View Full Version : Lowest safe

24th Jun 2006, 11:31
One for the more learned.....!
Would anyone here have good references or a good simple explanation for the question...
In international ops, When are Mora /Moca(t) /LSALT / MSA /and other lowest safes applied to obtain terrain clearances.?

Thank you very much.


25th Jun 2006, 05:27
There's actually no single generic rule for "international ops". Each country specifies the appropriate use for "Mora /Moca(t) /LSALT / MSA /" etc. in their own AIP. This may, or may not, completely conform to ICAO specifications.

I'd have thought that it would be usual for companies to spell out the differences in their Route Guide data, so that pilots have all the relevant information immediately available to them.

The Real Slim Shady
25th Jun 2006, 13:18
Extract from the EAG Guide:

Minimum Obstacle Clearance Altitude (MOCA)
The MOCA calculation described below is acc to the KSS method which is one of the JAA approved methods.
MOCA is the sum of:
- the maximum terrain or obstacle elevation whichever is highest, plus
- 1000 FT for elevation up to and including 6000 FT, or
- 2000 FT elevation exceeding 6000 FT rounded up to the next 100 FT.
MOCA in hundreds of feet, is given in the RFC, SID and STAR.
The lowest MOCA to be indicated is 2000 FT (20).
MOCA is valid for a corridor of variable width as indicated below.
From a VOR station, the corridor width is defined as a borderline starting 5 NM either side of the
VOR, diverging 4 from centreline until a width of 20 NM is reached at 70 NM out, thence parallels
the centreline until 140 NM out, thence again diverges 4 until a MAX width of 40 NM is reached at
280 NM out. Thereafter the width remains constant.
From an NDB similarly the corridor width is defined as a border line starting 5 NM either side of the
NDB diverging 7 until a width of 20 NM is reached 40 NM out, thence parallels the centreline until
80 NM out, thence again diverges 7 until a MAX width of 60 NM is reached 245 NM out. Thereafter
the width remains constant.
Overlapping area is determined only at track change, viz.:
- if less than 30 adequate coverage is obtained by considering elevations within the extended
corridor borderlines
- if 30 or more the MOCA corridor is extended by a 10 NM circle from the waypoint.
Note: At waypoints with different track changes, the most conservative value applies.
Minimum Off-Route Altitude (MORA)
MORA is the sum of:
- the maximum terrain or obstacle elevation whichever is highest, plus
- 1000 FT for elevation up to and including 6000 FT, or
- 2000 FT for elevation exceeding 6000 FT rounded up to the next 100 FT.
MORA in hundreds of feet is given in the RFC.
The lowest MORA to be indicated is 2000 FT (20).
MORA is valid in an area bounded by every second LAT/LONG squares on the RFC and TAC.

26th Jun 2006, 11:56
A subject close to my heart, on which I have posted before...

TOPC, have the previous replies answered your question? Because I understood you to say "When" are they used, not what is the definition.
Do you mean, for example, MSA within 25nm of the A/D and beyond that...what? One of the others - if so, which? How to choose which of the available terrain clearance methods you will use?

I, too, would like to see the answer to that one.

26th Jun 2006, 12:04
:ok: Yes Keith that is exactly the question .We have obviously discovered the same confusion requiring definition.... ;)

26th Jun 2006, 21:53
Yes, the reason I'm interested is that, in UK anyway, we were all quite clear about Minimum Safe Altitude as defined in CAP 360. Now CAP 360 has been withdrawn, all we have is JAROPS Minimum Flight Altitude. MFA encompasses several more things than just terrain clearance, and merely offers a selection of methods for calculating terrain clearance. I know they are all very similar, and I know its probably up to operators to pick one and tell their pilots to use it.

But I'm still interested in a wider perspective on what others do.

Acknowledged this is of less interest to non-UK types.

27th Jun 2006, 13:06
MOCA, LSALT and MSA all have one thing in common. They all provide a minimum level of obstacle clearance of AT LEAST 1,000 FT. Well, in this half of the planet anyway. MORA also provides a minimum amount of obstacle clearance but also GUARANTEES navaid reception.

MORA hasn't been implemented in this dark, dank part of the southern hemisphere because, with obstacles up to 13,000 feet quite close to all our major aerodromes, navaid reception can be problematic at the standard range of 60 NM for a VHF navaid such as VOR. Thus, we depend heavily on LSALT (aka MOCA) and MSA in the normal course of operations. I'm aware that some countries refer to LSALT/MOCA and MSA as emergency altitudes but, for the most part, those countries can afford to be generous because they don't have the obstacle problems (and therefore the navaid range limitations) that we have to contend with.

