BY ORDER OF THE SECRETARY OF THE AIR FORCE AIR FORCE ...

AFMAN 11-217 V1 3 JANUARY 2005 91
6.6.6. False Course Indications. False course indications may be received when the
aircraft is not within the depicted area of coverage. Therefore, localizer course
information received outside the area depicted in Figure 6.2 should be considered invalid
unless the procedure is published otherwise (for example, localizer type directional aid or
back course localizer). There is also a remote chance electromagnetic interference may
cause false course indications within the depicted area of coverage. For these reasons, it
is essential to confirm the localizer on course indication by reference to aircraft heading
and any other available navigation aids, such as an ADF bearing pointer, before
commencing final descent. Any abnormal indications experienced within 35 degrees of
the published front course or back course centerline of an ILS localizer should be
reported immediately to the appropriate ATC facility.
6.6.7. False Glideslope Indications. False glideslope indications may be received when
the aircraft is not within the depicted area of coverage, or the glide slope power status is
in alarm. There is also a chance that aircraft or vehicles parked in the ILS Critical Area
may interfere with the glideslope signal. For these reasons, it is essential to confirm
glideslope intercept altitudes and expected altitudes as depicted on the IAP. If indications
are suspect, transition to localizer procedures or execute a missed approach.
6.6.8. ILS Facilities with Associated DME. ILS facilities sometimes have associated
DME. These facilities are usually found at civilian fields. Some instrument approach
procedures require TACAN or VOR associated DME on the initial segment and the ILS
associated DME during the final portion of the approach. Pilots must exercise extreme
caution to ensure the proper DME channel is tuned to preclude premature descents.
6.6.8.1. NOTE: Due to angular dispersion of the localizer and glide slope signals, the
corrective inputs to return to "on course, on glide slope" become smaller as the
aircraft approaches the runway threshold (Figure 6.2).