Low or medium frequency. A frequency range between 190–535 kHz with the medium frequency above 300 kHz. Generally associated with nondirectional beacons transmitting a continuous carrier with either a 400 or 1,020 Hz modulation.
Lubber line. The reference line used in a magnetic compass or heading indicator.
MAA. See maximum authorized altitude.

Mach number. The ratio of the true airspeed of the aircraft to the speed of sound in the same atmospheric conditions, named in honor of Ernst Mach, late 19th century physicist.
Mach meter. The instrument that displays the ratio of the speed of sound to the true airspeed an aircraft is flying.
Magnetic bearing (MB). The direction to or from a radio transmitting station measured relative to magnetic north.
Magnetic heading (MH). The direction an aircraft is pointed with respect to magnetic north.
Mandatory altitude. An altitude depicted on an instrument approach chart with the altitude value both underscored and overscored. Aircraft are required to maintain altitude at the depicted value.
Mandatory block altitude. An altitude depicted on an instrument approach chart with two underscored and overscored altitude values between which aircraft are required to maintain altitude.
MAP. See missed approach point.
Margin identification. The top and bottom areas on an instrument approach chart that depict information about the procedure, including airport location and procedure identi. cation.
Marker beacon. A low-powered transmitter that directs its signal upward in a small, fan-shaped pattern. Used along the .ight path when approaching an airport for landing, marker beacons indicate both aurally and visually when the aircraft is directly over the facility.
Maximum altitude. An altitude depicted on an instrument approach chart with overscored altitude value at which or below aircraft are required to maintain altitude.
Maximum authorized altitude (MAA). A published altitude representing the maximum usable altitude or .ight level for an airspace structure or route segment.
MB. See magnetic bearing.
MCA. See minimum crossing altitude.
MDA. See minimum descent altitude.
MEA. See minimum en route altitude.
Mean sea level. The average height of the surface of the sea at a particular location for all stages of the tide over a 19-year period.
MFD. See multi-function display.
MH. See magnetic heading.
MHz. Megahertz.

Microwave landing system (MLS). A precision instrument approach system operating in the microwave spectrum which normally consists of an azimuth station, elevation station, and precision distance measuring equipment.
Mileage breakdown. A .x indicating a course change that appears on the chart as an “x” at a break between two segments of a federal airway.
Military operations area (MOA). Airspace established for the purpose of separating certain military training activities from IFR traf. c.