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localizer back course approach which can cause the glide
slope flag alarm to disappear and present unreliable glide
slope information. Disregard all glide slope signal
indications when making a localizer back course
approach unless a glide slope is specified on the approach
and landing chart.
2. The glide slope transmitter is located between
750 feet and 1,250 feet from the approach end of the
runway (down the runway) and offset 250 to 650 feet
from the runway centerline. It transmits a glide path
beam 1.4 degrees wide (vertically). The signal
provides descent information for navigation down to
the lowest authorized decision height (DH) specified
in the approved ILS approach procedure. The
glidepath may not be suitable for navigation below
the lowest authorized DH and any reference to
glidepath indications below that height must be
supplemented by visual reference to the runway
environment. Glidepaths with no published DH are
usable to runway threshold.
3. The glide path projection angle is normally
adjusted to 3 degrees above horizontal so that it
intersects the MM at about 200 feet and the OM at
about 1,400 feet above the runway elevation. The
glide slope is normally usable to the distance of
10 NM. However, at some locations, the glide slope
8/5/04 AIM
Air Navigation Aids 1−1−9
has been certified for an extended service volume
which exceeds 10 NM.
4. Pilots must be alert when approaching the
glidepath interception. False courses and reverse
sensing will occur at angles considerably greater than
the published path.
5. Make every effort to remain on the indicated
glide path.
CAUTION−
Avoid flying below the glide path to assure
obstacle/terrain clearance is maintained.
6. The published glide slope threshold crossing
height (TCH) DOES NOT represent the height of the
actual glide path on−course indication above the
runway threshold. It is used as a reference for
planning purposes which represents the height above
the runway threshold that an aircraft’s glide slope
antenna should be, if that aircraft remains on a
trajectory formed by the four−mile−to−middle
marker glidepath segment.
7. Pilots must be aware of the vertical height
between the aircraft’s glide slope antenna and the
main gear in the landing configuration and, at the DH,
plan to adjust the descent angle accordingly if the
published TCH indicates the wheel crossing height
over the runway threshold may not be satisfactory.
Tests indicate a comfortable wheel crossing height is
approximately 20 to 30 feet, depending on the type of
aircraft.
NOTE−
The TCH for a runway is established based on several
factors including the largest aircraft category that
normally uses the runway, how airport layout effects the
glide slope antenna placement, and terrain. A higher than
optimum TCH, with the same glide path angle, may cause
the aircraft to touch down further from the threshold if the
trajectory of the approach is maintained until the flare.
Pilots should consider the effect of a high TCH on the
runway available for stopping the aircraft.
e. Distance Measuring Equipment (DME)
1. When installed with the ILS and specified in
the approach procedure, DME may be used:
(a) In lieu of the OM;
(b) As a back course (BC) final approach fix
(FAF); and
(c) To establish other fixes on the localizer
course.
2. In some cases, DME from a separate facility
may be used within Terminal Instrument Procedures
(TERPS) limitations:
(a) To provide ARC initial approach segments;
(b) As a FAF for BC approaches; and
(c) As a substitute for the OM.
f. Marker Beacon
1. ILS marker beacons have a rated power
output of 3 watts or less and an antenna array
designed to produce an elliptical pattern with
dimensions, at 1,000 feet above the antenna, of
approximately 2,400 feet in width and 4,200 feet in
length. Airborne marker beacon receivers with a
selective sensitivity feature should always be
operated in the “low” sensitivity position for proper
reception of ILS marker beacons.
2. Ordinarily, there are two marker beacons
associated with an ILS, the OM and MM. Locations
with a Category II ILS also have an Inner
Marker (IM). When an aircraft passes over a marker,
the pilot will receive the indications shown in
TBL 1−1−3.
(a) The OM normally indicates a position at
which an aircraft at the appropriate altitude on the
localizer course will intercept the ILS glide path.
　
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