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MDA and DA are referenced to MSL and measured
with a barometric altimeter. CAT II and III
approach DHs are referenced to AGL and measured
with a radio altimeter.
The height above touchdown (HAT) for a CAT I
precision approach is normally 200 feet above
touchdown zone elevation (TDZE). When a HAT of
250 feet or higher is published, it may be the result
of the signal-in-space coverage, or there may be
penetrations of either the final or missed approach
obstacle clearance surfaces (OCSs). If there are
OCS penetrations, the pilot will have no indication
on the approach chart where the obstacles are
located. It is important for pilots to brief the MDA,
DA, or DH so that there is no ambiguity as to what
minimums are being used. These altitudes can be
restricted by many factors. Approach category,
inoperative equipment in the aircraft or on the
ground, crew qualifications, and company authorizations
are all examples of issues that may limit or change the
height of a published MDA, DA, or DH.
5-19
The primary authorization for the use of specific
approach minimums by an individual air carrier can be
found in Part C–Airplane Terminal Instrument
Procedures, Airport Authorizations and Limitations, of
its FAA approved OpsSpecs. This document lists the
lowest authorized landing minimums that the carrier
can use while conducting instrument approaches.
Figure 5-14 shows an example of a carrier’s OpsSpecs
that lists minimum authorized MDAs and visibilities
for nonprecision approaches.
Figure 5-14. Authorized Landing Minimums for Nonprecision Approaches.
5-20
As can be seen from the previous example, the
OpsSpecs of this company rarely restrict it from using
the published MDA for a nonprecision approach. In
other words, most, if not all, nonprecision approaches
that pilots for this company fly have published MDAs
that meet or exceed its lowest authorized minimums.
Therefore the published minimums are the limiting factor in these cases.
For many air carriers, OpsSpecs may be the limiting
factor for some types of approaches. NDB and circling
approaches are two common examples where the
OpsSpecs minimum listed altitudes may be more
restrictive than the published minimums. Many Part
121 and 135 operators are restricted from conducting
circling approaches below 1,000-feet MDA and 3 SM
visibility by Part C of their OpsSpecs, and many have
specific visibility criteria listed for NDB approaches
that exceed visibilities published for the approach
(commonly 2 SM). In these cases, flight crews must
determine which is the more restrictive of the two and
comply with those minimums.
In some cases, flight crew qualifications can be
the limiting factor for the MDA, DA, or DH for an
instrument approach. There are many CAT II and
III approach procedures authorized at airports
throughout the U.S., but Special Aircraft and
Aircrew Authorization Requirements (SAAAR)
restrict their use to pilots who have received specific
training, and aircraft that are equipped and authorized to conduct those approaches. Other rules pertaining to flight crew qualifications can also
determine the lowest usable MDA, DA, or DH for
a specific approach. Parts 121.652, 125.379, and
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
　
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