Minimum Safe Altitude (MSA) Standards

[O. Sami Saydjari]

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)

[Roy Smith]

In article >,
"O. Sami Saydjari" > wrote:

> 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?

There are rules on how many different sectors you can have, how big they
must be, etc. I don't remember the details, but you should be able to
find them in TERPS.

> 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.

I don't know of any way other than exhaustive search.

[Teacherjh]

>>
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?
<<

"at least" means "no less than, but maybe more than, though maybe not". So, it
does not necessarily mean that there is at least one obstacle 2000 ft tall. I
don't know the actual criteria for designing these things, but wherever there
is wiggle room, expect something to wiggle and you'll be safe.

>>
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
<<

Nope. And even if you found that one tower, you are still left with that other
tower that is two feet lower, but in a different area. And the hill that
doesn't have a tower, but it tall enough by itself to qualify as the third
highest elevation (by only fourteen feet), so only has a dot.

Besides, not all towers are indicated on the chart. They say so explicitly.

You'll need to study the sectional anyway to check for parachute areas, MOAs,
ATAs, landmarks, wires, frequencies, and all sorts of other things relevant to
VFR flight and even IFR flight for that matter, especially if you are going to
graze the trees, as I like to (for the view).

Jose



--
(for Email, make the obvious changes in my address)

[O. Sami Saydjari]

>
> There are rules on how many different sectors you can have, how big they
> must be, etc. I don't remember the details, but you should be able to
> find them in TERPS.
>

I am sorry to demonstrate my ignorance, but what are the "TERPS" and
where might I find them? I did notice that the AIM says that sectors
have to be no smaller than 90 degrees, so there can be no more than
four. It does not say the criteria for deciding when and whether to
subdivide the MSA circle.

-Sami

[Bob Noel]

In article >,
wrote:

> I am sorry to demonstrate my ignorance, but what are the "TERPS"

The United States Standards for Terminal Instrument Procedures.
iow, the FAA rules for creating airways, routes, and approaches.

> and where might I find them?

I don't have a link handy, but you can search www.faa.gov. Note
that you can't find all of the FAA regs/orders for the TERPS in one
document or one place, the main document likely be all that you'd
want to see.

--
Bob Noel

[Julian Scarfe]

"Bob Noel" > wrote in message
...

> > ... what are the "TERPS"

> I don't have a link handy, but you can search www.faa.gov. Note
> that you can't find all of the FAA regs/orders for the TERPS in one
> document or one place, the main document likely be all that you'd
> want to see.

http://av-info.faa.gov/terps/directives%20page.htm

now has virtually everything, I think. Beware file size though, 8260.3B is
about 30 MB.

Julian Scarfe

[Roy Smith]

In article >,
"O. Sami Saydjari" > wrote:

> >
> > There are rules on how many different sectors you can have, how big they
> > must be, etc. I don't remember the details, but you should be able to
> > find them in TERPS.
> >
>
> I am sorry to demonstrate my ignorance, but what are the "TERPS" and
> where might I find them? I did notice that the AIM says that sectors
> have to be no smaller than 90 degrees, so there can be no more than
> four. It does not say the criteria for deciding when and whether to
> subdivide the MSA circle.
>
> -Sami
>

TERPS is "United States for Terminal Instrument Procedures". You can
find a copy here....

http://av-info.faa.gov/terps/directives%20page.htm

It describes in gruesome detail what goes into constructing IFR
procedures such as instrument approaches.

[Bob Gardner]

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)
>

[Andrew Sarangan]

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
>
>

[[email protected]]

"O. Sami Saydjari" wrote:

>
> I am sorry to demonstrate my ignorance, but what are the "TERPS" and
> where might I find them? I did notice that the AIM says that sectors
> have to be no smaller than 90 degrees, so there can be no more than
> four. It does not say the criteria for deciding when and whether to
> subdivide the MSA circle.

In the case of RNAV approaches, there is only one MSA. It is not very useful
information, nor has it ever been.

[[email protected]]

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.

[Bob Gardner]

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
> >
> >

[Andrew Sarangan]

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?

[[email protected]]

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.

[Gary Drescher]

> 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

[[email protected]]

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.

[Gary Drescher]

> 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

[[email protected]]

>
>
> > 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.

[Gary Drescher]

> 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

[[email protected]]

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.

[Andrew Sarangan]

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.

[[email protected]]

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.

[[email protected]]

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.

[J Haggerty]

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?