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any distance from the transmitter site. Problems with signal
reception are usually caused by the airborne receiver not
being turned on, or by incorrect receiver sensitivity.
Some marker beacon receivers, to decrease weight and cost,
are designed without their own power supply. These units
utilize a power source from another radio in the avionics
stack, often the ADF. In some aircraft, this requires the
ADF to be turned on in order for the marker beacon receiver
to function, yet no warning placard is required. Another
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source of trouble may be the “High/Low/Off” three-position
switch, which both activates the receiver and selects receiver
sensitivity. Usually, the “test” feature only tests to see if
the light bulbs in the marker beacon lights are working.
Therefore, in some installations, there is no functional way
for the pilot to ascertain the marker beacon receiver is actually
on except to fly over a marker beacon transmitter, and see if
a signal is received and indicated (e.g., audibly, and visually
via marker beacon lights).
Operational Errors
1. Failure to understand the fundamentals of ILS ground
equipment, particularly the differences in course
dimensions. Since the VOR receiver is used on the
localizer course, the assumption is sometimes made
that interception and tracking techniques are identical
when tracking localizer courses and VOR radials.
Remember that the CDI sensing is sharper and faster
on the localizer course.
2. Disorientation during transition to the ILS due to poor
planning and reliance on one receiver instead of on all
available airborne equipment. Use all the assistance
available; a single receiver may fail.
3. Disorientation on the localizer course, due to the first
error noted above.
4. Incorrect localizer interception angles. A large
interception angle usually results in overshooting,
and possible disorientation. When intercepting,
if possible, turn to the localizer course heading
immediately upon the first indication of needle
movement. An ADF receiver is an excellent aid to
orient you during an ILS approach if there is a locator
or NDB on the inbound course.
5. Chasing the CDI and glide path needles, especially
when you have not sufficiently studied the approach
before the flight.
Simplified Directional Facility (SDF)
The SDF provides a final approach course similar to the ILS
localizer. The SDF course may or may not be aligned with
the runway and the course may be wider than a standard
ILS localizer, resulting in less precision. Usable off-course
indications are limited to 35° either side of the course
centerline. Instrument indications in the area between 35°
and 90° from the course centerline are not controlled and
should be disregarded.
The SDF must provide signals sufficient to allow satisfactory
operation of a typical aircraft installation within a sector
which extends from the center of the SDF antenna system
to distances of 18 NM covering a sector 10° either side of
centerline up to an angle 7° above the horizontal. The angle
of convergence of the final approach course and the extended
runway centerline must not exceed 30°. Pilots should note
this angle since the approach course originates at the antenna
site, and an approach continued beyond the runway threshold
would lead the aircraft to the SDF offset position rather than
along the runway centerline.
The course width of the SDF signal emitted from the
transmitter is fixed at either 6° or 12°, as necessary, to provide
maximum flyability and optimum approach course quality.
A three-letter identifier is transmitted in code on the SDF
frequency; there is no letter “I” (two dots) transmitted before
the station identifier, as there is with the LOC. For example,
the identifier for Lebanon, Missouri, SDF is LBO.
Localizer Type Directional Aid (LDA)
The LDA is of comparable utility and accuracy to a localizer
but is not part of a complete ILS. The LDA course width is
between 3° and 6° and thus provides a more precise approach
course than an SDF installation. Some LDAs are equipped
with a GS. The LDA course is not aligned with the runway,
but straight-in minimums may be published where the angle
between the runway centerline and the LDA course does not
exceed 30°. If this angle exceeds 30°, only circling minimums
are published. The identifier is three letters preceded by “I”
transmitted in code on the LDA frequency. For example, the
identifier for Van Nuys, California, LDA is I-BUR.
　
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