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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.



The act of removing ice accumulation from an
aircraft structure.



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.



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



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.



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



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.



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



Differential Global Positioning System


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.



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.


Direction 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 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.



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.



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