The Route Instructor was right !
I am aware of the so-called 100 nm Emergency Safe Altitude. I understand it's the minimum altitude within 100 nm of the field in question that provides 1000 ft clearance of all obstacles to 6000 ft and 2000 ft clearance to obstacles above 6000 ft within that radius (and that is a huge area !) The European IAL charts produced by the joint KLM/SAS/Swissair group have that data on them.
It would seem most have my interpretation of the 'practical' use of the 25nm MSA is common, but I have just found the reference I've been looking for, .... The Route Instructor would appear to be correct.... Dam !;
The Jeppesen, Introduction, Chart Glossary section reads as follows;
MINIMUM SAFE ALTITUDE (MSA) - Altitude depicted on an instrument approach chart and identified as the minimum safe altitude which provides 1000-foot obstacle clearance within a 25NM radius from the navigational facility upon which the MSA is predicated. If the radius limit is other than 25 NM, it is stated. This altitude is for EMERGENCY USE ONLY and does not necessarily guarantee NAVAID reception. When the MSA is divided into sectors, with each sector a different altitude, the altitudes in these sectors are referred to as "minimum sector altitudes".
So there it is ! The caveat seems to be the lack of guarantee of navaid reception and not to do with the quality of the survey within the 25nm.
How then can a pilot determine if the acceptable level of NAVAID reception is being achieved. eg what about a GPS position that's independant of the terrestrial NAVAID?
Interesting eh !