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.  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. 

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. 

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. 

DUATS.  See direct user access terminal system. 

Duplex.  Transmitting on one frequency and receiving on a separate frequency. 

Eddy currents.  Current induced in a metal cup or disc when it is crossed by lines of flux from a moving magnet. 

EFAS.  See En Route Flight Advisory Service. 

EFC.  See expect-further-clearance. 

Electronic flight display (EFD). For the purpose of standardization, any flight instrument display that uses LCD or other image-producing system (Cathode Ray Tube [CRT], etc.) 

Elevator illusion. The sensation of being in a climb or descent, caused by the kind of abrupt vertical accelerations that result from up- or downdrafts. 

Emergency.  A distress or urgent condition. 

Emphasis error.  The result of giving too much attention to a particular instrument during the cross-check, instead of relying on a combination of instruments necessary for attitude and performance information. 

EM wave.  Electromagnetic wave. 

Encoding altimeter.  A special type of pressure altimeter used to send a signal to the air traffic controller on the ground, showing the pressure altitude the aircraft is flying. 

En route facilities ring.  Depicted in the plan view of IAP charts, a circle which designates NAVAIDs, fixes, and intersections that are part of the en route low altitude airway structure. 

En Route Flight Advisory Service (EFAS).  An en route weather-only AFSS service. 

En route high-altitude charts.  Aeronautical charts for en route instrument navigation at or above 18,000 feet MSL. 

En route low-altitude charts.  Aeronautical charts for en route IFR navigation below 18,000 feet MSL. 

Equivalent airspeed. Airspeed equivalent to CAS in standard atmosphere at sea level. As the airspeed and pressure altitude increase, the CAS becomes higher than it should be, and a correction for compression must be subtracted from the CAS. 

