View Full Version : Difference between MSA and SSA
pre3l2s
17th Apr 2011, 18:58
Hi all,
Can anyone please enlighten me as to what the difference between MSA and SSA?
I cant find anything anywhere on SSA in either EU-OPs or PAN OPs.
When briefing I've always given the MSA, but apparently this is wrong, and the 25nm clearance from a fix is actually called the Sector Safe Altitude.
Thanks in advance.
seilfly
17th Apr 2011, 20:11
No; you are right. The 25nm* of 1000-ft-clearence above obstacles is called the MSA - Minimum Sector Altitude (ref PANS-OPS Vol II, Chapter 8 MSA).
*) Obstacles within a buffer zone of 9 km (5 NM) around the boundaries of any given sector shall be considered as well. (Verbatum from the above ref.)
What have you been told?
During my pilot training (in the US) I was told... "If there is one sector, it is called Minimum Safe Altitude. If there are more than one sector, it is called min sector altitude". Never heared about SSA (or is that a British thing? ;) )
Just to complicate your life further, 'MSA' also means 'Minimum Safe Altitude' and of course a SSA is an MSA - simples:)
FlightPathOBN
19th Apr 2011, 19:26
MSA can mean "Min Safe Alt" or "Min Sector Alt" and unfortunately, dont mean the same thing.
The Min Safe Alt version relates to the 25nm circle, with ROC additives 1000/2000 depending.
http://operationsbasednavigation.com/wordpress/wp-content/uploads/2011/04/MSA3.jpg
Min Sector Alt is usually a subset of the Min Safe Altitude. These are frequently noted on the charts with the individual sectors/arcs noting different minimums per sector, rather than the overall Min Safe Altitude...which may be far too restrictive for operations.
In regards to SSA, I have not heard of that...but well....
Pre312S,
Why do you (or does anybody) brief the MSA?
because msa could be considerable higher than ssa
What would you do with that information?
mustafagander
22nd Apr 2011, 04:40
Use it to avoid ramming the scenery.
I can't imaging a sensible, useful approach briefing which neglected sector safe altitudes. Piece of wee-wee at a coastal airport with no nearby hills, but rather different at some mountainous airport surrounds.
Huh? Why? Don't you almost always descend below the MSA while on radar vectors? Min vectoring altitude(MVA) is, after all, usually lower than MSA. Do you refuse a vector below MSA? If so, good luck ever getting vectored below an overcast layer and then being cleared for a visual approah. You'll be costing your company a lot of fuel...
So again. Why are you briefing MSA?
As far as I can tell, it is only useful in cases of lost comm while on a vector. That's about a 1/1,000,000 type thing. And when it happens, there's no way in hell you will have remembered from the brief 20 minutes ago that the NE quadrant MSA was 5200'.
There may be a good reason for briefing it. I just haven't heard it yet.
Reasons to brief msa....CFIT, airmanship, self preservation etc etc
Reasons not to...it's an unlikely event (jriv's words not mine). Oh well I'll stop briefing emergencies then using that logic - not!
mustafagander
22nd Apr 2011, 09:45
You've at least heard of situational awareness I hope. Well this is part of it knowing where the lumpy bits are in relation to your planned approach path.
Cows getting bigger
22nd Apr 2011, 10:29
The whole MSA thing is a bit of a dogs dinner. Depending on where you look, there are numerous different 'definitions' and acronyms. Colloquially, pilots, companies and regulators use the term for safety altitudes that have nothing to do with the 25nm (PANS-Ops described) approach scenario.
Firstly, the FAA (FAR 91.119) describes 'minimum safe altitude' which loosely correlates to the UK Rule 5/6 (ie not scaring the public with low flying).
Folklore also has a part to play and we have people banding around the term MSA with regards to en-route (ie not the approach or departure phase) of flight. This argument is fuelled by companies such as Garmin shoving a software derived MSA on their GPS; in their case the figure appears to be derived from an MEF grid with an additional 10nm buffer. Why you would want such a broadly calculated altitude depicted on a piece of equipment that tells you exactly where you are, within a matter of feet, is beyond me :). Alternatively, third party navigation software providers have a different interpretation of MSA. For example, SkyDemon (a VFR flight planning application) indicates:
The MSA for a leg is calculated by looking at the terrain and obstructions 4nm either side of the leg, with 1000ft clearance for obstructions and 800ft clearance for terrain.
