It would probably help if it was explained how the MDA is established by
AVN.
First, the MDA is established between the FAF and MAP/threshold
(whichever is last). This is the basic MDA for the procedure.
Second, the specialist will determine if it's possible to provide a
lower MDA at some point on final by adding another fix after the FAF
where the pilot can "step down" to a lower altitude. This can only be
done if there is either at least a 60 ft reduction in the MDA, or a
reduction in visibility.
Third, there has to be a suitable fix to be used as a stepdown.

If the above criteria is met, then the specialist can add a lower MDA
that is applicable after the stepdown fix. Whether the first MDA is
actually listed as an MDA in the minimums section is dependent on
whether the stepdown fix will "always" be received, or if it will only
be received by some aircraft. For RNAV (GPS) approaches, it's considered
to always be receivable by the aircraft, so only the one MDA is
published as an MDA. If it's a procedure with /DME in the procedure
name, then it's also considered always receivable by the aircraft, since
you're required to have DME to conduct the procedure due to the
procedure title.

For other procedures, such as a VOR or LOC or NDB (not xxx/DME) with a
DME or crossing radial/bearing stepdown fix, the stepdown altitude will
also be shown as the MDA, since it is the MDA for those aircraft not
capable of receiving the stepdown fix. In those cases, you will see 2
MDA's published on the procedure, since some aircraft can't benefit from
the lower MDA.

You're right, the MDA is the lowest altitude, expressed in MSL, to which
descent is authorized on final approach. And the lowest altitude,
expressed in MSL, to which descent is authorized on final prior to the
stepdown fix is the minimum altitude shown at the stepdown fix. That is
your MDA until you reach the stepdown fix.

The complication is that the selected altitude may be computed
differently depending on whether it's "always received" or just
"sometimes received". If it can only be received sometimes, it will
actually be the lowest altitude that will clear obstructions on that
segment of final between the FAF and stepdown fix. This is to benefit
those that won't be able to receive the stepdown fix.
If it can always be received, it may be artificially adjusted higher for
various reasons (provide an optimum descent gradient, provide a 300'
buffer above the floor of controlled airspace, ensure NAVAID reception
in an otherwise poor reception area, give an even 100' increment
altitude, etc)

There's an example of what can happen when an aircraft descends below
the stepdown altitude on final (and also below the MDA) at this website.
http://www.avweb.com/news/airman/184931-1.html
This particular aircraft had reported the runway in sight prior to the
stepdown fix and about a minute before he impacted rising terrain on final.


JPH

Ron Rosenfeld wrote:
On Wed, 05 Jan 2005 16:05:08 -0600, J Haggerty
wrote:


The minimum altitude prior to reaching the stepdown fix is also the MDA
unless and until the stepdown fix is received.


You must be looking at a different P/C glossary than I am:

MINIMUM DESCENT ALTITUDE- The lowest altitude, expressed in feet above mean
sea level, to which descent is authorized on final approach or during
circle-to-land maneuvering in execution of a standard instrument approach
procedure where no electronic glideslope is provided.


Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)