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Minimum Sector Altitude
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The Minimum Sector Altitude (MSA) is 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 centred on a radio aid to navigation. (ICAO PANS-OPS/I - definitions)
Minimum sector altitudes or terminal arrival altitudes are established for each aerodrome and provide at least 300m (1 000 ft) obstacle clearance within 46 km (25 NM) of the navigation aid, initial approach fix, or intermediate fix associated with the approach procedure for that aerodrome. (ICAO PANS-OPS/1 - Section 5, Chapter 2, §2.3)
Note. Sometimes the acronym MSA is also used to represent minimum safe altitude, which has a different meaning from minimum sector altitude, so care should always be taken to establish which term the abbreviation MSA is referring to in any particular circumstance.