Aeronautical Terms beginning with D
D-value
Departure of true altitude from pressure altitude; obtained by algebraically subtracting true altitude from pressure altitude; thus it may be plus or minus. On a constant pressure chart, the difference between actual height and standard atmospheric height of a constant pressure surface.
Danger Area [ICAO]
An airspace of defined dimensions within which activities dangerous to the flight of aircraft may exist at specified times. Note: The term “Danger Area” is not used in reference to areas within the United States or any of its possessions or territories.
Dark adaptation
Physical and chemical adjustments of the eye that make vision possible in relative darkness.
Data Block
Alphanumeric Display.
Database Columns
The spaces for data entry on each record. One column can accommodate one character.
Database Field
The collection of characters needed to define one item of information.
Database Identifier
A specific geographic point in space identified on an aeronautical chart and in an aviation database, officially designated by the controlling state authority or derived by Jeppesen. It has no ATC function and should not be used in filing flight plans nor used when communicating with ATC.
Database Record
A single line of computer data made up of the fields necessary to define fully a single useful piece of data.
Datum (Reference Datum)
An imaginary vertical plane or line from which all measurements of moment arm are taken. The datum is established by the manufacturer. Once the datum has been selected, all moment arms the location of CG range are measured from this point.
Dead reckoning
Navigation of an airplane solely by means of computations based on airspeed, course, heading, wind direction and speed, groundspeed, and elapsed time.
Deceleration error
A magnetic compass error that occurs when the aircraft decelerates while flying on an easterly or westerly heading, causing the compass card to rotate toward South.
Decision Altitude (DA)
A specified altitude in the precision approach at which a missed approach must be initiated if the required visual reference to continue the approach has not been established. The term Decision Altitude (DA) is referenced to mean sea level and the term Decision Height (DH)is referenced to the threshold elevation. Even though DH is charted as an altitude above MSL, the U.S. has adopted the term DAas a step toward harmonization of the United States and international terminology. At some point, DA will be published for all future instrument approach procedures with vertical guidance.
Decision Altitude/Decision Height [ICAO]
A specified altitude or height (A/H) in the precision approach at which a missed approach must be initiated if the required visual reference to continue the approach has not been established. Note 1: Decision altitude [DA] is referenced to mean sea level [MSL] and decision height [DH] is referenced to the threshold elevation. Note 2: The required visual reference means that section of the visual aids or of the approach area which should have been in view for sufficient time for the pilot to have made an assessment of the aircraft position and rate of change of position, in relation to the desired flight path.
Decision Height
With respect to the operation of aircraft, means the height at which a decision must be made during an ILS, MLS, or PAR instrument approach to either continue the approach or to execute a missed approach.
Decision Height (DH)
See Decision Altitude
Decoder
The device used to decipher signals received from ATCRBS transponders to effect their display as select codes.
Decompression Sickness
A condition where the low pressure at high altitudes allows bubbles of nitrogen to form in the blood and joints causing severe pain. Also known as the bends.
Deepening
A decrease in the central pressure of a pressure system; usually applied to a low rather than to a high, although technically, it is acceptable in either sense.
Defense mechanisms
Subconscious ego-protecting reactions to unpleasant situations.
Defense Visual Flight Rules
Rules applicable to flights within an ADIZ conducted under the visual flight rules in 14 CFR Part 91.
Deice
The act of removing ice accumulation from an aircraft structure.
Deicer Boots
Inflatable rubber boots attached to the leading edge of an airfoil. They can be sequentially inflated and deflated to break away ice that has formed over their surface.
Deicing
Removing ice after it has formed.
Delamination
The separation of layers.
Delay Assignment (DAS)
Delays are distributed to aircraft based on the traffic management program parameters. The delay assignment is calculated in 15-minute increments and appears as a table in Traffic Flow Management System (TFMS).
Delay Indefinite (Reason If Known) Expect Further Clearance
Used by ATC to inform a pilot when an accurate estimate of the delay time and the reason for the delay cannot immediately be determined; e.g., a disabled aircraft on the runway, terminal or center area saturation, weather below landing minimums, etc.
Delay Time
The amount of time that the arrival must lose to cross the meter fix at the assigned meter fix time. This is the difference between ACLT and VTA.
Delta
A Greek letter expressed by the symbol ∆ to indicate a change of values. As an example, ∆CG indicates a change (or movement) of the CG.
Delta ∆
This symbol, ∆, means a change in something. ∆CG means a change in the center of gravity location.
Demonstration-performance method
An educational presentation where an instructor first shows the student the correct way to perform an activity and then has the student attempt the same activity.
Density
(l) The ratio of the mass of any substance to the volume it occupies—weight per unit volume. (2) The ratio of any quantity to the volume or area it occupies, i.e., population per unit area, power density.
