Page 273 - Planning and Design of Airports

234 Airp o r t D e sign

Sight Distance and Longitudinal Profile
As in the case of runways, the number of changes in longitudinal
profile for taxiways is limited by sight distance and minimum dis-
tance between vertical curves.
The FAA does not specify line of sight requirements for taxiways
other than those discussed earlier related to runway and taxiway
intersections. However, the sight distance along a runway from an
intersecting taxiway needs to be sufficient to allow a taxiing aircraft to
enter or cross the runway safely. The FAA specifies that from any point
on the taxiway centerline the difference in elevation between that
point and the corresponding point on a parallel runway, taxiway, or
apron edge is 1.5 percent of the shortest distance between the points.
ICAO requires that the surface of the taxiway should be seen for
a distance of 150 m from a point 1.5 m above the taxiway for aero-
drome code letter A runways, for a distance of 200 m from a point 2 m
above the taxiway for aerodrome code letter B runways, and for a
distance of 300 m from a point 3 m above the taxiway for aerodrome
code letter C, D, or E runways.
In regard to longitudinal profile of taxiways, the ICAO does not
specify the minimum distance between the points of intersection of
vertical curves. The FAA specifies that the minimum distance for both
utility and transport category airports should be not less than the
product of 100 ft multiplied by the sum of the absolute percentage
values of change in slope.

Exit Taxiway Geometry
The function of exit taxiways, or runway turnoffs as they are some-
times called, is to minimize runway occupancy by landing aircraft.
Exit taxiways can be placed at right angles to the runway or some
other angle to the runway. When the angle is on the order of 30°, the
term high-speed exit is often used to denote that it is designed for
higher speeds than other exit taxiway configurations. In this chapter,
specific dimensions for high-speed exit, right-angle exit (low-speed)
taxiways are presented. The dimensions presented here are the results
obtained from research conducted many years ago [13] and subse-
quent research conducted by the FAA.
The earlier tests [13] were conducted on wet and dry concrete and
asphalt pavement with various types of civil and military aircraft in
order to determine the proper relationship between exit speed and
radii of curvature and the general configuration of the taxiway. A sig-
nificant finding of the tests was that at high speeds a compound curve
was necessary to minimize tire wear on the nose gear and, therefore,
the central or main curve radius R should be preceded by a much
2
larger radius curve R .
1
Aircraft paths in the test approximated a spiral. A compound
curve is relatively easy to establish in the field and begins to approach