Minimum Safe Altitude (MSA) Standards

The context of minimum sector altitudes is important...they are to be used
only in emergencies. I see many postings in which a pilot is trying to fit
an MSA into an approach procedure.

Bob Gardner

"O. Sami Saydjari" wrote in message
...
MSA is defined in the AIM as "altitudes depicted on approach charts
which provide at least 1,000 feet of obstacle clearance." So, if an MSA
is 3000 ft, does that necesarily mean that there is at least one
obstacle in the area that is 2000 ft tall or could there be some other
reason for the 3000 ft setting? If there are only one or two towers in
the north part of the MSA circle and the rest of the area is completely
flat at 1000 ft (MSL), then would they always break the sector into
pieces are create a sector at 2000 ft, and just put the northern half at
3000 ft, or is that too much trouble in general?

While I am at it, is there any easy way to find the obstacle in a quad
of VFR sectional that makes the quad's Maximum Elevation Feature (MEF)
at the level that is at. It is a bit of a pain to search the quad's
entire area to find that one tower that makes the MEF way above the
surrounding terrain. It seems that they could mark the highest feature
in some distinctive way. OK, so maybe I am lazy.


-Sami (N2057M, Piper Turbo Arrow III)

I realize that that is what the AIM says, but why are they to be used
for emergencies only? I always interpreted the MSA as the minimum
altitude to use when flying off-feeder routes (direct to the IAF),
sort of like the OROCA when flying off-airways.





"Bob Gardner" wrote in message news:EJJ2c.135940$4o.172500@attbi_s52...
The context of minimum sector altitudes is important...they are to be used
only in emergencies. I see many postings in which a pilot is trying to fit
an MSA into an approach procedure.

Bob Gardner

Andrew Sarangan wrote:

I realize that that is what the AIM says, but why are they to be used
for emergencies only? I always interpreted the MSA as the minimum
altitude to use when flying off-feeder routes (direct to the IAF),
sort of like the OROCA when flying off-airways.


In many countries MSAs are operational altitudes. They are not in the United States. And, in a
designated mountainous area you are required by 91.177 to have 2,000 feet vertically or 4 miles
laterally when off a published route or segment. MSAs are not IFR altitudes, routes, or segments.

wrote in message ...
Andrew Sarangan wrote:

I realize that that is what the AIM says, but why are they to be used
for emergencies only? I always interpreted the MSA as the minimum
altitude to use when flying off-feeder routes (direct to the IAF),
sort of like the OROCA when flying off-airways.


In many countries MSAs are operational altitudes. They are not in the United States. And, in a
designated mountainous area you are required by 91.177 to have 2,000 feet vertically or 4 miles
laterally when off a published route or segment. MSAs are not IFR altitudes, routes, or segments.

I agree that MSA does not satisfy the altitudes in mountainous areas.
But in nonmountainous areas MSA does satisfy the altitude
requirements. What am I missing here?

Andrew Sarangan wrote:



I agree that MSA does not satisfy the altitudes in mountainous areas.
But in nonmountainous areas MSA does satisfy the altitude
requirements. What am I missing here?

That they are not published as IFR altitudes under either of the pertinent IFR alitude regulations, Part
95 or Part 97.

wrote in message ...
Andrew Sarangan wrote:

I agree that MSA does not satisfy the altitudes in mountainous areas.
But in nonmountainous areas MSA does satisfy the altitude
requirements. What am I missing here?

That they are not published as IFR altitudes under either of the pertinent
IFR alitude regulations, Part 95 or Part 97.

True, but under the circumstances Andrew described (flying off-route direct
to an IAF), Parts 95 and 97 do not prescribe a minimum altitude. Hence,
91.177a2 applies. In a non-mountainous area, 91.177a2ii specifies a
condition that can be assured by reference to the MSA.

--Gary

Gary Drescher wrote:

wrote in message ...
Andrew Sarangan wrote:

I agree that MSA does not satisfy the altitudes in mountainous areas.
But in nonmountainous areas MSA does satisfy the altitude
requirements. What am I missing here?

That they are not published as IFR altitudes under either of the pertinent
IFR alitude regulations, Part 95 or Part 97.

True, but under the circumstances Andrew described (flying off-route direct
to an IAF), Parts 95 and 97 do not prescribe a minimum altitude. Hence,
91.177a2 applies. In a non-mountainous area, 91.177a2ii specifies a
condition that can be assured by reference to the MSA.

--Gary

Your responsibilities under the off-route provisions of 91.177 you site are
absolute. OTOH, the design of MSAs by the FAA are done with sectional charts
and are not assessed with the precision that are accorded IFR altitudes.

MSA's do not ensure NAVAID or radio reception, and may even contain
sectors that are known areas of no signal coverage. They just ensure 951
feet of obstacle clearance within the specified distance from the NAVAID.

JPH

Andrew Sarangan wrote:
wrote in message ...

Andrew Sarangan wrote:


I realize that that is what the AIM says, but why are they to be used
for emergencies only? I always interpreted the MSA as the minimum
altitude to use when flying off-feeder routes (direct to the IAF),
sort of like the OROCA when flying off-airways.


In many countries MSAs are operational altitudes. They are not in the United States. And, in a
designated mountainous area you are required by 91.177 to have 2,000 feet vertically or 4 miles
laterally when off a published route or segment. MSAs are not IFR altitudes, routes, or segments.


I agree that MSA does not satisfy the altitudes in mountainous areas.
But in nonmountainous areas MSA does satisfy the altitude
requirements. What am I missing here?

J Haggerty wrote:

MSA's do not ensure NAVAID or radio reception, and may even contain
sectors that are known areas of no signal coverage. They just ensure 951
feet of obstacle clearance within the specified distance from the NAVAID.

True, except 951 is no longer permitted per AFS-420. No more rounding down on TERPs ROC.

I defer to sammy, who says what I would have said.

Bob

"Andrew Sarangan" wrote in message
om...
I realize that that is what the AIM says, but why are they to be used
for emergencies only? I always interpreted the MSA as the minimum
altitude to use when flying off-feeder routes (direct to the IAF),
sort of like the OROCA when flying off-airways.





"Bob Gardner" wrote in message
news:EJJ2c.135940$4o.172500@attbi_s52...
The context of minimum sector altitudes is important...they are to be
used
only in emergencies. I see many postings in which a pilot is trying to
fit
an MSA into an approach procedure.

Bob Gardner