Density Altitude
This altitude is pressure altitude corrected for variations from standard temperature. When conditions are standard, pressure altitude and density altitude are the same. If the temperature is above standard, the density altitude is higher than pressure altitude. If the temperature is below standard, the density altitude is lower than pressure altitude. This is an important altitude because it is directly related to the airplane’s performance.
Departure Center
The ARTCC having jurisdiction for the airspace that generates a flight to the impacted airport.
Departure Control
A function of an approach control facility providing air traffic control service for departing IFR and, under certain conditions, VFR aircraft.
Departure End of Runway (DER)
The end of runway available for the ground run of an aircraft departure. The end of the runway that is opposite the landing threshold, sometimes referred to as the stop end of the runway.
Departure procedure (DP)
Preplanned IFR ATC departure, published for pilot use, in textual and graphic format.
Departure Sequencing Program
A program designed to assist in achieving a specified interval over a common point for departures.
Departure Time
The time an aircraft becomes airborne.
Deposition
The direct transformation of a gas to a solid state, in which the liquid state is bypassed. Some sources use sublimation to describe this process instead of deposition.
Depression
In meteorology, an area of low pressure; a low or trough. This is usually applied to a certain stage in the development of a tropical cyclone, to migratory lows and troughs, and to upper-level lows and troughs that are only weakly developed.
Descend Via
A descend via clearance instructs you to follow the altitudes published on a STAR. You are not authorized to leave your last assigned altitude unless specifically cleared to do so. If ATC amends the altitude or route to one that is different from the published procedure, the rest of the charted descent procedure is canceled. ATC will assign you any further route, altitude, or airspeed clearances, as necessary.
Descent Speed Adjustments
Speed deceleration calculations made to determine an accurate VTA. These calculations start at the transition point and use arrival speed segments to the vertex.
Description of the skill or behavior
The first part of a performance-based objective which explains the desired outcome of instruction in concrete terms that can be measured.
Designated Pilot Examiner (DPE)
An individual designated by the FAA to administer practical tests to pilot applicants.
Desired Course
Either True Course or Magnetic Course.
Desired Track
The planned or intended track between two waypoints. It is measured in degrees from either magnetic or true north. The instantaneous angle may change from point to point along the great circle track between waypoints.
Determiners
In test items, words which give a clue to the answer. Words such as “always” and “never” are determiners in true-false questions. Since absolutes are rare, such words usually make the statement false.
Detonation
The sudden release of heat energy from fuel in an aircraft engine caused by the fuel-air mixture reaching its critical pressure and temperature. Detonation occurs as a violent explosion rather than a smooth burning process.
Detresfa (Distress Phase) [ICAO]
The code word used to designate an emergency phase wherein there is reasonable certainty that an aircraft and its occupants are threatened by grave and imminent danger or require immediate assistance.
Deviation
A magnetic compass error caused by local magnetic fields within the aircraft. Deviation error is different on each heading.
Deviations
A. A departure from a current clearance, such as an off course maneuver to avoid weather or turbulence.
B. Where specifically authorized in the CFRs and requested by the pilot, ATC may permit pilots to deviate from certain regulations.
Dew
Water condensed onto grass and other objects near the ground, the temperatures of which have fallen below the initial dew point temperature of the surface air, but is still above freezing. Compare with frost.
Dew point (or dew-point temperature)
The temperature to which a sample of air must be cooled, while the mixing ratio and barometric pressure remain constant, in order to attain saturation with respect to water.
Dewpoint
The temperature at which air can hold no more water.
DF Approach Procedure
Used under emergency conditions where another instrument approach procedure cannot be executed. DF (Direction Finding) guidance for an instrument approach is given by ATC facilities with DF capability.
DF Fix
The geographical location of an aircraft obtained by one or more direction finders.
DF Guidance
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.
DF Steer
DF (Direction Finding) Guidance. 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.
Differential Ailerons
Control surface rigged such that the aileron moving up moves a greater distance than the aileron moving down. The up aileron produces extra parasite drag to compensate for the additional induced drag caused by the down aileron. This balancing of the drag forces helps minimize adverse yaw.
Differential Global Positioning System (DGPS)
A system that improves the accuracy of Global Navigation Satellite Systems (GNSS) by measuring changes in variables to provide satellite positioning corrections.
Differential pressure
A difference between two pressures. The measurement of airspeed is an example of the use of differential pressure.
Diffusion
Reducing the velocity of air causing the pressure to increase.
Digital ATIS (D-ATIS)
An alternative method of receiving ATIS reports by aircraft equipped with datalink services capable of receiving information in the cockpit over their Aircraft Communications Addressing and Reporting System (ACARS) unit.
