Aeronautical Information Manual (AIM), page 63

Index   62 -- Page 63 -- 64

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.


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

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

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-

(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.

(b) The MM indicates a position approxim-
ately 3,500 feet from the landing threshold. This is
also the position where an aircraft on the glide path

will be at an altitude of approximately 200 feet above

the elevation of the touchdown zone.
(c) The IM will indicate a point at which an
aircraft is at a designated decision height (DH) on the
glide path between the MM and landing threshold.

TBL 1-1-3
Marker Passage Indications
Marker Code Light



Navigation Aids 1-1-9

Page 63 of the Aeronautical Information Manual (AIM.pdf)
AIM: Official Guide to Basic Flight Information and ATC Procedures

Index   62 -- Page 63 -- 64