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(see Types of Approaches later in this chapter). It
normally consists of two fixes (the FAF and the
landing runway threshold), a FAF crossing altitude,
a vertical descent angle (VDA), and may provide a
visual descent point (VDP). [Figure 5-15] The published VDA is for information only, advisory in
nature, and provides no additional obstacle protection below the MDA. Operators can be approved to
add a height loss value to the MDA, and use this
derived decision altitude (DDA) to ensure staying
above the MDA. Operators authorized to use a
VNAV DA in lieu of the MDA must commence a
missed approach immediately upon reaching the
VNAV DA if the required visual references to continue the approach have not been established.
A constant-rate descent has many safety advantages
over nonprecision approaches that require multiple
level-offs at stepdown fixes or manually calculating
rates of descent. A stabilized approach can be maintained from the FAF to the landing when a constantrate descent is used. Additionally, the use of an
electronic vertical path produced by onboard avionics can serve to reduce CFIT, and minimize the
effects of visual illusions on approach and landing.
WIDE AREA AUGMENTATION SYSTEM
In addition to the benefits that VNAV information provides for conventional nonprecision approaches,
VNAV has a significant effect on approaches that are
designed specifically for RNAV systems. Using an
FMS or GPS that can provide both lateral navigation
(LNAV) and VNAV, some RNAV approaches allow
descents to lower MDAs or DAs than when using
LNAV alone. The introduction of the Wide Area
Augmentation System (WAAS), which became operational on July 10, 2003, provides even lower minimums for RNAV approaches that use GPS by
providing electronic vertical guidance and increased
accuracy.
The Wide Area Augmentation System, as its name
implies, augments the basic GPS satellite constellation with additional ground stations and enhanced
5-21
position integrity information transmitted from
geostationary satellites. This capability of augmentation enhances both the accuracy and integrity of
basic GPS, and may support electronic vertical
guidance approach minimums as low as 200 feet
HAT and 1/2 SM visibility. In order to achieve the
lowest minimums, the requirements of an entire
electronic vertical guidance system, including
satellite availability; clear obstruction surfaces; AC
150/5300-13, Airport Design; and electronic vertical guidance runway and airport requirements,
must be satisfied. The minimums are shown as DAs
since electronically computed glidepath guidance
is provided to the pilot. The electronically computed
guidance eliminates errors that can be introduced
when using barometric altimetry.
RNAV (GPS) approach charts presently can have up to
four lines of approach minimums: LPV, LNAV/VNAV,
LNAV, and Circling. Figure 5-16 shows how these minimums might be presented on an approach chart, with
the exception of GLS.
• GLS — The acronym GLS stands for The Global
Navigation Satellite System [GNSS] Landing
System (GLS). GLS is a satellite based navigation system that provides course and glidepath
information meeting the precision standards of
ICAO Annex 10. Procedures based on the local
area augmentation system (LAAS) will be
charted separately under the GLS title as these
systems are implemented.
NOTE: On RNAV approach charts the GLS minima line has been used as a placeholder only. As
WAAS procedures are developed, LPV lines of
minima will replace the “GLS DA-NA” lines of
minima.
• LPV — APV minimums that take advantage of
WAAS to provide electronic lateral and vertical
guidance capability. The term “LPV” (localizer
performance with vertical guidance) is used for
approaches constructed with WAAS criteria
where the value for the vertical alarm limit is
more than 12 meters and less than 50 meters.
WAAS avionics equipment approved for LPV
approaches is required for this type of approach.
The lateral guidance is equivalent to localizer
accuracy, and the protected area is considerably
smaller than the protected area for the present
LNAV and LNAV/VNAV lateral protection.
Aircraft can fly this minima line with a statement
in the Aircraft Flight Manual that the installed
equipment supports LPV approaches. Notice the
WAAS information shown in the top left corner
of the pilot briefing information on the chart
depicted. Below the term WAAS is the WAAS
channel number (CH 50102), and the WAAS
approach identifier (W17A), indicating Runway
17R in this case, and then a letter to designate the
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Instrument Procedures Handbook (IPH)仪表程序手册上(117)