When challenged as to how they have decided upon these criteria their response is that they have responded to customer feedback.
Equally, some counties (UK for example) have IFR that talk about safety altitude (Rule 33) which is the 1000ft/5nm scenario.
I suppose much of the argument comes down to whether a pilot knows where he is, or isn't. Personally, I shiver at the loose use of terminology which, in reality, is rather important. To me, MSA is to do with the approach phase of flight which can be refined to SSAs and, if available, MVAs (RMACs) .
Confused? :rolleyes:
I don't think jriv is serious!? Also he/she demonstrates a short memory span (20 minutes?) for important stuff and, as mustafagander and BBK say, appears not to be concerned about SA!
I'm actually quite serious.
Are you really refusing a lower altitude while on a vector?
If I lose comm while on a vector, the MSA is the first thing I will check. There is no way I am just going to start climbing without checking the chart (a 2 second procedure), as some MSAs are broken into several sectors, all with different altitudes - a very easy thing to mix up.
As for SA, I agree it is important to know where the high terrain is, but I have never heard anybody refuse a lower altitude because it was below MSA.
I understand what you guys are saying, and if the other pilot I am flying with wants to brief it, so be it. I really don't care. I'm just trying to understand what I'm missing.
FlightPathOBN
22nd Apr 2011, 15:39
Software derived MSA is not a good idea. There are additives for mountainousness terrain, precipitous terrain, and other issues to take into account.
Some regulators adjust the MSA with airspace separation in mind as well.
With the MVA, you are on approach clearances, and they could be below the MSA as now you are dealing with different ROC parameters and separation issues. This also allows for ATC do give you a direct with a go-around, rather than going back to the STAR
Are you really refusing a lower altitude while on a vector - hang on! Your question was why do we 'BRIEF' the MSA. Not would we refuse a radar clearance below it. No-one will refuse a 'lower altitude while on a vector', unless, of course, they consider it dangerous and it is, after all, literally in the Captain's lap if ATC get it wrong. That is one of the reasons why I and all the rest brief it. That is why when given a vector BELOW MSA I ensure that I and the rest of my crew are happy with it - that we are radar identified.
Also, by briefing it (within memory span...) if comms are lost I do not need to grab for a map to establish MSA nor during a GPS pull up do I need to do that. Are you telling me that in a GPS pull up' you and/or the other crew members are going to be grabbing a map?
The MSA is on the approach chart which is sitting right in front of me. There is no digging required.
As for a GPWS pullup: that has nothing to do with MSA. Why on earth would anybody look at a chart when the system is telling you to "pull up or die?"
I'm not trying to upset anybody here. No need for the silly hypotheticals.
Avenger
23rd Apr 2011, 12:36
JRIV,
For what it's worth, we brief MSA why:
1) Situational awareness, in case of Emergency descent, or Engine Out Cruise or Loss of ATC when under vectors, the list goes on, comes under the general heading " Airmanship"
2) Ops manual specify max rate of descent within xx feet of MSA, for instance 3000 fpm within 3000 feet of the MSA, 2000, within 2000, 1000 within 1000 etc.
3) If we descend in VNAV, which is normal, with a tailwind or late descent cleared we may well be over the allowed ROD for the sector and rsik the hard warnings.
4) Even under radar vectors PM must state " below MSA" we have to know this for many reasons, windshear go-arounds, RA,s etc navigation with loss of FMS etc, ( this is not a clue for the raw data brigade to get on high horses)
In many parts of the world where there is a non-radar environment, Turkey for instance, you will not be cleared to a level below MSA until XXX DME and the controller know where you say you are. In this position the descent planning in the geometric path VNAV can be useless and again, " airmanship" comes into play.
Fair enough. Thanks for the thoughtful reply.
I guess flying around the US spoils me.
I guess flying around the US spoils me. - let's hope it doesn't 'spoil' your passengers and crew.
I know. It's a miracle I've survived this long.
