Minimum Safe Altitude (MSA) Standards

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 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

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.

wrote in message ...
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

Which is typically how I'd ascertain an off-route minimum altitude in accord
with 91.177a2.

and are not assessed with the precision that are accorded IFR altitudes.

I'm not sure what you mean by "precision" here. The parameters are clearly
specified: 1000' above any obstacle in the designated sector. (The MSA
doesn't assure navaid reception, though, so that has to be assessed
separately.)

--Gary

and are not assessed with the precision that are accorded IFR altitudes.

I'm not sure what you mean by "precision" here. The parameters are clearly
specified: 1000' above any obstacle in the designated sector. (The MSA
doesn't assure navaid reception, though, so that has to be assessed
separately.)

The folks who design approach procedures at the FAA use very precise
topographical information to design the published segments of an instrument
approach procedure. For MSAs, though, they simply use sectionals, which may not
provide the required obstacle clearance at all times, simply because sectionals
do not have the accuracy that USGS 1:24,000 topos have.

Plus, when the FAA assesses the published segments they add 200 feet of assumed
adverse obstacle ("AAO") pad, because folks can construct towers, etc, up to 200
feet high without notifying the FAA, unless the towers are within certain
distances of an airport. There is no AAO assessment made for MSAs, though.
Also, spot elevations on sectionals can be off by a fair abount, without
adversly affecting their stated purpose; i.e., VFR navigation charts. And,
contours on sectionals are very coarse, although that isn't usually an issue in
non-mountainous areas.

wrote in message ...
and are not assessed with the precision that are accorded IFR
altitudes.

I'm not sure what you mean by "precision" here. The parameters are
clearly
specified: 1000' above any obstacle in the designated sector. (The MSA
doesn't assure navaid reception, though, so that has to be assessed
separately.)

The folks who design approach procedures at the FAA use very precise
topographical information to design the published segments of an
instrument
approach procedure. For MSAs, though, they simply use sectionals, which
may not
provide the required obstacle clearance at all times, simply because
sectionals
do not have the accuracy that USGS 1:24,000 topos have.

Plus, when the FAA assesses the published segments they add 200 feet of
assumed
adverse obstacle ("AAO") pad, because folks can construct towers, etc, up
to 200
feet high without notifying the FAA, unless the towers are within certain
distances of an airport. There is no AAO assessment made for MSAs,
though.
Also, spot elevations on sectionals can be off by a fair abount, without
adversly affecting their stated purpose; i.e., VFR navigation charts.
And,
contours on sectionals are very coarse, although that isn't usually an
issue in
non-mountainous areas.

Hm, so you're saying that the MSA doesn't necessarily provide the obstacle
clearance that it's advertised to provide. I hadn't considered that
possibility. Are you familiar with any example of an obstacle that's less
than 1000' below a current MSA?

Thanks,
Gary

Gary Drescher wrote:


Hm, so you're saying that the MSA doesn't necessarily provide the obstacle
clearance that it's advertised to provide. I hadn't considered that
possibility. Are you familiar with any example of an obstacle that's less
than 1000' below a current MSA?

Thanks,
Gary

Well, it's an emperical conclusion based on the certain knowledge that some
TRACON MVA charts have been found to have less than required obstacle
clearance, because sectional charts were used in their construction.

The difference there is that ATC assigns you the MVA, so that is the FAA's
"fault," not your's. But, if you elect to use an MSA as an IFR altitude, that
is strictly your deal. Are you going to be at risk because of this? No, I
don't think so. The point I am trying to make is that MSAs are almost an
afterthought to the procedures designer. A lot of folks in the charting
business would like to do away with them entirely. That has pretty much
happened with RNAV MSAs that do not have sectors.