Pilots must confer to the minimum safe altitude (MSA) as well as the minimum en-route altitude (MEA). The former is used to denote altitudes below which it would be unsafe to fly due to the presence of obstacles or terrains. The expression is also commonly used in association with the MSAW function of Air Traffic Control systems.
Meanwhile, MEA refers to the altitude for the en-route subdivision that offers adequate reception of relevant ATS communications and navigation facilities, provides obligatory obstacle clearance, and complies with the structure of the airspace.
Seasoned instructors at Momentum Flight Training, an institute for aircraft simulator training programs, highlight some of the primary types of minimum IFR altitudes to know before you start your career as a regional airline pilot.
(RNAV) Area Navigation Minimum En-Route Altitude
These are depicted on certain en-route IFR low altitude charts, enabling both non-RNAV and RNAV navigators to use the same chart for instrument navigation.
(MRA) Minimum Reception Altitude
The FAA flight inspection determines the minimum reception altitude traversing the complete flight route to establish the minimum altitude with a navigational signal received for off-course NAVAID facilities and routes that determine the fix. If the Minimum Reception Altitude fix is greater than the minimum en-route altitude, an MRA will be established for the fix. It becomes the lowest altitude for which the intersection can be determined.
(MOCA) Minimum Obstruction Clearance Altitude
The Minimum Obstruction Clearance Altitude is the lowermost published altitude between off-airway routes, VOR airways, and route segment fixes that meet obstacle clearance requirements for all route segments. The altitude assures conventional navigational signal coverage within 22 nautical miles of the Very High-Frequency Omni-Directional Range.
(MAA) Maximum Authorized Altitude
This maximum authorized altitude represents the maximum flight level or operational altitude for a route segment or airspace structure. It's the highest altitude on a jet route, Federal airway, RNAV high or low route, and other direct routes for which the maximum en-route altitude is designated. Sufficient reception of navigation signals is guaranteed.
(MCA) Minimum Crossing Altitude
The minimum crossing altitude refers to the lowermost altitude at certain fixes at which the aviator should cross when proceeding in the higher IFR MEA direction.
(OROCA) Off-Route Obstruction Clearance Altitude
An off-route obstruction clearance altitude (OROCA) is an off-route altitude that provides obstruction clearance with a 1,000-foot buffer in non-mountainous terrain areas and a 2,000-foot buffer in designated mountainous areas. This altitude doesn't guarantee signal coverage from ground-based NAVAIDs, ATC radar, or communications coverage.
(MTA) Minimum Turning Altitude
MTA refers to the chartered altitude providing lateral and vertical obstruction clearance based on turn principles over certain waypoints, NAVAIDs, fixes, and charted route segments. It provides a turning area with obstacle clearance for both flyover protection and turn anticipation without violating the requirement concerning flying the airway's centerline.
(MVA) Minimum Vectoring Altitudes
Minimum vectoring altitudes are used by the air traffic control when radar ATC is exercised. It provides 2000 feet above the highest obstacle in mountainous areas and 1,000 feet of clearance above the highest obstacles in non-mountainous terrains.
Owing to their ability to isolate particular obstacles, some minimum vectoring altitudes can be lower than MOCAs, MEAs, and other minimum altitudes represented on charts for any given position. ATC's IFR altitude assignments, when radar vectored, are typically above or at the minimum vectoring altitudes.
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