Digital Target
A computer-generated symbol representing an aircraft’s position, based on a primary return or radar beacon reply, shown on a digital display.
Digital Terminal Automation System (DTAS)
A system where digital radar and beacon data is presented on digital displays and the operational program monitors the system performance on a real-time basis.
Digital-Automatic Terminal Information Service (D-ATIS)
The service provides text messages to aircraft, airlines, and other users outside the standard reception range of conventional ATIS via landline and data link communications to the cockpit. Also, the service provides a computer-synthesized voice message that can be transmitted to all aircraft within range of existing transmitters. The Terminal Data Link System (TDLS) D-ATIS application uses weather inputs from local automated weather sources or manually entered meteorological data together with preprogrammed menus to provide standard information to users. Airports with D-ATIS capability are listed in the Airport/Facility Directory.
Digitized Target
A computer-generated indication shown on an analog radar display resulting from a primary radar return or a radar beacon reply.
Dihedral
The positive acute angle between the lateral axis of an airplane and a line through the center of a wing or horizontal stabilizer. Dihedral contributes to the lateral stability of an airplane.
Diluter-demand oxygen system
An oxygen system that delivers oxygen mixed or diluted with air in order to maintain a constant oxygen partial pressure as the altitude changes.
Direct
Straight line flight between two navigational 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.
Direct Altitude And Identity Readout
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 Identity 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 ARTCCs.
Direct indication
The true and instantaneous reflection of aircraft pitch-and-bank attitude by the miniature aircraft, relative to the horizon bar of the attitude indicator.
Direct question
A question used for follow-up purposes, but directed at a specific individual.
Direct User Access Terminal System (DUATS)
A system that provides current FAA weather and flight plan filing services to certified civil pilots, via personal computer, modem, or telephone access to the system. Pilots can request specific types of weather briefings and other pertinent data for planned flights.
Direction Finder
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 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.
Directional Stability
Stability about the vertical axis of an aircraft, whereby an aircraft tends to return, on its own, to flight aligned with the relative wind when disturbed from that equilibrium state. The vertical tail is the primary contributor to directional stability, causing an airplane in flight to align with the relative wind.
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.
Discontinuity
A zone with comparatively rapid transition of one or more meteorological elements.
Discrete Beacon 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.]
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.
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.
Discrimination
The degree to which a test distinguishes the differences between students.
Displaced Threshold
A threshold that is located at a point on the runway other than the designated beginning of the runway.
Distance circle
The circle depicted in the plan view of an IAP chart that typically has a 10 NM radius, within which chart the elements are drawn to scale. (also, reference circle)
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.
Distance measuring equipment (DME)
A pulse-type electronic navigation system that shows the pilot, by an instrument-panel indication, the number of nautical miles between the aircraft and a ground station or waypoint.
Distant Early Warning Identification Zone (DEWIZ)
An ADIZ over the coastal waters of the State of Alaska.
Distractors
Incorrect responses to a multiple-choice test item.
Distress
A condition of being threatened by serious and/or imminent danger and of requiring immediate assistance.
Disturbance
In meteorology, applied rather loosely: (1) any low pressure or cyclone, but usually one that is relatively small in size; (2) an area where weather, wind, pressure, etc., show signs of cyclonic development; (3) any deviation in flow or pressure that is associated with a disturbed state of the weather, i.e., cloudiness and precipitation; and (4) any individual circulatory system within the primary circulation of the atmosphere.
Disuse
A theory of forgetting that suggests a person forgets those things that are not used.
Ditching
Emergency landing in water.
Diurnal
Daily, especially pertaining to a cycle completed within a 24-hour period, and which recurs every 24 hours.
Dive Brakes
Speed Brakes.
Divergence
The condition that exists when the distribution of winds within a given area is such that there is a net horizontal flow of air outward from the region. In divergence at lower levels, the resulting deficit is compensated for by subsidence of air from aloft; consequently the air is heated and the relative humidity lowered making divergence a warming and drying process. Low-level divergent regions are areas unfavorable to the occurrence of clouds and precipitation. The opposite of convergence.
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.
Diverse Vector Area (DVA)
An airport may establish a diverse vector area if it is necessary to vector aircraft below the minimum vectoring altitude to assist in the efficient flow of departing traffic. DVA design requirements are outlined in TERPS and allow for the vectoring of aircraft immediately off the departure end of the runway below the MVA.
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 arc
A flight track that is a constant distance from the station or waypoint.
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.
DME Separation
Spacing of aircraft in terms of distances (nautical miles) determined by reference to distance measuring equipment (DME).
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).
Doghouse
A turn-and-slip indicator dial mark in the shape of a doghouse.
Doldrums
The equatorial belt of calm or light and variable winds between the two tradewind belts. Compare intertropical convergence zone.
