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intersects the MM at about 200 feet and the OM at
2/14/08 AIM
Navigation Aids 1-1-9
about 1,400 feet above the runway elevation. The
glide slope is normally usable to the distance of
10NM. 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.
CAUTIONAvoid
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.
NOTEThe
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 2,400feet 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 approximately
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
OM BLUE
MM AMBER
IM WHITE
BC WHITE
AIM 2/14/08
1-1-10 Navigation Aids
3. A back course marker normally indicates the
ILS back course final approach fix where approach
descent is commenced.
g. Compass Locator
1. Compass locator transmitters are often
situated at the MM and OM sites. The transmitters
have a power of less than 25 watts, a range of at least
15miles and operate between 190 and 535 kHz. At
some locations, higher powered radio beacons, up to
400 watts, are used as OM compass locators. These
generally carry Transcribed Weather Broadcast
(TWEB) information.
2. Compass locators transmit two letter identification
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