MINIMUM SAFE ALTITUDE (MSA) is a generic expression, used in various cases to denote an altitude below which it is unsafe to fly, owing to presence of terrain or obstacles.
An ICAO definition of the expression or term Minimum Safe Altitude (MSA) does not exist. However, ICAO has an acronym MSA which stands for Minimum Sector Altitude; and refers to the lowest altitude which may be used, which will provide a minimum clearance of 300 m (1 000 ft) above all objects located in the area contained within a sector of a circle of 46 km (25 NM) radius, centered on a radio aid to navigation.
Since the acronym MSA is used interchangeably for Minimum Safe Altitude, which has a different meaning from Minimum Sector Altitude, care should always be taken to establish which term the abbreviation MSA is referring to in any particular circumstance.
Minimum altitudes are calculated in relation to the highest terrain or obstacle within a specified area, allowing a buffer for error, and adding a specified margin.
Minimum altitudes associated with Controlled Airspace may be published in the states’ AIPs.
The expression “Minimum Safe Altitude” is also used in relation to the Minimum Safe Altitude Warning (MSAW) function of ATC systems.
REFERENCES
ICAO PANS-OPS/1