From now on I'll be like you. Every time I descend below MSA I'll check with the whole crew.
SonarKing
24th Apr 2011, 23:13
The Recorders will not be difficult to find at this point. Most are found and only a few haven't been. A couple of the missing ones are: one of the A/C that flew into the world trade center and a Chilean airliner that ran out of fuel (positioning flight) off the coast of Newfoundland. In both cases the causes of the accidents were well known....no need for recorders.
I suspect the large sonar target out to the side of the debris field in the sonar data is the empennage, which structure is often largely intact in over-water-accidents. Also the French now have data from the site survey that strongly indicates the A/C impacted the sea surface with very little horizontal velocity.
Just my $00.02
Starbear
25th Apr 2011, 19:19
jriv
Having flown a bit in the USA I know of what you speak. I never met one local pilot who ever mentioned terrain clearance in any brief, just obeyed ATC all the way.
A simple question if you don't mind:
IF, as a result of srcew up (either by crew or even ATC with radar) you do receive a genuine GPWS "Pull UP", at what point in the subsequent recovery will you level off?
Climb until the warning stops and clear of terrain. I know what you're getting at, but I'm certainly not going to level off at the MSA I think is valid for the quadrant I think I'm in when my heart is pounding and trying to remember the MSA from the briefing 20 minutes ago. I'll simply look at the chart right in front of me as I'm climbing.
Look, I'm not suggesting that I fly around with no idea where the terrain is. My point is that as it is briefed, it is simply thrown out there. Then when we are cleared lower and lower as we're vectored onto final, every single pilot I have ever flown with, Captain or FO, always accepts and descends below MSA. A conscientious pilot would ask what the MVA is, but I've simply NEVER heard that request. Not a single time. I have asked for it once, as I was given a vector to intercept final 100' BELOW the charted altitude for the segment I was about to intercept. But that's it.
I'd love to hear from somebody who actually refused a lower altitude because it was below MSA and that action saved them.
Rubber Dog
26th Apr 2011, 07:51
Good stuff there from BOAC and Avenger and a good prod from Starbear. We always brief MSA et al. You must always be aware of your terrain clearance.
You should also not rely on the fact that you are under radar vectors. This discussion came up in the sim just last week and the trainer had an experience where vectoring would have resulted in the a unfortunate outcome. He was aware of the terrain and was able to do something about it.
jriv, yes you do have the GPWS to aid you should that event occur and yes you can glance down at your chart but I would rather know this info before an occurrence. I would not like to be glancing and wondering in the heat of the moment. You don't have to remember it from 20 minutes ago. You can update your self every few moments. We also update each other throughout the whole approach with altitude and vector changes unless blindingly not necessary.
Safe flying everyone
Climb until the warning stops - hmm! That's useful! Normally that happens the moment you pull up so forget that one. I don't think I would contemplate 'stopping' then - nor rely on looking at my chart in the pull-up, but then I can remember important things for MORE than 20 minutes.
MSA is the only one at which you would consider easing off the pull up IMC/night.
EMB170
26th Apr 2011, 09:24
jriv :
do you brief the approach minimum ?
i guess not, as it is on the chart in front of you !
i spend a lot of time in India , where they have no radar on most regional airports and descent clearance is as previously mentioned based on radial and DME distance.
for T/O, very often you get a clearance to your desired flight level , visibility is poor most of the time so if you have , let's say an engine failure at 500' , which altitude would you climb to ? i stop at MSA rather than let's say FL300, if you ever get there with one engine .
as part of the line up check, we have a review of the T/O briefing, ie the MSA, or what ever has changed since the clearance and briefing was given.
Fellow Aviators,
I didn't bring up GPWS. BOAC did. I certainly don't use that as my "safety net." I also don't blindly trust ATC to give me a safe altitude to descend to.
MSA covers an area almost 2000 square miles, or 500 square miles per quadrant. So I'm not neccesarily going to climb to MSA if I get a GPWS warning. MSA is, after all, 1000' above the highest terrain in that 500 square mile quadrant. ATC could very well have traffic at or below MSA because they have an MVA that is lower right where you are. That's why I said "Climb until the warning stops AND YOU ARE CLEAR OF TERRAIN." That's the part that BOAC deliberately left out to try to make me look foolish. If it's night/IMC and the frequency is saturated, of course I would climb to MSA. But let's be realistic, we aren't going to remember that from 20 minutes prior and we aren't going to be rebriefing every few minutes. I have simply NEVER seen that happen.
