Minimum Safe Altitude (MSA) Standards

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.

This is some good info about MSA that I did not realize. But I am
still a bit confused. The AIM says that MSA provides 1000 ft clearance
over all obstacles, implying that it satisfies the minimum IFR
altitudes (in non-mountainous areas). Is this statement incorrect?
Also, if the MSA is not accurate for IFR, how much extra work is it
anyway to produce a chart with an MSA that conforms to IFR standards?
The numbers are already available from the enroute charts, so it can't
be that difficult, right?



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.

Andrew Sarangan wrote:

This is some good info about MSA that I did not realize. But I am
still a bit confused. The AIM says that MSA provides 1000 ft clearance
over all obstacles, implying that it satisfies the minimum IFR
altitudes (in non-mountainous areas). Is this statement incorrect?
Also, if the MSA is not accurate for IFR, how much extra work is it
anyway to produce a chart with an MSA that conforms to IFR standards?
The numbers are already available from the enroute charts, so it can't
be that difficult, right?


The AIM parrots the cartographic standard set forth for MSAs. What I explained is the
reality of how they are constructed. They are intended for emergency use, and with
today's technology their value for that use is questionable.

In any case, the altitude you really want is the center's minimum instrument altitude
sectors on a moving map. Or, approach control's MVA sectors on a moving map when in a
TRACON's airspace. Those are coming, because the FAA recently relented and agreed to
release them (well, the MVAs anyway). It will be a while though.