Minimum safe altitudes (MSAs) are published for
emergency use on IAP charts. For conventional navigation
systems, the MSA is normally based on the
primary omnidirectional facility on which the IAP is
predicated. The MSA depiction on the approach chart
contains the facility identifier of the NAVAID used to
determine the MSA. For RNAV approaches, the MSA is
based on either the runway waypoint (RWY WP) or the
missed approach waypoint (MAWP) for straight-in
approaches, or the airport waypoint (APT WP) for circling
only approaches.For RNAV (GPS) approaches
with a terminal arrival area (TAA) the MSA is based on
the IAF waypoint.
MSAs are expressed in feet above MSL and normally
have a 25 NM radius. This radius may be expanded to
30 NM if necessary to encompass the airport landing
surfaces. Ideally, a single sector altitude is established
and depicted on the planview of approach charts. When
necessary to maintain clearance from obstructions, the
area may be further sectored and as many as four MSAs
established. When established, sectors may be no less
than 90°in spread. MSAs provide 1,000 feet clearance
over all obstructions but do not necessarily assure
acceptable navigation signal coverage.
FINAL APPROACH FIX ALTITUDE
Another important altitude that should be briefed
during an IAP briefing is the FAF altitude, designated
by the cross on a nonprecision approach, and the lightning
bolt symbol designating the glide slope intercept
altitude on a precision approach. Adherence to and
crosscheck of this altitude can have a direct effect on
the success of an approach.
Proper airspeed, altitude, and configuration, when
crossing the FAF of a nonprecision approach, are
extremely important no matter what type of aircraft is
being flown. The stabilized approach concept, implemented
by the FAA within the SOPs of each air carrier,
suggests that crossing the FAF at the published altitude
is often a critical component of a successful nonprecision
approach, especially in a large turbojet
The glide slope intercept altitude of a precision
approach should also be included in the IAP briefing.
Awareness of this altitude when intercepting the glide
slope can ensure the flight crew that a “false glide
slope” or other erroneous indication is not inadvertently
followed. Many air carriers include a standard
callout when the aircraft passes over the FAF of the
nonprecision approach underlying the ILS. The pilot
monitoring (PM) states the name of the fix and the
charted glide slope altitude, thus allowing both pilots to
crosscheck their respective altimeters and verify the
MINIMUM DESCENT ALTITUDE, DECISION ALTITUDE,
AND DECISION HEIGHT
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.
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.
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.