Minimum obstacle clearance altitude

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Minimum Obstacle Clearance Altitude, or MOCA, is the lowest altitude that an aircraft can fly on an airway in Instrument meteorological conditions (IMC) and still keep safe clearance from terrain and obstacles (like buildings or towers). MOCA is often lower than 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.