descent below minimums

On Tue, 04 Jan 2005 23:09:47 GMT, wrote:

Since the "MDA rule" only refers to MDA, and the MDA only exists on
the final segment, I think that if you were operating below an
altitude specified for some other segment, and had some kind of
problem as a result, a case could be made against you if there were
an enforcement action of some kind.

Assuming that statement is true, PATER is within the final segment, so the
MDA rule would still apply.


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

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

JPH

Ron Rosenfeld wrote:
On Tue, 04 Jan 2005 23:09:47 GMT, wrote:


Since the "MDA rule" only refers to MDA, and the MDA only exists on
the final segment, I think that if you were operating below an
altitude specified for some other segment, and had some kind of
problem as a result, a case could be made against you if there were
an enforcement action of some kind.


Assuming that statement is true, PATER is within the final segment, so the
MDA rule would still apply.


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

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)

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)

On Thu, 06 Jan 2005 21:42:04 -0600, J Haggerty
wrote:

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.

There seems to be a disconnect between TERPs, training, AIM, etc. Not that
that is unusual.

Your explanation is clear.

However, as I mentioned in a later post, it reinforces the idea that it is
legal to descend below a SDF that occurs after the FAF, provided the
requirements of 91.175 are met.

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.

First of all what is safe is not necessarily legal.

In the particular instance you cite, according to the report, the crew
flying was not following procedures that had been set up for the safe
operation of this aircraft. And, from the recordings, it does not appear
as if they really met all of the requirements of 91.175 before they
descended below the MDA.

Taking shortcuts (not following proper procedures) at an airport like Aspen
is fraught with hazard, moreso than at flatland airports.


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

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.

I should add that on the approach in question, there is no published
alternate MDA to be used if the last stepdown fix is not identified.


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

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.

It'd be nice if I could get all my thoughts together for one message. And
it'd also be nice if I did not always respond as if a positive comment were
argumentative :-(

Your statement is supportive of the idea that it is OK to descend below the
stepdown fix, provided the requirements of 91.175 for operating below the
MDA are met.


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