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flexible approach aid that, due to its inherent design,
provides many applications for a variety of needs in
instrument flying. An ILS glide slope installation may
be impossible due to surrounding terrain. For whatever
reason, the localizer is able to provide four separate
applications from one approach system:
• Localizer Approach.
• Localizer/DME Approach.
Figure 5-53. Asheville Regional (KAVL), Asheville, NC, Radar Instrument Approach Minimums.
Obstacle Clearance Surface (OCS)
975 10,000 40,000
Glide Slope 2°– 3°
Ground Point of
Interception (GPI)
Figure 5-54. PAR Final Approach Area Criteria.
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• Localizer Back Course Approach.
• Localizer-type Directional Aid (LDA).
LOCALIZER AND LOCALIZER DME
The localizer approach system can provide both
precision and nonprecision approach capabilities to
a pilot. As a part of the ILS system, the localizer
provides horizontal guidance for a precision
approach. Typically, when the localizer is discussed, it is thought of as a nonprecision approach
due to the fact that either it is the only approach
system installed, or the glide slope is out of service
on the ILS. In either case, the localizer provides a
nonprecision approach using a localizer transmitter
installed at a specific airport. [Figure 5-55]
TERPS provide the same alignment criteria for a localizer approach as it does for the ILS since it is essentially
the same approach without vertical guidance stemming
from the glide slope. A localizer is always aligned within
Figure 5-55. Vicksburg Tallulah Regional (KTVR), Tallulah/Vicksburg, Louisiana, LOC RWY 36.
5-65
3 degrees of the runway, and it is afforded a minimum of
250 feet obstacle clearance in the final approach area. In
the case of a localizer DME (LOC DME) approach, the
localizer installation has a collocated DME installation
that provides distance information required for the
approach. [Figure 5-56]
LOCALIZER BACK COURSE
In cases where an ILS is installed, a back course may
be available in conjunction with the localizer. Like
the localizer, the back course does not offer a glide
slope, but remember that the back course can project
a false glide slope signal and the glide slope should
be ignored. Reverse sensing will occur on the back
course using standard VOR equipment. With an
HSI (horizontal situation indicator) system,
reverse sensing is eliminated if it is set appropriately to the front course. [Figure 5-57 on page 5-66]
LOCALIZER-TYPE DIRECTIONAL AID
An LDA is a NAVAID that provides nonprecision
approach capabilities. The LDA is essentially a localizer. It is termed LDA because the course alignment
with the runway exceeds 3 degrees. Typically, an LDA
Figure 5-56. Davidson County (KEXX), Lexington, North Carolina, LOC DME RWY 6.
5-66
installation does not incorporate a glide slope component. However, the availability of a glide slope associated with an LDA is noted on the approach chart. This
type of NAVAID provides an approach course between
3 and 6 degrees, making it similar in accuracy to a
localizer, but remember that the LDA is not as closely
aligned with the runway and it does not offer a navigable back course. Currently there are less than 30 LDA
installations in the U.S., and as a result, most pilots are
not familiar with this type of instrument approach.
[Figure 5-58]
SIMPLIFIED DIRECTIONAL FACILITY
The SDF is another instrument approach system that is
not as accurate as the LOC approach facilities. Like the
LOC type approaches, the SDF is an alternative
approach that may be installed at an airport for a variety of reasons, including terrain. The final approach
Figure 5-57. Baton Rouge Metro/Ryan (KBTR), Baton Rouge, Louisiana, LOC BC RWY 4L.
5-67
Figure 5-58. Hartford-Brainard (KHFD), Hartford, Connecticut, LDA RWY 2.
5-68
course width of an SDF system is set at either 6 or 12
degrees. The SDF is a nonprecision approach since it
only provides lateral guidance to the runway.
For straight-in SDF approaches, the angle of convergence for the final approach course and the extended
runway centerline is 30 degrees or less, and if the
angle of convergence is beyond 30 degrees, the SDF
will only have circling minimums. An SDF approach
is provided a minimum of 250 feet obstacle clearance
for straight-in approaches while in the final approach
area, which is an area defined for a 6 degrees course:
1,000 feet at or abeam the runway threshold expanding to 19,228 feet 10 NM from the threshold. The
same final approach area for a 12 degrees course is
larger. This type of approach is also designed with a
　
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