Aviation Dictionary: D

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DA Density altitude
DALR Dry adiabatic lapse rate
DAME Designated aviation medical examiner
Danger Areas An airspace area which has some form of danger to aircraft. You may enter a danger area, however you will need to take into consideration what danger may be present.
DAP Departure and approach
Debrief A portion of time at the end of a flight that is used to discuss how the flight went, what was done well, what can be improved on etc. As an instructor debriefing a student, this should be done outside the aircraft, away from noise and in a private place as some information may embarrass a student. A solo pilot can debrief himself on a flight to improve on parts that they feel did not meet their expectations. This will improve you as a pilot.
Decision Altitude (DA) A specified altitude in the precision approach, or approach with vertical guidance, at which a missed approach must be initiated if visual reference has not been established. DA is referenced in mean sea level.
Decision Height (DH) A specified height in the precision approach, or approach with vertical guidance, at which a missed approach must be initiated if visual reference has not been established. DH is referenced to the threshold elevation.
Density Altitude (DA) This is the amount of air in a given space above mean sea level (AMSL). In other words, the more dense the air is, the more the amount of air in a given amount of space. Density altitude is pressure altitude corrected for temperature. As density altitude increases, aircraft performance decreases.
Descend A procedure which will cause an intentional loss of altitude usually at a planned rate of descent.
Deviation This is the difference between Magnetic North and the actual direction the compass is pointing to. A difference exists because of interference around the compass which can alter the direction the arrow points to slightly and a correction is noted on a Deviation Card.
Deviation Card A card that is placed near the compass in the aircraft, showing the difference between magnetic north and the direction the compass is pointing, and also showing the correction to use in order to fly the correct compass heading.
DFDR Digital flight data recorder
DFE Design feature endorsement
Dihedral The upward sloping tendency of the wings which provides lateral stability. This is most common on any low wing aircraft and can be seen when viewing an aircraft from the front.
Displaced Threshold A threshold that has been moved further up the runway. Usually this is done because a portion of the runway is unusable
DME Distance measuring Equipment. This equipment automatically provides the distance between an aircraft and a VOR station. The DME does not send the distance over the ground, but rather the actual distance between the two stations (slant range), so there will generally be a slight difference if comparing to a gps distance
Downwind A position in the aerodrome circuit parallel to the runway in use, but travelling in the opposite direction of intended landing.
Downwind Checks Checks done when on the downwind leg of a circuit. These may vary slightly between different training organisations, but comprise of a BUMFH, BUMPFISH or similar. The purpose of this checklist is to ensure you are ready to land the aircraft, for example, ensuring that your landing gear is down is quite important.
DR Low Drifting
Drag An object moving through air experiences resistance which is known as drag. To overcome drag, aircraft are designed to be streamlined.
Drift This is the difference between where the aircraft is pointing to, to where the actual direction the aircraft is flying or travelling over the ground. This is known as the aircraft heading and aircraft track.
DS Duststorm 
DU Widespread Dust
Dynamic Pressure This is the pressure experienced by any moving object as it passes through the air. On the forwards facing surface of the object, air pressure will build up. Dynamic pressure is = to ½ pV2 (part of the lift formula)
DZ Drizzle

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