Page 343 - Planning and Design of Airports

Airport Lighting, Marking, and Signage 301

Sequenced-flashing high-intensity lights are available for airport
use and are installed as supplements to the standard approach lighting
system at those airports where very low visibilities occur frequently.
These lights operate from the stored energy in a capacitor which is dis-
charged through the lamp in approximately 5 ms and may develop as
much as 30 million cd of light. They are mounted in the same pedestals
as the light bars. The lights are sequence-fired, beginning with the unit
farthest from the runway. The complete cycle is repeated every 2 s. This
results in a brilliant ball of light continuously moving toward the run-
way. Since the very bright light can interfere with the eye adaptation of
the pilot, condenser discharge lamps are usually omitted in the 1000 ft
of the approach lighting system nearest the runway.

Visual Approach Slope Aids
Visual approach slope aids are lighting systems designed to provide
a measure of vertical guidance to aircraft approaching a particular
runway. The principle of these aids is to provide color-based identifi-
cation to the pilot indicating their variation from a desired altitude
and descent rate while on approach. The two most common visual
approach slope aids are the visual approach slope indicator (VASI),
and the precision approach path indicator (PAPI).

Visual Approach Slope Indicator
The visual approach slope indicator (VASI) is a system of lights which
acts as an aid in defining the desired glide path in relatively good
weather conditions. VASI lighting intensities are designed to be visi-
ble from 3 to 5 mi during the day and up to 20 mi at night.
There are a number of different VASI configurations depending
on the desired visual range, the type of aircraft, and whether large
wide bodied aircraft will be using the runway. Each group of lights
transverse to the direction of the runway is referred to as a bar. The
downwind bar is typically located between 125 and 800 ft from the
runway threshold, each subsequent bar is located between 500 and
1000 ft from the previous bar. A bar is made up of one, two, or three
light units, referred to as boxes. The basic VASI-2 system, illustrated in
Fig. 8-6, is a two-bar system consisting of four boxes. The bar that is
nearest to the runway threshold is referred to as the downwind bar,
and the bar that is farthest from the runway threshold is referred to as
the upwind bar. As illustrated in Fig. 8-6, if pilots are on the proper
glide path, the downwind bar appears white and the upwind bar
appears red; if pilots are too low, both bars appear red; and if they are
too high both bars appear white.
In order to accommodate large wide bodied aircraft where the
height of the eye of the pilot is much greater than in smaller jets, a third
upwind bar is added. For wide bodied aircraft the middle bar becomes