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.  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.  See 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.  See distance measuring equipment.  G-10  DME arc.  A flight track that is a constant distance from the  station or waypoint.  DOD.  Department of Defense.  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.  DP.  See departure procedure.  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.  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.  DRVSM.  See Domestic Reduced Vertical Separation  Minimum.  DUATS.  See direct user access terminal system.  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. 