D Delta

data - Information that is collected from an experiment. For example, an engineer in a wind tunnel may collect data about how much lift is created by a certain wing shape.

data block - The information sent by an aircraft's transponder to an air traffic controller's radar scope that moves adjacent to the aircraft's image on the screen. this includes the aircraft's call sign, altitude, computer ID number and its speed.

datalink - The continuous transmission of the most recent data to/from the airborne and ground systems.

DA (drift angle) - The horizontal angle between the longitudinal axis of an aircraft and its path relative to the ground.

dead reckoning - See deductive reckoning

decelerate - To slow down. When an airplane comes in to land, it decelerates and rolls to a stop.

decibels (dB) - A unit for expressing relative difference in power usually between acoustic or electric signals.

decision support tools - Software programs that very quickly collect and analyze a massive amount of data about the airspace and aircraft within it.

deductive reckoning - The navigation of an airplane solely by means of computations based on airspeed, course, heading, wind direction, and speed, groundspeed, and elapsed time.

delay - A delay is incurred when any action is taken by a controller that prevents an aircraft from proceeding normally to its destination for an interval of 15 minutes or more.

Delta - Designator for the letter "D" in the International Phonetic Alphabet.

delta wing - A sweepback wing that looks like a triangle from above. The trailing edge of the wing is the base of the triangle. The XB-70A is an example of an airplane that has a delta wing.

density altitude - The altitude in the standard atmosphere at which the air has the same density as the air at the point in question.

Depart - The exit of an aircraft by taking off.

departure - The number of aircraft takeoffs actually performed in domestic and international scheduled and non-scheduled passenger/cargo and all-cargo revenue services.

departure airport - The airport from which an aircraft takes off.

departure controller - The air traffic control personnel responsible for monitoring all departing flights and for guiding departing aircraft to their initial navigational fix.

departure leg - The flight path which begins after takeoff and continues straight ahead along the runway centerline.

deplane - The process of passengers exiting the aircraft.

descend - To come down under control from a higher to a lower altitude.

descent - The action carried out in flying an aircraft from a higher to lower altitude.

Descent control - The air traffic control operation that directs aircraft from the end of their en route flight phase to their approach phase by instructing pilots from a higher to a lower altitude.

destination - The place toward which one is going.

destination airport - The airport to which an aircraft is flying.

dew point - The temperature to which air must be cooled to become saturated.

DGPS (Differential Global Position System) - This system uses a ground station to correct the code received from satellite for a 5 meter accuracy.

dihedral angle - The upward angle of the wings that is formed where the wings connect to the fuselage.

diurnal variation - Pertaining to actions or events that occur during a twenty-four hour cycle or recurs every twenty-four hours. Meteorological elements that are measured diurnally include clouds, precipitation, pressure, relative humidity, temperature and wind.

DME (Distance Measuring Equipment) - A combination of ground and airborne equipment which gives a continuous slant range distance-from-station readout by measuring time-lapse of a signal transmitted by the aircraft to the station and responded back. DMEs can also provide groundspeed and time-to-station readouts by differentiation.

downdraft - A sudden descent of cool or cold air to the ground, usually with precipitation, and associated with a thunderstorm or shower.

downwind leg - A flight path parallel to the landing runway in the opposite direction of landing.

drag - The force that resists the motion of the aircraft through the air. One type of drag is caused by air molecules. As the aircraft flies through the molecules, they resist the motion of the aircraft. This resistance is due to friction between the air molecules and the surface of the aircraft. Airplanes are streamlined to decrease the drag force.

DTK (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.

dust devil - A small vigorous whirlwind, usually of short duration, rendered visible by dust, sand, and debris picked up from the ground.

dust storm - A severe weather condition characterized by strong winds and dust-filled air over a large area.