Delta airplane.  An airplane with a triangular-shaped wing.  This wing has an extreme amount of sweepback on its leading  edge, and a trailing edge that is almost perpendicular to the  longitudinal axis of the airplane. Delta connection (electrical connection).  A method of  connecting three electrical coils into a ring or, as they are  drawn on a schematic diagram as a triangle, a delta (D). Denier.  A measure of the fineness of the yarns in a fabric. Density altitude.  The altitude in standard air at which the  density is the same as that of the existing air. Density ratio ( σ ).  The ratio of the density of the air at a  given altitude to the density of the air at sea level under  standard conditions. Derated (electrical specification).  Reduction in the rated  voltage or current of an electrical component. Derating is  done to extend the life or reliability of the device. Desiccant (air conditioning component).  A drying agent  used in an air conditioning system to remove water from  the refrigerant. A desiccant is made of silica-gel or some  similar material. Detent.  A spring-loaded pin or tab that enters a hole or groove  when the device to which it is attached is in a certain position.  Detents are used on a fuel valve to provide a positive means  of identifying the fully on and fully off position of the valve. Detonation.  An explosion, or uncontrolled burning of the  fuel-air mixture inside the cylinder of a reciprocating engine.  Detonation occurs when the pressure and the temperature  inside the cylinder become higher than the critical pressure  and temperature of the fuel. Detonation is often confused  with preignition.  Deviation error.  An error in a magnetic compass caused  by localized magnetic fields in the aircraft. Deviation error,  which is different on each heading, is compensated by the  technician “swinging” the compass. A compass must be  compensated so the deviation error on any heading is no  greater than 10 degrees. Dewar bottle.  A vessel designed to hold liquefied gases. It  has double walls with the space between being evacuated to  prevent the transfer of heat. The surfaces in the vacuum area  are made heat-reflective. Divergent oscillation.  Oscillation whose amplitude increases  with time. Disc area (helicopter specification).  The total area swept  by the blades of a helicopter main rotor. Dirigible.  A large, cigar-shaped, rigid, lighter-than-air flying  machine. Dirigibles are made of a rigid truss structure covered  with fabric. Gas bags inside the structure contain the lifting  gas, which is either helium or hydrogen. Dipole antenna.  A half wavelength, center-fed radio antenna.  The length of each of the two arms is approximately one  fourth of the wavelength of the center frequency for which  the antenna is designed. Diluter-demand oxygen system.  A popular type of oxygen  system in which the oxygen is metered to the mask, where  it is diluted with cabin air by an airflow-metering aneroid  assembly which regulates the amount of air allowed to dilute  the oxygen on the basis of cabin altitude. The mixture of  oxygen and air flows only when the wearer of the mask  inhales. The percentage of oxygen in the air delivered to the  mask is regulated, on the basis of altitude, by the regulator. A diluter-demand regulator has an emergency position which  allows 100 percent oxygen to flow to the mask, by-passing  the regulating mechanism. Dihedral.  The positive angle formed between the lateral axis  of an airplane and a line that passes through the center of the  wing or horizontal stabilizer. Dihedral increases the lateral  stability of an airplane. Digital multimeter.  An electrical test instrument that can be  used to measure voltage, current, and resistance. The indication  is in the form of a liquid crystal display in discrete numbers. Differential-voltage reverse-current cutout.  A type of  reverse-current cutout switch used with heavy-duty electrical  systems. This switch connects the generator to the electrical  bus when the generator voltage is a specific amount higher  than the battery voltage. Differential pressure.  The difference between two pressures.  An airspeed indicator is a differential-pressure gauge. It  measures the difference between static air pressure and pitot  air pressure. Differential aileron travel.  Aileron movement in which the  upward-moving aileron deflects a greater distance than the  one moving downward. The up aileron produces parasite drag  to counteract the induced drag caused by the down aileron.  Differential aileron travel is used to counteract adverse yaw.  G-12