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approaches and include the use of a wide variety of
equipment such as DME and TACAN. Due to the wide
variety of options included in a VOR approach, TERPS
outlines design criteria for both on and off airport VOR
facilities as well as VOR approaches with and without
a FAF. Despite the various configurations, all VOR
approaches are nonprecision approaches, require the
presence of properly operating VOR equipment, and
can provide MDAs as low as 250 feet above the runway. VOR also offers a flexible advantage in that an
approach can be made toward or away from the
navigational facility.
The VOR approach into Missoula International in
Missoula, Montana, is an example of a VOR approach
where the VOR facility is on the airport and there is no
specified FAF. [Figure 5-49] For a straight-in approach,
the final approach course is typically aligned to intersect
the extended runway centerline 3,000 feet from the runway threshold, and the angle of convergence between
the two does not exceed 30 degrees. This type of VOR
approach also includes a minimum of 300 feet of obstacle
clearance in the final approach area. The final approach
area criteria include a 2 NM wide primary area at the
facility that expands to 6 NM wide at a distance of 10 NM
from the facility. Additional approach criteria are established for courses that require a high altitude teardrop
approach penetration.
When DME is included in the title of the VOR
approach, operable DME must be installed in the aircraft in order to fly the approach from the FAF. The
use of DME allows for an accurate determination of
position without timing, which greatly increases situational awareness throughout the approach. Alexandria,
Louisiana, is an excellent example of a VOR/DME
approach in which the VOR is off the airport and a FAF
is depicted. [Figure 5-50 on page 5-60] In this case,
the final approach course is a radial or straight-in final
approach and is designed to intersect the runway centerline at the runway threshold with the angle of convergence not exceeding 30 degrees.
The criteria for an arc final approach segment associated
with a VOR/DME approach is based on the arc being
beyond 7 NM and no farther than 30 NM from the VOR,
Figure 5-49. Missoula International, Missoula, Montana (KMSO), VOR–C.
and depends on the angle of convergence between the
runway centerline and the tangent of the arc. Obstacle
clearance in the primary area, which is considered the
area 4 NM on either side of the arc centerline, is
guaranteed by at least 500 feet.
NDB APPROACH
Like the VOR approach, an NDB approach can be
designed using facilities both on and off the airport,
with or without a FAF, and with or without DME availability. At one time it was commonplace for an instrument student to learn how to fly an NDB approach, but
with the growing use of GPS, many pilots no longer
use the NDB for instrument approaches. New RNAV
approaches are also rapidly being constructed into airports that are served only by NDB. The long-term plan
includes the gradual phase out of NDB facilities, and
eventually, the NDB approach will become nonexistent.
Until that time, the NDB provides additional availability
for instrument pilots into many smaller, remotely located
airports.
The NDB Runway 9 approach at Charleston Executive
Airport, is an example of an NDB approach established
5-60
Figure 5-50. Alexandria International, Alexandria, Louisiana (KAEX), VOR/DME RWY 32.
5-61
with an on-airport NDB that does not incorporate a
FAF. [Figure 5-51] In this case, a procedure turn or
penetration turn is required to be a part of the approach
design. For the NDB to be considered an on-airport
facility, the facility must be located within one mile of
any portion of the landing runway for straight-in
approaches and within one mile of any portion of
usable landing surface for circling approaches. The
final approach segment of the approach is designed
with a final approach area that is 2.5 NM wide at the
facility, and increases to 8 NM wide at 10 NM from the
facility. Additionally, the final approach course and the
extended runway centerline angle of convergence cannot exceed 30 degrees for straight-in approaches. This
type of NDB approach is afforded a minimum of 350
feet obstacle clearance.
When a FAF is established for an NDB approach, the
approach design criteria changes. It also takes into
account whether or not the NDB is located on or off the
airport. Additionally, this type of approach can be made
both moving toward or away from the NDB facility.
The St. Mary’s, Alaska, NDB DME RWY 16 [Figure
　
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