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?
