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Minimum Safe Altitude (MSA)
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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 term "minimum safe altitude" as such does not exist.
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.
The expression "minimum safe altitude" is also used in relation to the Minimum Safe Altitude Warning (MSAW) function of ATC systems.
Note. The acronym MSA is also often used as a substitute for "minimum safe altitude". However, the International Civil Aviation Organisation (ICAO) definition of the acronym MSA is Minimum Sector Altitude (MSA). When the abbreviation MSA is encountered, care should be taken to establish which term it refers to.