Gospel according to the FAA (from the Instrument Procedures Handbook). Your aviation authority may vary.
Minimum Descent Altitude (MDA), Decision
Altitude (DA), And Decision Height (DH)
MDA—the lowest altitude, expressed in feet MSL, to which
descent is authorized on final approach or during circle-toland
maneuvering in execution of a standard instrument
approach procedure (SIAP) where no electronic glideslope
is provided.
DA—a specified altitude in the precision approach at
which a missed approach must be initiated if the required
visual reference to continue the approach has not been
established.
DH—with respect to the operation of aircraft, means the
height at which a decision must be made during an ILS, MLS,
or PAR IAP to either continue the approach or to execute a
missed approach.
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 has 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.
For many air carriers, OpSpecs may be the limiting factor
for some types of approaches. NDB and circling approaches
are two common examples where the OpSpecs 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 OpSpecs,
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 United States, but
RNP AR restricts 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.
14 CFR Part 121, section 121.652, 14 CFR Part 125, section
125.379, and 14 CFR Part 135, section 135.225 require
that some PICs, 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
OpSpecs. 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.
Pilots are cautioned to fully understand and abide by
the guidelines set forth in 91.175(c) regarding proper
identification of the runway and runway environment when
electing to continue any approach beyond the published
DA/DH or MDA.
It is imperative to recognize that any delay in making a
decision to execute the Missed Approach Procedure at
the DA/DH or MDA/Missed Approach Point will put the
aircrew at risk of impacting any obstructions that may be
penetrating the visual obstacle clearance surface
The visual segment of an IAP begins at DA or MDA and
continues to the runway. There are two means of operating
in the visual segment, one is by using natural vision under
14 CFR Part 91, section 91.175 (c) and the other is by using
an Enhanced Flight Vision System under 14 CFR Part 91,
section 91.175 (l).