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around the country, it is highly unlikely that most
pilots will ever encounter the MLS approach, and
if they do, it is even less likely that the proper
equipment would be installed in the aircraft.
Like the ILS, the basic MLS approach requires the
final approach course alignment to be within 3
degrees of the extended runway centerline. This type
of approach uses a glide slope between 3 and 6.40
degrees and provides precision landing minimums to
200 feet HAT. Obstacle clearance is based on the glide
slope angle used in the approach design. The design
criteria differ for each type of MLS approach and
incorporate numerous formulas for the derivation of
specific course criteria. This information is contained
in FAA Order 8260.3 Volume 3, Chapters 2 and 3.
In the front of the TPP, there is a page containing additional information pertaining to the use of an MLS system. The MLS Channeling and Frequency Pairing
Table cross references the appropriate MLS channel
with its paired VHF and TACAN frequencies. Ground
equipment associated with the MLS operates on the
MLS channels, while the MLS angle/data and DME is
required to operate using one of the paired VHF or
TACAN frequencies.
5-57
Figure 5-46. Simultaneous Offset Instrument Approach Procedure.
5-58
15° to
100°
Figure 5-47. Converging Approach Criteria.
Figure 5-48. Dallas/Fort Worth (KDFW), Dallas/Fort Worth, Texas, CONVERGING ILS RWY 35C.
5-59
VOR APPROACH
The VOR is one of the most widely used nonprecision
approach types in the NAS. VOR approaches use VOR
facilities both on and off the airport to establish
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
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Instrument Procedures Handbook (IPH)仪表程序手册下(135)
