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.) 

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.) 

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.) 

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 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). 

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. 

DOMESTIC NOTICES- A special notice or notice containing graphics or plain language text pertaining to almost every aspect of aviation, such as military 

