Minimum En Route Altitude (MEA). The MEA is the lowest published altitude between radio fixes that assures acceptable navigational signal coverage and meets obstacle clearance requirements between those fixes. The MEA prescribed for a Federal Airway or segment, RNAV low or high route, or other direct route applies to the entire width of the airway, segment, or route between the radio fixes defining the airway, segment, or route.

Minimum IFR Altitude (MIA). Minimum altitudes for IFR operations are prescribed in 14 CFR Part 91. These MIAs are published on IFR charts and prescribed in 14 CFR Part 95 for airways and routes, and in 14 CFR Part 97 for standard instrument approach procedures.

Minimum Navigation Performance Specifications (MNPS). A set of standards that require aircraft to have a minimum navigation performance capability in order to operate in MNPS designated airspace. In addition, aircraft must be certified by their State of Registry for MNPS operation. Under certain conditions, non-MNPS aircraft can operate in MNPS airspace, however, standard oceanic separation minima is provided between the non-MNPS aircraft and other traffic.

Minimum Obstruction Clearance Altitude (MOCA). The MOCA is the lowest published altitude in effect between radio fixes on VOR airways, off-airway routes, or route segments that meets obstacle clearance requirements for the entire route segment. This altitude also assures acceptable navigational signal coverage only within 22 NM of a VOR.

Minimum Reception Altitude (MRA). An MRA is determined by FAA flight inspection traversing an entire route of flight to establish the minimum altitude the navigation signal can be received for the route and for off-course NAVAID facilities that determine a fix. When the MRA at the fix is higher than the MEA, an MRA is established for the fix, and is the lowest altitude at which an intersection can be determined.

Minimum Safe Altitude (MSA). MSAs are published for emergency use on IAP charts. For conventional navigation systems, the MSA is normally based on the primary omnidirectional facility on which the IAP is predicated. For RNAV approaches, the MSA is based on the runway waypoint (RWY WP) for straight-in approaches, or the airport waypoint (APT WP) for circling approaches. For GPS approaches, the MSA center is the Missed Approach Waypoint (MAWP).

Minimum Vectoring Altitude (MVA). Minimum vectoring altitude charts are developed for areas where there are numerous minimum vectoring altitudes due to variable terrain features or man-made obstacles. MVAs are established for use by ATC when radar ATC is exercised.

Missed Approach Holding Waypoint. An approach waypoint sequenced during the holding portion of the missed approach procedure that is a fly-over waypoint.

Missed Approach Waypoint (MAWP). An approach waypoint sequenced during the missed approach procedure that is a fly-over waypoint.

National Airspace System (NAS). Consists of a complex collection of facilities, systems, equipment, procedures, and airports operated by thousands of people to provide a safe and efficient flying environment.

Navigational Gap. A navigational course guidance gap, referred to as an MEA gap, describes a distance along an airway or route segment where a gap in navigational signal coverage exists. The navigational gap may not exceed a specific distance that varies directly with altitude.

Next Generation Air Transportation System (NextGen). Ongoing, wide-ranging transformation of the National Airspace System (NAS). NextGen represents an evolution from a ground-based system of air traffic control to a satellite-based system of air traffic management.

Non-Directional Radio Beacon (NDB). An L/MF or UHF radio beacon transmitting nondirectional signals whereby the pilot of an aircraft equipped with direction finding equipment can determine bearing to or from the radio beacon and “home” on or track to or from the station. When the radio beacon is installed in conjunction with the ILS marker, it is normally called a compass locator.

Non-RNAV DP. A DP whose ground track is based on ground-based NAVAIDS and/or dead reckoning navigation.

Obstacle Clearance Surface (OCS). An inclined or level surface associated with a defined area for obstruction evaluation.

Obstacle Departure Procedure (ODP). A procedure that provides obstacle clearance. ODPs do not include ATC related climb requirements. In fact, the primary emphasis of ODP design is to use the least erroneous route of flight to the en route structure while attempting to accommodate typical departure routes.

Obstacle Identification Surface (OIS). The design of a departure procedure is based on FAA Order 8260.3, a living document that is updated frequently. Departure design criteria assumes an initial climb of 200 Ft/NM after crossing the departure end of the runway (DER) at a height of at least 35 feet above the ground. Assuming a 200 Ft/NM climb, the departure is structured to provide at least 48 Ft/NM of clearance above objects that do not penetrate the obstacle slope. The slope, known as the OIS, is based on a 40 to 1 ratio, which is the equivalent of a 152 Ft/NM slope.

Off-Airway Routes. The FAA prescribes altitudes governing the operation of aircraft under IFR for fairway routes in a similar manner to those on federal airways, jet routes, area navigation low or high altitude routes, and other direct routes for which an MEA is designated.