Localizer (LOC).  The portion of an ILS that gives left/right guidance information down the centerline of the instrument runway for final approach. 

Localizer-type directional aid (LDA).  A NAVAID used for nonprecision instrument approaches with utility and accuracy comparable to a localizer but which is not a part of a complete ILS and is not aligned with the runway. Some LDAs are equipped with a glide slope. 

Locator middle marker (LMM). Nondirectional radio beacon (NDB) compass locator, collocated with a middle marker (MM). 

Locator outer marker (LOM). NDB compass locator, collocated with an outer marker (OM). 

LOM.  See locator outer marker. 

Long range navigation (LORAN). An electronic navigational system by which hyperbolic lines of position are determined by measuring the difference in the time of reception of synchronized pulse signals from two fixed transmitters. LORAN A operates in the 1750 to 1950 kHz frequency band. LORAN C and D operate in the 100 to 110 kHz frequency band. 

LORAN.  See long range navigation. 

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 identification. 

Marker beacon.  A low-powered transmitter that directs its signal upward in a small, fan-shaped pattern. Used along the flight 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 flight 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. 

