 50 to 1 along the runway 
centerline extended until it reaches an 
elevation of 500 feet above the estab-
lished airport elevation. It then con-
tinues horizontally at this elevation to 
a point 50,000 feet from the point of be-
ginning. The width of this surface at 
the runway end is the same as the pri-
mary surface, it flares uniformly, and 
the width at 50,000 is 16,000 feet. 

(4) 

Transitional surfaces. These sur-

faces connect the primary surfaces, the 
first 200 feet of the clear zone surfaces, 
and the approach clearance surfaces to 
the inner horizontal surface, conical 
surface, outer horizontal surface or 
other transitional surfaces. The slope 
of the transitional surface is 7 to 1 out-
ward and upward at right angles to the 
runway centerline. 

§ 77.23

Heliport imaginary surfaces. 

(a) 

Primary surface. The area of the 

primary surface coincides in size and 
shape with the designated take-off and 
landing area. This surface is a hori-
zontal plane at the elevation of the es-
tablished heliport elevation. 

(b) 

Approach surface. The approach 

surface begins at each end of the heli-
port primary surface with the same 
width as the primary surface, and ex-
tends outward and upward for a hori-
zontal distance of 4,000 feet where its 
width is 500 feet. The slope of the ap-
proach surface is 8 to 1 for civil heli-
ports and 10 to 1 for military heliports. 

(c) 

Transitional surfaces. These sur-

faces extend outward and upward from 
the lateral boundaries of the primary 
surface and from the approach surfaces 
at a slope of 2 to 1 for a distance of 250 
feet measured horizontally from the 
centerline of the primary and approach 
surfaces. 

Subpart D—Aeronautical Studies 

and Determinations 

§ 77.25

Applicability. 

(a) This subpart applies to any aero-

nautical study of a proposed construc-
tion or alteration for which notice to 
the FAA is required under § 77.9. 

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