Do I brief the approach? Of course! I don't brief the MSA because in my experience it is NEVER used. Now if I fly into an unfamiliar airport with high terrain at night/IMC, I will, but other than that I think it simply clutters the briefing and is thrown out there and instantly forgotten because it is NEVER used.
FlightPathOBN
26th Apr 2011, 15:27
note on all of the failures and warnings...wouldnt you just climb to the hold or follow the instructions for communications loss?
I am aware that with the procedures in AUS, the MVA chart looks like a mottled pattern.
With the RNP approaches, multiple transitions to a common final, there are many entry points. These waypoints will each have a MVA, which will be to join the procedure, irrespective of the MSA.
http://operationsbasednavigation.com/wordpress/wp-content/uploads/2011/04/RNPtransitions.jpg
AND YOU ARE CLEAR OF TERRAIN." That's the part that BOAC deliberately left out to try to make me look foolish. - not left out to make you look 'foolish' - left out because you would not know when that is!
Two further points to ponder:
1) 'MSA' is NOT a '2000sq mile area' - it is the RELEVANT MSA for where you are. It can cover quite a small area, actually, for those of us who know where we are.
2) Since 20 minutes appears to be your limit for retaining safety information, do you also have to 'look down' at the paperwork when an engine goes bang 21 minutes after you briefed/were briefed on the EOSID?
FlightPathOBN
27th Apr 2011, 01:40
you briefed/were briefed on the EOSID
after reading multiple posts on multiple threads...
I am wary on how many operators have any clue with regards to EO procedures...
BOAC,
This whole thread is about the original poster's question about the MSA on the approach chart. in the US that is the only definition of MSA. I don't know where to find the definition you're talking about.
And yes, I will absolutely look to verify the EOSID or have the FO call it out to me. When the one-in-million happens I don't know how I will react. Neither do you, unless you've been there. I'll use the tools I have instead of bravado.
- not left out to make you look 'foolish' - left out because you would not know when that is!
BOAC,
You again deliberately take my words out of context. This is becoming a pattern.
I specifically said that if I could not contact ATC I would, in fact, climb to MSA. However, automatically climbing to MSA may cause me to climb into another airplane flying around at MVA.
Finally, I never said 20 minutes was my limit for retaining information. I simply said that I don't think it is reasonable to assume that any person would retain a random number for 20 minutes. It would be far more effective to brief where the high terrain was than all four MSA sectors. As I am capable of using this computer to type this message, it should be clear to any reader that I am capable of retaining information for at least 20 minutes. I believe there is something called "long term memory", or something like that. Or maybe you heard of that and just "forgot?"
kick the tires
27th Apr 2011, 05:00
jriv - you need to back off a bit here.
You appear to be the only pilot who doesnt want to take 2 seconds and brief an MSA. For the rest of us it is an SA tool: 'MSA is 5300, mountains are to the west and sea is to the east' What a great way to have the big picture in your mind as to what to do if the sh!t hits the fan.
It doesnt take much.
Nobody has once mentioned refusing a decent below MSA, you are the one harping on about this - I presume you check you min radar vector altitude chart when you get such an instruction - having first realised you are below MSA from the altitude you havent previously checked!
Its all about holes in the cheese. You rely on GPWS if you want, personally, I prefer to use the safeguards that are available to me before I get to that stage.
Complacency about being spoiled by a familiar environment is NOT one of my tools!!!
I agree. I started out asking why guys brief it, looking to learn. BOAC got a little nasty and I probably dug in my heels. Your example of mentioning MSA is as it relates to the mountains/sea makes perfect sense. The guys who I fly with who throw it out there and never give it a second thought do not make sense to me.
But once again, I never brought up GPWS. I specifically said I would never use that as a safet net.
potkettleblack
27th Apr 2011, 13:24
BOAC will you be commencing your L-1011 course soon?
No - not too keen on final destination.