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the ground facility radiation pattern. The glide path is the
straight, sloped line the aircraft should fly in its descent from
where the GS intersects the altitude used for approaching the
FAF, to the runway touchdown zone. The GS equipment
is housed in a building approximately 750 to 1,250 feet
down the runway from the approach end of the runway, and
between 400 and 600 feet to one side of the centerline.
7-40
Figure 7-36. Localizer receiver indications and aircraft
displacement.
The course projected by the GS equipment is essentially the
same as would be generated by a localizer operating on its
side. The GS projection angle is normally adjusted to 2.5°
to 3.5° above horizontal, so it intersects the MM at about
200 feet and the OM at about 1,400 feet above the runway
elevation. At locations where standard minimum obstruction
clearance cannot be obtained with the normal maximum GS
angle, the GS equipment is displaced farther from the approach
end of the runway if the length of the runway permits; or, the
GS angle may be increased up to 4°.
Unlike the localizer, the GS transmitter radiates signals only
in the direction of the final approach on the front course. The
system provides no vertical guidance for approaches on the
back course. The glide path is normally 1.4° thick. At 10
NM from the point of touchdown, this represents a vertical
distance of approximately 1,500 feet, narrowing to a few feet
at touchdown.
Marker Beacons
Two VHF marker beacons, outer and middle, are normally
used in the ILS system. [Figure 7-36] A third beacon, the
inner, is used where Category II operations are certified. A
marker beacon may also be installed to indicate the FAF on
the ILS back course.
The OM is located on the localizer front course 4–7 miles
from the airport to indicate a position at which an aircraft, at
the appropriate altitude on the localizer course, will intercept
the glide path. The MM is located approximately 3,500 feet
from the landing threshold on the centerline of the localizer
front course at a position where the GS centerline is about 200
feet above the touchdown zone elevation. The inner marker
(IM), where installed, is located on the front course between
the MM and the landing threshold. It indicates the point at
which an aircraft is at the decision height on the glide path
during a Category II ILS approach. The back-course marker,
where installed, indicates the back-course FAF.
Compass Locator
Compass locators are low-powered NDBs and are received
and indicated by the ADF receiver. When used in conjunction
with an ILS front course, the compass locator facilities are
collocated with the outer and/or MM facilities. The coding
identification of the outer locator consists of the first two
letters of the three-letter identifier of the associated LOC.
For example, the outer locator at Dallas/Love Field (DAL) is
identified as “DA.” The middle locator at DAL is identified
by the last two letters “AL.”
Approach Lighting Systems (ALS)
Normal approach and letdown on the ILS is divided into two
distinct stages: the instrument approach stage using only radio
guidance, and the visual stage, when visual contact with the
ground runway environment is necessary for accuracy and
safety. The most critical period of an instrument approach,
particularly during low ceiling/visibility conditions, is the
point at which the pilot must decide whether to land or
execute a missed approach. As the runway threshold is
approached, the visual glide path will separate into individual
lights. At this point, the approach should be continued by
reference to the runway touchdown zone markers. The ALS
provides lights that will penetrate the atmosphere far enough
from touchdown to give directional, distance, and glide path
information for safe visual transition.
Visual identification of the ALS by the pilot must be
instantaneous, so it is important to know the type of ALS
before the approach is started. Check the instrument approach
chart and the A/FD for the particular type of lighting facilities
at the destination airport before any instrument flight. With
reduced visibility, rapid orientation to a strange runway can
be difficult, especially during a circling approach to an airport
with minimum lighting facilities, or to a large terminal airport
located in the midst of distracting city and ground facility
lights. Some of the most common ALS systems are shown
in Figure 7-37.
A high-intensity flasher system, often referred to as “the
rabbit,” is installed at many large airports. The flashers consist
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