699 

Federal Aviation Administration, DOT 

§ 77.21 

(iv) The width of the primary surface 

of a runway will be that width pre-
scribed in this section for the most pre-
cise approach existing or planned for 
either end of that runway. 

(d) 

Approach surface. 

A surface longi-

tudinally centered on the extended 
runway centerline and extending out-
ward and upward from each end of the 
primary surface. An approach surface 
is applied to each end of each runway 
based upon the type of approach avail-
able or planned for that runway end. 

(1) The inner edge of the approach 

surface is the same width as the pri-
mary surface and it expands uniformly 
to a width of: 

(i) 1,250 feet for that end of a utility 

runway with only visual approaches; 

(ii) 1,500 feet for that end of a runway 

other than a utility runway with only 
visual approaches; 

(iii) 2,000 feet for that end of a utility 

runway with a non-precision instru-
ment approach; 

(iv) 3,500 feet for that end of a non- 

precision instrument runway other 
than utility, having visibility mini-
mums greater that three-fourths of a 
statute mile; 

(v) 4,000 feet for that end of a non- 

precision instrument runway, other 
than utility, having a non-precision in-
strument approach with visibility 
minimums as low as three-fourths stat-
ute mile; and 

(vi) 16,000 feet for precision instru-

ment runways. 

(2) The approach surface extends for 

a horizontal distance of: 

(i) 5,000 feet at a slope of 20 to 1 for 

all utility and visual runways; 

(ii) 10,000 feet at a slope of 34 to 1 for 

all non-precision instrument runways 
other than utility; and 

(iii) 10,000 feet at a slope of 50 to 1 

with an additional 40,000 feet at a slope 
of 40 to 1 for all precision instrument 
runways. 

(3) The outer width of an approach 

surface to an end of a runway will be 
that width prescribed in this sub-
section for the most precise approach 
existing or planned for that runway 
end. 

(e) 

Transitional surface. 

These sur-

faces extend outward and upward at 
right angles to the runway centerline 
and the runway centerline extended at 

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 

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