FAA Acronyms and Initials Use to Abbreviate Altitude

It can be easy to get lost in the seemingly endless list of acronyms and initials used to abbreviate the many different altitudes associated with VFR and IFR operations. Here are a few brief descriptions of some of the common acronyms and initials the FAA has chosen to use to describe different VFR and IFR altitudes.

MSA - Minimum Safe Altitudes  
Published for emergency use on instrument approach charts. For conventional navigation systems, the MSA is normally based on the primary omnidirectional facility on which the instrument approach is predicated. MSA's are expressed in MSL and normally have a 25 NM radius. They provide a 1,000 feet of clearance over all obstructions but do not assure acceptable navigation signal coverage. Found on Instrument Approach Charts. (See FAA-H-8261-1 Instrument Procedures Handbook)

ESA - Emergency Safe Altitude
Similar to an MSA but will provide 1000 of feet clearance in flat terrain and 2000 feet in designated mountainous terrain within 100 NM of the fix. Found on Instrument Approach Charts. (See AIM Glossary)

MEA - Minimum Enroute Altitude  
The lowest published altitude that ensures a navigation signal strong enough for adequate reception by the aircraft navigation (NAV) receiver and adequate obstacle clearance along the airway. Communication is not necessarily guaranteed with MEA compliance. The obstacle clearance, within the limits of the airway, is typically 1,000 feet in non-mountainous areas and 2,000 feet in designated mountainous areas. Found on IFR Enroute Low Altitude Map. (See FAA Instrument Flying Handbook)


MOCA - Minimum Obstacle Clearance Altitude
Provides the same obstruction clearance as an MEA. However, navigation signal reception is only ensured within 22 NM of the closest NAVAID defining the route. The MOCA is listed below the MEA with an asterisk to the right of the altitude (*3400).    Found on IFR Enroute Low Altitude charts. (See FAA Instrument Flying Handbook)

OROCA - Off-Route Obstacle Clearance Altitude  
An OROCA is an off-route altitude that provides obstruction clearance with a 1,000-foot buffer in non-mountainous terrain and a 2,000-foot buffer in designated a mountainous areas within the U.S. This altitude may not provide signal coverage from ground-based navigation aids, air traffic control radar, or communications coverage. OROCAs are intended primarily as a pilot tool for emergencies and situational awareness. The large number represents thousands while the small number is representative of hundreds. Found on IFR Enroute Low Altitude charts. (See FAA-H-8261-1 Instrument Procedures Handbook)

MEF - Maximum Elevation Figure    
The MEF represents the highest elevation, including terrain and other vertical obstacles (towers, trees, etc.), within a quadrant. A MEF is determined by taking the highest point within a quadrangle, adding 100 feet for vertical error, then adding the height of the highest obstacle in the quadrangle or 200 feet (whichever is higher), and then rounding up to the next hundred feet. Found on VFR Sectional Charts. (See Explanation of VFR Terms and Symbols)

MAA - Maximum Authorized Altitude
The highest altitude at which the airway can be flown without receiving conflicting navigation signals from NAVAIDs operating on the same frequency. Found on IFR Enroute Low Altitude charts. (See FAA Instrument Flying Handbook)

MCA - Minimum Crossing Altitude
The MCA is usually indicated prior to approaching steeply rising terrain, a changing obstacle clearance or to prevent signal reception being compromised. When indicated, the pilot is required to initiate a climb to reach the MCA by the time the intersection is crossed. Found on IFR Enroute Low Altitude charts. (See FAA Instrument Flying Handbook)

MRA - Minimum Reception Altitude

It is the lowest altitude on an airway segment at which an aircraft can be assured of receiving signals from navigation aids. If the reception is line-of-sight based, signal coverage will only extend to the MRA or above. (See FAA Instrument Flying Handbook)