DIRECT- Straight line flight between two naviga- tional aids, fixes, points, or any combination thereof.  When used by pilots in describing off-airway routes,  points defining direct route segments become  compulsory reporting points unless the aircraft is  under radar contact.  DIRECTLY BEHIND- An aircraft is considered to  be operating directly behind when it is following the  actual flight path of the lead aircraft over the surface  of the earth except when applying wake turbulence  separation criteria.  DISCRETE BEACON CODE- (See DISCRETE CODE.)  DISCRETE CODE- As used in the Air Traffic  Control Radar Beacon System (ATCRBS), any one  of the 4096 selectable Mode 3/A aircraft transponder  codes except those ending in zero zero; e.g., discrete  codes: 0010, 1201, 2317, 7777; nondiscrete codes:  0100, 1200, 7700. Nondiscrete codes are normally  reserved for radar facilities that are not equipped with  discrete decoding capability and for other purposes  such as emergencies (7700), VFR aircraft (1200), etc.  (See RADAR.)  (Refer to AIM.)  DISCRETE FREQUENCY- A separate radio  frequency for use in direct pilot-controller commu- nications in air traffic control which reduces  frequency congestion by controlling the number of  aircraft operating on a particular frequency at one  time. Discrete frequencies are normally designated  for each control sector in en route/terminal ATC  facilities. Discrete frequencies are listed in the Chart  Supplement U.S. and the DOD FLIP IFR En Route  Supplement.  (See CONTROL SECTOR.)  DISPLACED THRESHOLD- A threshold that is  located at a point on the runway other than the  designated beginning of the runway.  (See THRESHOLD.)  (Refer to AIM.)  DISTANCE MEASURING EQUIPMENT (DME)- Equipment (airborne and ground) used to measure, in  nautical miles, the slant range distance of an aircraft  from the DME navigational aid.  (See TACAN.)  (See VORTAC.)  DOMESTIC NOTICES- A special notice or notice  containing graphics or plain language text pertaining  to almost every aspect of aviation, such as military  DOMESTIC AIRSPACE- Airspace which overlies  the continental land mass of the United States plus  Hawaii and U.S. possessions. Domestic airspace  extends to 12 miles offshore.  DOD FLIP- Department of Defense Flight Informa- tion Publications used for flight planning, en route,  and terminal operations. FLIP is produced by the  National Geospatial-Intelligence Agency (NGA) for  world-wide use. United States Government Flight  Information Publications (en route charts and  instrument approach procedure charts) are incorpo- rated in DOD FLIP for use in the National Airspace  System (NAS).  (See DISTANCE MEASURING EQUIPMENT.)  DME SEPARATION- Spacing of aircraft in terms of  distances (nautical miles) determined by reference to  distance measuring equipment (DME).  DME FIX- A geographical position determined by  reference to a navigational aid which provides  distance and azimuth information. It is defined by a  specific distance in nautical miles and a radial,  azimuth, or course (i.e., localizer) in degrees  magnetic from that aid.  (See DISTANCE MEASURING EQUIPMENT.)  (See FIX.)  DISTRESS- A condition of being threatened by  serious and/or imminent danger and of requiring  immediate assistance.  DIVE BRAKES- (See SPEED BRAKES.)  DIVERSE VECTOR AREA- In a radar environ- ment, that area in which a prescribed departure route  is not required as the only suitable route to avoid  obstacles. The area in which random radar vectors  below the MVA/MIA, established in accordance with  the TERPS criteria for diverse departures, obstacles  and terrain avoidance, may be issued to departing  aircraft.  DIVERSION (DVRSN)- Flights that are required to  land at other than their original destination for  reasons beyond the control of the pilot/company, e.g.  periods of significant weather.  DME- (See DISTANCE MEASURING EQUIPMENT.)  Pilot/Controller Glossary Pilot/Controller Glossary  8/15/19 7/16/20  PCG D-3 