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j. Inoperative ILS Components
1. Inoperative localizer. When the localizer
fails, an ILS approach is not authorized.
2. Inoperative glide slope. When the glide
slope fails, the ILS reverts to a non-precision
See the inoperative component table in the U.S. Government Terminal
Procedures Publication (TPP), for adjustments to minimums due to
inoperative airborne or ground system equipment.
k. ILS Course Distortion
1. All pilots should be aware that disturbances to
ILS localizer and glide slope courses may occur when
surface vehicles or aircraft are operated near the
localizer or glide slope antennas. Most ILS
installations are subject to signal interference by
either surface vehicles, aircraft or both. ILS
CRITICAL AREAS are established near each
localizer and glide slope antenna.
2. ATC issues control instructions to avoid
interfering operations within ILS critical areas at
controlled airports during the hours the Airport
Traffic Control Tower (ATCT) is in operation as
(a) Weather Conditions. Less than ceiling
800 feet and/or visibility 2 miles.
(1) Localizer Critical Area. Except for
aircraft that land, exit a runway, depart, or execute a
missed approach, vehicles and aircraft are not
authorized in or over the critical area when an arriving
aircraft is inside the outer marker (OM) or the fix
used in lieu of the OM. Additionally, when conditions
are less than reported ceiling 200 feet or RVR less
than 2,000 feet, do not authorize vehicles or aircraft
operations in or over the area when an arriving
aircraft is inside the MM, or in the absence of a MM,
½ mile final.
(2) Glide Slope Critical Area. Do not
authorize vehicles or aircraft operations in or over the
area when an arriving aircraft is inside the ILS outer
marker (OM), or the fix used in lieu of the OM, unless
the arriving aircraft has reported the runway in sight
and is circling or side-stepping to land on another
(b) Weather Conditions. At or above ceil-
ing 800 feet and/or visibility 2 miles.
(1) No critical area protective action is
provided under these conditions.
(2) A flight crew, under these conditions,
should advise the tower that it will conduct an
AUTOLAND or COUPLED approach.
Denver Tower, United 1153, Request Autoland/Coupled
ATC replies with:
United 1153, Denver Tower, Roger, Critical Areas not
3. Aircraft holding below 5,000 feet between
the outer marker and the airport may cause localizer
signal variations for aircraft conducting the ILS
approach. Accordingly, such holding is not author-
ized when weather or visibility conditions are less
than ceiling 800 feet and/or visibility 2 miles.
4. Pilots are cautioned that vehicular traffic not
subject to ATC may cause momentary deviation to
ILS course or glide slope signals. Also, critical areas
are not protected at uncontrolled airports or at airports
with an operating control tower when weather or
visibility conditions are above those requiring
protective measures. Aircraft conducting coupled or
autoland operations should be especially alert in
monitoring automatic flight control systems.
(See FIG 1-1-7.)
Unless otherwise coordinated through Flight Standards,
ILS signals to Category I runways are not flight inspected
below the point that is 100 feet less than the decision
altitude (DA). Guidance signal anomalies may be
encountered below this altitude.
1-1-10. Simplified Directional Facility
a. The SDF provides a final approach course
similar to that of the ILS localizer. It does not provide
glide slope information. A clear understanding of the
ILS localizer and the additional factors listed below
completely describe the operational characteristics
and use of the SDF.
b. The SDF transmits signals within the range of
108.10 to 111.95 MHz.
c. The approach techniques and procedures used
in an SDF instrument approach are essentially the
same as those employed in executing a standard
localizer approach except the SDF course may not be
Navigation Aids 1-1-11