Just thought I'd throw that in for the benefit of me mate keithl, as a non-UK perspective. :E

28th Jun 2006, 10:46
Cheers, OzEx, old chap.
Yes, the 1000ft clearance comes from PANSOPS (where its called Minimum Obstacle Clearance) and so you'd expect to see that worldwide - as you know, of course.
But nothing seems to tell us which distance off track to take in. I even wonder if an operator is free to make up his own formula (obviously with the agreement of his local Regulator). Its actually something we're considering doing as none of the methods offered in JAROPS quite suits us...

29th Jun 2006, 12:44
G'day keithl, I don't know why there seems to be such a problem with this because ICAO has issued standards for the dimensions of enroute protection area in Pans Ops. We've issued our standards for enroute dimensions in CAR Part 91 of our Civil Aviation Rules and have also issued advice in the form of Advisory Circulars.

This is because we understand that our published, formal route network may not suit everyone. Thus, we've provided fairly extensive guidance material that allows operators to use the same standards to work out LSALT for any route that isn't covered by our network.

This has always been the case here (i.e. a Third World Country), so I don't know why the UK has a problem with providing the same sort of information. The very little I know about the UK CAA suggests that they might wish to use their own standards, in preference to the recommendations provided by ICAO in Pans Ops. I don't have a problem with them doing that, but they really should publish the criteria so that everyone under their control is singing from the same hymn book page.

Have I missed something here? :confused:

29th Jun 2006, 13:36
No, you haven't missed anything, OzEx, the UK used to have a very clear sytem, which is the one I referred to, called MSA for Minimum Safe Altitude. The trouble is, the book that defined that has now been withdrawn and MSA now only has the meaning of Minimum Sector Altitude (i.e. the 25nm circle).

We want to use the old system, which suited our operations (low level, off airways) and I guess we still could, but there's no authoritative reference for it any more. I should say (because pPruners are so quick on the draw!) that there's no reference that I'm aware of. The new reference we have, in JAROPS, I've already described.

29th Jun 2006, 19:14
hello keithl & others, 29 06 2006

with reference to jeppesen approach charts:

msa= min safe altititude, if there is only one altitude published in the middle of the 25nm circle(always also check the location of this middle point).

msa= min sector altitude, if the 25nm circle is sectorized with arrows,(qdm's), pointing towards the middlepoint, with different altitudes in each sector, somewhat alike to the french camembertcheese(but without the smelling)( no offense to the french as i very much like their cheese).

mora(min off route altitude) is an invention of jeppesen & is only related to terrain clearance 10nm left/right of airway centerline: obstacle clearance is 1000ft if reference(obstacle) point is 5000ft or lower & 2000ft if ref point is 5001ft or greater. navaid/ radio reception is not taken into account.

and, please, i fully agree that jaa/jar ops/fcl etc. is far from perfect, but at least, we now have a more or less common standard to operate in. with the massive increase in traffic over the last decades, it would be a nightmare if every other state would still go its own way.

29th Jun 2006, 22:56

I didn't think the terrain had anything to do with lack of Navaid reception in your part of the woods, isn't it more that none of them work (as evidenced by 3 pages of Notams on Moresby):)

30th Jun 2006, 08:52
Sounds like a serious problem, keithl, unless of course you're using a route where there are airports, with declared MSAs, every 25 miles. I guess that I can understand the UK CAA withdrawing the original reference source (eg if there was an error or two in it, or maybe it didn't comply with ICAO specifications), but I can't understand why they haven't issued a replacement reference.

Well spotted blackmail! I was confusing that idiotic MORA term with the American MEA expression. Thanks for that. :ok:

Touche ftrplt. Time was when navaids were U/S because maintenance wasn't being done. These days, we can't get through 2 consecutive maintenance cycles due to sabotage and/or removal of the solar power equipment. We can't win and we can't afford to keep replacing navaids and equipment because they'll just be stolen/sabotaged again.

However, the situation I'd been thinking about with my comments was the typical AYPY-AYNZ route, where there are working VOR, DME and NDB facilities at each end.

30th Jun 2006, 10:26
Blackmail, thanks for clarifying that MORA is a Jepp invention. And I agree with your MSA definitions as they now stand. The old MSA I referred to used to be 10 nm either side of track (not necessarily airway centreline) and not limited to 25 nm (with various caveats and exceptions).