(2) Where specifically authorized in the Federal Air Regulations and requested by the pilot Air Traffic Control may permit pilots to deviate from certain regulations.
DEVIATION (JCS, NATO) - The angular difference between magnetic and compass headings.
DF APPROACH PROCEDURE - Used under emergency conditions where another instrument approach procedure cannot be executed. Direction Finder guidance for an instrument approach is given by Air Traffic Control facilities with Direction Finder capability. (See DF Guidance, Direction Finder).
DF FIX - The geographical location of an aircraft obtained by one or more Direction Finders. (See Direction Finder).
DF GUIDANCE/DF STEER - Headings provided to aircraft by facilities equipped with direction finding equipment. These headings, if followed, will lead the aircraft to a predetermined point such as the DF station or an airport. DF guidance is given to aircraft in distress or to other aircraft which request the service. Practice DF guidance is provided when workload permits. (See Direction Finder, DF Fix).
DIGITAL AERONAUTICAL FLIGHT INFORMATION FILE (NGA) - The Digital Aeronautical Flight Information File (DAFIF) provides worldwide digital flight information selected from the DoD Flight Information Publications (FLIP). The DAFIF is used for existing and developing automated applications such as flight planning systems, flight simulators and flight management computer systems (mission computers).
DIRECT - Straight line flight between two navigational aids, fixes, point 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.
DIRECTION ALTITUDE AND IDENTITY READOUT/DAIR - The DAIR system is a modification to the AN/TPX-42 Interrogator System. The Navy has two adaptations of the DAIR system, Carrier Air Traffic Control Direct Altitude and Identification Readout System for Aircraft Carriers and Radar Air Traffic Control Facility Direct Altitude and Identify Readout System for land based terminal operations. The DAIR detects, tracks, and predicts secondary radar aircraft targets. Targets are displayed by means of computer generated symbols and alphanumeric characters depicting flight identification, altitude, ground speed, and flight plan data. The DAIR System is capable of interfacing with ARTCC's.
DIRECTION FINDER/DF/UDF/VDF/UVDF - A radio receiver equipped with a directional sensing antenna used to take bearings on a radio transmitter. Specialized radio Direction Finders are used in aircraft as air navigation aids. Others are ground based, primarily to obtain a "fix" on a pilot requesting orientation assistance, or to locate downed aircraft. A location "fix" is established by the intersection of two or more bearing lines plotted on a navigational chart using either two separately located Direction Finders to obtain a fix on an aircraft or by a pilot plotting the bearing indication of his Direction Finder on two separately located ground based transmitters both of which can be identified on his chart. Ultra High Direction Finders receive signals in the Ultra High Frequency radio broadcast band; Very High Direction Finders in the Very High Frequency band, UVDFs in both bands. Air Traffic Control provides Direction Finder service at those Air Traffic Control towers and Flight Service Stations listed in Airport/Facility Directory and DoD FLIP IFR Enroute Supplement. (See DF Guidance, DF Fix).
DISCRETE CODE/DISCRETE BEACON 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; non-discrete codes: 0100, 1200, 7700. Non-discrete codes are normally reserved for radar facilities that are not equipped with discrete decoding capability and for other purposes such as emergency (7700), VFR aircraft (1200), etc. (See Radar).
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