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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
indications of his/her DF on two separately located
ground‐based transmitters, both of which can be
identified on his/her chart. UDFs receive signals in
the ultra high frequency radio broadcast band; VDFs
in the very high frequency band; and UVDFs in both
bands. ATC provides DF service at those air traffic
control towers and flight service stations listed in the
Airport/Facility Directory and the DOD FLIP IFR En
Route Supplement.
(See DF FIX.)
(See DF GUIDANCE.)
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 communications
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
Airport/Facility Directory 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- Equipment
(airborne and ground) used to measure, in
nautical miles, the slant range distance of an aircraft
from the DME navigational aid.
(See MICROWAVE LANDING SYSTEM.)
(See TACAN.)
(See VORTAC.)
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 environment,
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 2/14/08
PCG D-4
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.)
(See MICROWAVE LANDING SYSTEM.)
DME SEPARATION- Spacing of aircraft in terms of
distances (nautical miles) determined by reference to
distance measuring equipment (DME).
(See DISTANCE MEASURING EQUIPMENT.)
DOD FLIP- Department of Defense Flight Information
Publications used for flight planning, en route,
and terminal operations. FLIP is produced by the
National Imagery and Mapping Agency (NIMA) for
world‐wide use. United States Government Flight
Information Publications (en route charts and
instrument approach procedure charts) are incorporated
in DOD FLIP for use in the National Airspace
System (NAS).
DOMESTIC AIRSPACE- Airspace which overlies
the continental land mass of the United States plus
Hawaii and U.S. possessions. Domestic airspace
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