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135.225 require that some pilots-in-command,
with limited experience in the aircraft they are
operating, increase the approach minimums and
visibility by 100 feet and one-half mile respectively. Rules for these “high-minimums” pilots
are usually derived from a combination of federal regulations and the company’s OpsSpecs.
There are many factors that can determine the actual
minimums that can be used for a specific approach.
All of them must be considered by pilots during the
preflight and approach planning phases, discussed,
and briefed appropriately.
VERTICAL NAVIGATION
One of the advantages of some GPS and multi-sensor FMS RNAV avionics is the advisory VNAV
capability. Traditionally, the only way to get vertical path information during an approach was to use
a ground-based precision NAVAID. Modern RNAV
avionics can display an electronic vertical path that
provides a constant-rate descent to minimums.
Since these systems are advisory and not primary
guidance, the pilot must continuously ensure the
aircraft remains at or above any published altitude
constraint, including step-down fix altitudes, using
the primary barometric altimeter. The pilots, airplane, and operator must be approved to use advisory VNAV inside the FAF on an instrument
approach.
VNAV information appears on selected conventional nonprecision, GPS, and RNAV approaches
(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
　
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