Domestic Air Defense Identification Zone
An ADIZ within the United States along an international boundary of the United States.
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 Reduced Vertical Separation Minimum (DRVSM)
Additional flight levels between FL 290 and FL 410 to provide operational, traffic, and airspace efficiency.
Double gimbal
A type of mount used for the gyro in an attitude instrument. The axes of the two gimbals are at right angles to the spin axis of the gyro, allowing free motion in two planes around the gyro.
Downburst
A strong downdraft which induces an outburst of damaging winds on or near the ground. Damaging winds, either straight or curved, are highly divergent. The sizes of downbursts vary from 1/2 mile or less to more than 10 miles. An intense downburst often causes widespread damage. Damaging winds, lasting 5 to 30 minutes, could reach speeds as high as 120 knots.
Downdraft
A relative small scale downward current of air; often observed on the lee side of large objects restricting the smooth flow of the air or in precipitation areas in or near cumuliform clouds.
Downwash
Air deflected perpendicular to the motion of the airfoil.
Downwind Leg
A flight path parallel to the landing runway in the direction opposite to landing. The downwind leg normally extends between the crosswind leg and the base leg.
Drag
An aerodynamic force on a body acting parallel and opposite to the relative wind. The resistance of the atmosphere to the relative motion of an aircraft. Drag opposes thrust and limits the speed of the airplane.
Drag Chute
A parachute device installed on certain aircraft which is deployed on landing roll to assist in deceleration of the aircraft.
Drag Curve
A visual representation of the amount of drag of an aircraft at various airspeeds.
Drift Angle
Angle between heading and track.
Drifting snow
A type of hydrometeor composed of snow particles picked up from the surface, but carried to a height of less than 6 feet.
Drill and practice method
A time-honored training delivery method based on the learning principle that connections are strengthened with practice.
Drizzle
A form of precipitation. Very small water drops that appear to float with the air currents while falling in an irregular path (unlike rain, which falls in a comparatively straight path, and unlike fog droplets which remain suspended in the air).
Dropsonde
A radiosonde dropped by parachute from an aircraft to obtain soundings (measurements) of the atmosphere below.
Dry adiabatic lapse rate
The rate of decrease of temperature with height when unsaturated air is lifted adiabatically (due to expansion as it is lifted to lower pressure).
Dry bulb
A name given to an ordinary thermometer used to determine temperature of the air; also used as a contraction for dry-bulb temperature. Compare wet bulb.
Dry-bulb temperature
The temperature of the air.
Ducted-Fan Engine
An engine-propeller combination that has the propeller enclosed in a radial shroud. Enclosing the propeller improves the efficiency of the propeller.
Due Regard
A phase of flight wherein an aircraft commander of a State-operated aircraft assumes responsibility to separate his/her aircraft from all other aircraft.
Duplex
Transmitting on one frequency and receiving on a separate frequency.
Dust
A type of lithometeor composed of small earthen particles suspended in the atmosphere.
Dust devil
A small, vigorous whirlwind, usually of short duration, rendered visible by dust, sand, and debris picked up from the ground.
Dust storm (also called duster, black blizzard)
An unusual, frequently severe weather condition characterized by strong winds and dust-filled air over an extensive area.
Duster
Same as dust storm—an unusual, frequently severe weather condition characterized by strong winds and dust-filled air over an extensive area.
Dutch Roll
A combination of rolling and yawing oscillations that normally occurs when the dihedral effects of an aircraft are more powerful than the directional stability. Usually dynamically stable but objectionable in an airplane because of the oscillatory nature.
Duty Runway
Runway In Use/Active Runway/Duty Runway.
DVFR Flight Plan
A flight plan filed for a VFR aircraft which intends to operate in airspace within which the ready identification, location, and control of aircraft are required in the interest of national security.
Dynamic
Continuous review, evaluation, and change to meet demands.
Dynamic Hydroplaning
A condition that exists when landing on a surface with standing water deeper than the tread depth of the tires. When the brakes are applied, there is a possibility that the brake will lock up and the tire will ride on the surface of the water, much like a water ski. When the tires are hydroplaning, directional control and braking action are virtually impossible. An effective anti-skid system can minimize the effects of hydroplaning.
Dynamic Load
The actual weight of the aircraft multiplied by the load factor, or the increase in weight caused by acceleration.
Dynamic Magnetic Variation
A field which is simply a computer model calculated value instead of a measured value contained in the record for a waypoint.
Dynamic Restrictions
Those restrictions imposed by the local facility on an “as needed” basis to manage unpredictable fluctuations in traffic demands.
Dynamic Stability
The property of an aircraft that causes it, when disturbed from straight-and level flight, to develop forces or moments that restore the original condition of straight and level.
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