MSA or SSA?

[Buffy Summers]

What is the difference between SSA and MSA?
There are two different names so there must be two separate definitions, but I am unclear as to what the practical difference is.

[exeng]

MSA is the minimum altitude en-route. SSA is the minimum altitude on an instrument approach chart, it is published for each quadrant within 25nm of the aerodrome reference point.


Regards
Exeng

[ManaAdaSystem]

Wrong!
MSA=Minimum Sector Altitude. Not an enroute altitude.
What is SSA? Safe Sector Altitude? New to me.

[Sick Squid]

Buffy,

MSA is Minimum Safe Altitude, and gives you the company/state approved vertical clearance over all terrain and obstacles in the specified area. If the area is between two en-route points, then a bandwidth 20nm either side of track is applied.

SSA is Sector Safe Altitude, applicable as Exeng says to a quadrant of an instrument approach chart, and gives company/state approved clearance as above within that quadrant. The range for the quadrant is usually 25nm, with a 5nm buffer applied both outside, and into adjacent quadrants.

Approved vertical clearances are based on the elevation of the obstacle, and are of the order 1000ft for obstacles up to and including 5000ft, and 2000ft above that.

Some companies advise increasing the MSA by an arbitrary amount if the wind at the MSA is above a certain strength (increase MSA by 2000 feet if 50kts or greater at the MSA) or the atmosphere significantly colder than ISA (4% of height per 10 degree C below ISA comes to mind, but I can't get at the books right now to confirm.)

Stay safe!

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Disappears in a cloud of black ink... should use some of it to spell correctly in the first place, then wouldn't need to edit!

[This message has been edited by Sick Squid (edited 17 November 2000).]

[askop]

Have not the MSA also a 5 nm buffer?

[Vmu]

Maybe we're using different documentation. In my company MSA is Minimum Sector/Safe Altitude. It gives you a safe altitude within 25nm of an approach navaid.

For enroute use, we have other official and/or company calculated min altitudes.

I am not familiar with SSA. Is it a "Jeppesen-altitude"?

BTW: All minimum altitudes should be corrected for low temperatures (and wind if in mountainous terrain), regardless of company policy. When it's cold you are lower than indicated altitude. If you forget this, you might get some unpleasant surprises.

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"Recovery was marginal..."

[Buffy Summers]

OK, so technically they are based on the same criteria, but apply to different areas. But ultimately they are the same thing surely? The MSA for the approach area should be the same as the SSA shouldn't it?
So why have 2 different terms?
I understand that we have the 2 terms, I am really just asking why, what is different about them?

[Royan]

When the the figure is the same from the specified navaid (25 miles radius) it is called min safe alt. and when the figure vary coming from different direction it is then min sector alt

[exeng]

This is taken more or less directly from our ops manual.

MSAs are shown as a screened overprint in relation to the Latitude/Longitude graticule as
follows:
a) LOW and LOW/HIGH charts -I deg Lat x I deg Long.
b) NAT charts below 56 deg N - I deg Lat x I deg Long.
c) NAT charts above 56 deg N - I deg Lat x 2 deg Long.
d) HIGH charts -2 deg Lat x 2 deg Long.
e) POLAR CHART -5 deg Lat x 5 deg Long

Aerodrome Related Charts
MSAs:
On applicable charts, e.g. Instrument Approach Charts, Arrivals Charts, Area Charts and Terrain Arrivals/Departures charts, MSAs are shown by means of Contour Envelopes.
Contour Envelope MSA figures apply to the whole of the envelope area.
The lowest (background) MSA value shown on the chart is derived by taking the
aerodrome elevation rounded up to the next 100 ft, adding 500ft and then adding the
appropriate clearance. (Up to and including 5000ft clearance of 1000ft, above 5000ft clearance of 2000ft.)

SSAs
SSAs are quoted for each of the four quadrants, centred on the Aerodrome Reference
Point and aligned on the true cardinal points N, S, E and W. Each figure is quoted as "SSA 25nm" but a 5nm margin is added to this radius. The SSA for each quadrant also
takes account of a 5nm overlap between quadrants.
The SSA is derived by taking the elevation of the highest obstacle in the quadrant
rounded up to the next lOOft, then adding the appropriate clearance. (Up to and including 5000ft clearance of 1000ft, above 5000ft clearance of 2000ft.)
NOTE: On the revised (2000 specification) Racal Aerodrome Charts quadrant SSA's are
discontinued and MSA's are specified based upon the airfield reference point (UK
standard), or the relevant radio aid as identified on the plate (Non UK) and may be split into segments.


Hope this is of help to you.


Regards
Exeng

[This message has been edited by exeng (edited 17 November 2000).]

[ManaAdaSystem]

Sorry Exeng, i should of course have realised that documentation and procedures vary from one airline/area to another.
Your SSA is our MSA (Minimum Sector Altitude). Your MSA is our MEA (Minimum Enroute Altitude), and so on.
Have a nice weekend!