Minimum En-route Altitude (MEA)

MEA is the lowest published altitude between two radio fixes or way points that meets obstacle clearance requirements between those fixes or waypoints and in many countries assures acceptable navigational signal coverage.

Minimum Obstruction Clearance Altitude (MOCA)

MOCA is lowest published altitude in effect between radio fixes on VOR airways, off airway routes, or route segments which meets obstacle clearance requirements for the entire route segment.

MOCA is published with and “*” asterisk on charts.

Minimum Off-Route Altitude (MORA)

This altitude is derived by Jeppesen. MORA provides known obstruction clearance 10NM either side of the route centerline including 10NM radius beyond the radio fix reporting or mileage break defining the route segment.

Grid Minimum Off-Route Altitude (Grid MORA)

It is an altitude derived by Jeppesen or provided by State Authorities. Grid MORA provides terrain and man-made structure clearance within section outlined by latitude and longitude lines. MORA does not provide for navaid signal coverage or communication coverage.

• Grid MORA values derived by Jeppesen clear all terrain and man-made structures by 1000ft areas where the highest elevations are 5000ft MSL or lower. Grid MORA values clear all terrain and man-made structures by 2000ft in areas where the highest elevations are 5001ft MSL or higher. Grid MORA values followed by a +/- denote doubtful accuracy, but are believed to provide sufficient reference point clearance.
• Grid MORA values supplied by the State Authorities provide 2000ft clearance in mountainous areas and 1000ft in non-mountainous areas.
• Check footer of this article for the definition of mountainous terrain.

Maximum Authorized Altitude (MAA)

An MAA is a published altitude representing the maximum usable altitude or flight level for an airspace structure or route segment. MAA are very common due to flexibility of navigation system and direct routing.

Minimum Sector Altitude (MSA)

It is the lowest altitude which provide a minimum clearance of 1000ft above all objects located in an area contained within a sector of a circle 25 NM radius centered on a radio aid to navigation.

Mountainous Area (ICAO)

It is an area of changing terrain profile where the changes of terrain elevation exceed 3000 ft within a distance of 10 NM.

In this definition, the term “changes” refers to the difference between the minimum and maximum terrain elevations in the specified area. If the variation in height (from the lowest point to the highest point) is greater than 3,000 feet within that 10 NM distance, the area is classified as mountainous.

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