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AIM

10/12/17

1−1−10

Navigation Aids

d. Glide Slope/Glide Path

1. 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.

CAUTION−

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.

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

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 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)

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 seg-

ments;

(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