Page 338 - Planning and Design of Airports

296 Airp o r t D e sign

value of α − β of 12° results in H being about 200 ft when h is 200 ft.
However, when h is 100 ft, H is 687 ft. Consequently, lighting systems
designed to aid in aircraft approaching to land on a runway have
been designed to provide optimal visual guidance when aircraft are
at relatively low altitudes on approach, and are angled to be consis-
tent with the downward approach angle of arriving aircraft.

Approach Lighting
Approach lighting systems (ALS) are designed specifically to provide
guidance for aircraft approaching a particular runway under night-
time or other low-visibility conditions. While under nighttime condi-
tions it may be possible to view approach lighting systems from several
miles away, under other low-visibility conditions, such as fog, even
the most intense ALS systems may only be visible from as little as
2500 ft from the runway threshold.
Studies of the visibility in fog [3] have shown that for a visual range
of 2000 to 2500 ft it would be desirable to have as much as 200,000 can-
delas (cd) available in the outermost approach lights where the slant
range is relatively long. Under these same conditions the optimum
intensity of the approach lights near the threshold should be on the
order of 100 to 500 cd. A transition in the intensity of the light that is
directed toward the pilot is highly desirable in order to provide the best
visibility at the greatest possible range and to avoid glare and the loss
of contrast sensitivity and visual acuity at short range.

System Configurations
The configurations which have been adopted are the Calvert system
[3] shown in Fig. 8-3 which has been widely used in Europe and other
parts of the world, the ICAO category II and category III system shown
in Fig. 8-4, and the four system configurations which have been adopted
by the FAA in the United States shown in Fig. 8-5. The FAA publishes
criteria for the establishment of the approach lighting systems [13]
and other navigation facilities at airports [6]. Approach lights are nor-
mally mounted on frangible pedestals of varying height to improve
the perspective of the pilot in approaching a runway.
The first approach lighting system was known as the Calvert sys-
tem. In this system, developed by E. S. Calvert in Great Britain in 1949,
includes a line of single bulb lights spaced on 100-ft centers along the
extended runway centerline and six transverse crossbars of lights of
variable length spaced on 500-ft centers, for a total length of 3000 ft.
The Calvert system was the first approach lighting system to be certi-
fied by ICAO, and is also commonly known as the ICAO category I
approach lighting system. An illustration of the Calvert system is found
in Fig. 8-3. The Calvert system is still used in developing countries.