Minimum Altitude Defns

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MORA

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There are two types of Minimum Off Route Altitudes (MORAs) - one is called a route MORA and the other is the grid MORA.

Minimum Off Route Altitudes give at least 1,000 feet altitude clearance above terrain, and 2,000 feet in mountainous (an area of changing terrain were the changes of terrain elevation exceed 3000 feet within a distance of 10NM) terrain.

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Route MORA

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Route MORAs provided an obstacle clearance within 10 nautical miles (19 km) on both sides of the airways and within a 10-nautical-mile (19 km) radius around the ends of the airways.

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Grid MORA

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The Grid MORA altitude provides terrain and man-made structure clearance within the section outlined by latitude and longitude lines, usually of one degree by one degree. They are presented in feet (ft), omitting the last two figures. Example: 7,600 feet is given as 76.

MORA does not provide for navaid signal coverage or communication coverage.

For Elevation 5000ft and below it provides 1000ft, for 5001ft and above it provides 2000ft.

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MOCA

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Minimum Obstacle Clearance Altitude, or MOCA, is the lowest published altitude in effect between fixes on VOR airways or route segments that meets obstacle (like a building or a tower) clearance requirements for the entire route segment. Within the United States, this altitude also assures acceptable navigational signal coverage only within 22 nm of a VOR.

The MOCA is based upon obstacle clearance over the terrain or over manmade objects, adequacy of navigation facility performance, and communications requirements. The MOCA is always at or below the Minimum enroute altitude (MEA), and may put an aircraft below air traffic control RADAR coverage and also below Minimum reception altitude (MRA) for navigation aids; as a result, it is typically used only in emergencies, especially to get below icing.

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MEA

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Minimum en route altitude (MEA) is the lowest published altitude between radio navigation fixes that assures acceptable navigational signal coverage (see MRA) and meets obstacle clearance requirements (see MOCA) between those fixes.

Basically, the higher of MRA or MOCA

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MRA

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Minimum reception altitude (MRA) is the lowest altitude on an airway segment where an aircraft can be assured of receiving signals from navigation aids like VOR or NDB.

Typically, the greater the distance between navigation aids, the higher the MRA.