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Navigation Aids

d. Glide Slope/Glide Path


The UHF glide slope transmitter, operating

on one of the 40 ILS channels within the frequency
range 329.15 MHz, to 335.00 MHz radiates its signals
in the direction of the localizer front course. The term
“glide path” means that portion of the glide slope that
intersects the localizer.


False glide slope signals may exist in the area of the
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.


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.


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
has been certified for an extended service volume
which exceeds 10 NM.


Pilots must be alert when approaching the

glidepath interception. False courses and reverse
sensing will occur at angles considerably greater than
the published path.


Make every effort to remain on the indicated

glide path.


Avoid flying below the glide path to assure
obstacle/terrain clearance is maintained.


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


marker glidepath segment.


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


The TCH for a runway is established based on several
factors including the largest aircraft category that
normally uses the runway, how airport layout affects 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)


When installed with the ILS and specified in

the approach procedure, DME may be used:


In lieu of the OM;


As a back course (BC) final approach fix

(FAF); and


To establish other fixes on the localizer



In some cases, DME from a separate facility

may be used within Terminal Instrument Procedures
(TERPS) limitations:


To provide ARC initial approach seg-



As a FAF for BC approaches; and


As a substitute for the OM.

f. Marker Beacon


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