Aeronautical Terms beginning with D

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.


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 Height (DH)

See Decision Altitude


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.


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.


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.


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.


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.


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.


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.


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.


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.


Designated Pilot Examiner (DPE)

An individual designated by the FAA to administer practical tests to pilot applicants.


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.


Dewpoint

The temperature at which air can hold no more water.


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.


Diffusion

Reducing the velocity of air causing the pressure to increase.


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.


Ditching

Emergency landing in water.


Downwash

Air deflected perpendicular to the motion of the airfoil.


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 Curve

A visual representation of the amount of drag of an aircraft at various airspeeds.


Drift Angle

Angle between heading and track.


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.


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.


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 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.


Dark adaptation

Physical and chemical adjustments of the eye that make vision possible in relative darkness.


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, charted in feet MSL, at which a missed approach must be initiated if the required visual reference to continue the approach has not been established.


Decision height (DH)

A specified altitude in the precision approach, charted in height above threshold elevation, at which a decision must be made either to continue the approach or to execute a missed approach.


Deice

The act of removing ice accumulation from an aircraft structure.


Density altitude

Pressure altitude corrected for nonstandard temperature. Density altitude is used in computing the performance of an aircraft and its engines.


Departure procedure (DP)

Preplanned IFR ATC departure, published for pilot use, in textual and graphic format.


Deviation

A magnetic compass error caused by local magnetic fields within the aircraft. Deviation error is different on each heading.


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.


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 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.


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 (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.


DME arc

A flight track that is a constant distance from the station or waypoint.


Doghouse

A turn-and-slip indicator dial mark in the shape of a doghouse.


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.


Drag

The net aerodynamic force parallel to the relative wind, usually the sum of two components: induced drag and parasite drag.


Drag curve

The curve created when plotting induced drag and parasite drag.


Duplex

Transmitting on one frequency and receiving on a separate frequency.


Defense mechanisms

Subconscious ego-protecting reactions to unpleasant situations.


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.


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.


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.


Direct question

A question used for follow-up purposes, but directed at a specific individual.


Discrimination

The degree to which a test distinguishes the differences between students.


Distractors

Incorrect responses to a multiple-choice test item.


Disuse

A theory of forgetting that suggests a person forgets those things that are not used.


Drill and practice method

A time-honored training delivery method based on the learning principle that connections are strengthened with practice.


Decision altitude (DA)

A specified altitude in an instrument approach procedure at which the pilot must decide whether to initiate an immediate missed approach if the pilot does not see the required visual reference, or to continue the approach. Decision altitude is expressed in feet above mean sea level.


Decision height (DH)

A specified height above the ground in an instrument approach procedure at which the pilot must decide whether to initiate an immediate missed approach if the pilot does not see the required visual reference, or to continue the approach. Decision height is expressed in feet above ground level.


Delta ∆

This symbol, ∆, means a change in something. ∆CG means a change in the center of gravity location.


Dynamic Load

The actual weight of the aircraft multiplied by the load factor, or the increase in weight caused by acceleration.


Density altitude

The altitude in the standard atmosphere at which the air has the same density as the air at the point in question. An aircraft will have the same performance characteristics as it would have in a standard atmosphere at this altitude.


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.


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.


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.


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.


Discontinuity

A zone with comparatively rapid transition of one or more meteorological elements.


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.


Diurnal

Daily, especially pertaining to a cycle completed within a 24-hour period, and which recurs every 24 hours.


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.


Doldrums

The equatorial belt of calm or light and variable winds between the two tradewind belts. Compare intertropical convergence zone.


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.


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.


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.


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.


Duster

Same as dust storm—an unusual, frequently severe weather condition characterized by strong winds and dust-filled air over an extensive area.


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.


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.


Datum (Reference Datum)

An imaginary vertical plane or line from which all measurements of arm are taken. The datum is established by the manufacturer. Once the datum has been selected, all moment arms and the location of CG range are measured from this point.


Dark adaptation

Physical and chemical adjustments of the eye that make vision possible in relative darkness.


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, charted in feet MSL, at which a missed approach must be initiated if the required visual reference to continue the approach has not been established.


Decision height (DH)

A specified altitude in the precision approach, charted in height above threshold elevation, at which a decision must be made either to continue the approach or to execute a missed approach.


Deice

The act of removing ice accumulation from an aircraft structure.


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.


Density altitude

Pressure altitude corrected for nonstandard temperature. Density altitude is used in computing the performance of an aircraft and its engines.


Departure procedure (DP)

Preplanned IFR ATC departure, published for pilot use, in textual and graphic format.


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.


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.


Deviation

A magnetic compass error caused by local magnetic fields within the aircraft. Deviation error is different on each heading.


Dew

Moisture that has condensed from water vapor. Usually found on cooler objects near the ground, such as grass, as the near-surface layer of air cools faster than the layers of air above it.


Dewpoint

The temperature at which air reaches a state where it can hold no more water.


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.


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 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 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.


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.


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 (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.


DME arc

A flight track that is a constant distance from the station or waypoint.


Doghouse

A turn-and-slip indicator dial mark in the shape of a doghouse.


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.


Drag

The net aerodynamic force parallel to the relative wind, usually the sum of two components: induced drag and parasite drag.


Drag curve

The curve created when plotting induced drag and parasite drag.


Drift angle

Angle between heading and track.


Duplex

Transmitting on one frequency and receiving on a separate frequency.


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.


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 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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