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of a series of brilliant blue-white bursts of light flashing in
sequence along the approach lights, giving the effect of a ball
of light traveling towards the runway. Typically, “the rabbit”
makes two trips toward the runway per second.
Runway end identifier lights (REIL) are installed for rapid and
positive identification of the approach end of an instrument
runway. The system consists of a pair of synchronized
flashing lights placed laterally on each side of the runway
threshold facing the approach area.
7-41
ALSF-2 ALSF-1 SSALR/MALSR MALSF
ODALS
REIL
THRESHOLD
Figure 7-37. Precision and Nonprecision ALS Configuration.
The visual approach slope indicator (VASI) gives visual
descent guidance information during the approach to a
runway. The standard VASI consists of light bars that
project a visual glide path, which provides safe obstruction
clearance within the approach zone. The normal GS angle
is 3°; however, the angle may be as high as 4.5° for proper
obstacle clearance. On runways served by ILS, the VASI
angle normally coincides with the electronic GS angle.
Visual left/right course guidance is obtained by alignment
with the runway lights. The standard VASI installation
consists of either 2-, 3-, 4-, 6-, 12-, or 16-light units arranged
in downwind and upwind light bars. Some airports serving
long-bodied aircraft have three-bar VASIs which provide two
visual glidepaths to the same runway. The first glide path
encountered is the same as provided by the standard VASI.
The second glide path is about 25 percent higher than the first
and is designed for the use of pilots of long-bodied aircraft.
The basic principle of VASI is that of color differentiation
between red and white. Each light projects a beam having
a white segment in the upper part and a red segment in the
lower part of the beam. From a position above the glide path
the pilot sees both bars as white. Lowering the aircraft with
respect to the glide path, the color of the upwind bars changes
from white to pink to red. When on the proper glide path,
the landing aircraft will overshoot the downwind bars and
undershoot the upwind bars. Thus the downwind (closer)
bars are seen as white and the upwind bars as red. From a
position below the glide path, both light bars are seen as
red. Moving up to the glide path, the color of the downwind
7-42
Figure 7-38. Standard two-bar VASI.
bars changes from red to pink to white. When below the
glide path, as indicated by a distinct all-red signal, a safe
obstruction clearance might not exist. A standard two-bar
VASI is illustrated in Figure 7-38.
ILS Airborne Components
Airborne equipment for the ILS system includes receivers
for the localizer, GS, marker beacons, ADF, DME, and the
respective indicator instruments.
The typical VOR receiver is also a localizer receiver with
common tuning and indicating equipment. Some receivers
have separate function selector switches, but most switch
between VOR and LOC automatically by sensing if odd
tenths between 108 and 111.95 MHz have been selected.
Otherwise, tuning of VOR and localizer frequencies is
accomplished with the same knobs and switches, and the CDI
indicates “on course” as it does on a VOR radial.
Though some GS receivers are tuned separately, in a typical
installation the GS is tuned automatically to the proper
frequency when the localizer is tuned. Each of the 40 localizer
channels in the 108.10 to 111.95 MHz band is paired with a
corresponding GS frequency.
When the localizer indicator also includes a GS needle, the
instrument is often called a cross-pointer indicator. The
crossed horizontal (GS) and vertical (localizer) needles are
free to move through standard five-dot deflections to indicate
position on the localizer course and glide path.
When the aircraft is on the glide path, the needle is horizontal,
overlying the reference dots. Since the glide path is much
narrower than the localizer course (approximately 1.4° from
full up to full down deflection), the needle is very sensitive
to displacement of the aircraft from on-path alignment. With
the proper rate of descent established upon GS interception,
very small corrections keep the aircraft aligned.
The localizer and GS warning flags disappear from view on
the indicator when sufficient voltage is received to actuate the
needles. The flags show when an unstable signal or receiver
malfunction occurs.
The OM is identified by a low-pitched tone, continuous dashes
at the rate of two per second, and a purple/blue marker beacon
　
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