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. G-11