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when no other instrument approach procedure can be executed.
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DF fix. The geographic location of an aircraft obtained by the use of one or more direction finders.
DF guidance (air traffic control). Headings provided an aircraft by a ground facility using direction
finding equipment. DF headings, if followed, will lead the aircraft to a predetermined location.
diac (semiconductor device). A semiconductor used as a triggering device for a triac. A diac acts in much
the same way as two zener diodes installed back to back, with one forward biased and the other reverse
biased. No current can flow through the diac until the breakdown voltage of the reverse-biased diode is
reached. When this breakdown voltage is reached, the diac immediately turns on and allows the gate
current for the triac to flow.
diagonal cutting pliers. A type of wire cutters commonly used by aircraft maintenance technicians. The
short, strong jaws are ground so they cut the wire by a chiseling action, rather than by shearing.
diagram. A line drawing used to represent a system or process. A diagram can be used to trace a flow of
electrons, fluid, or components in a manufacturing process.
dial (instrument face). The face of an instrument on which calibration numbers are printed. The indication
of the instrument is read as the number on the dial to which the pointer is pointing.
dial-indicating torque wrench (precision hand tool). A type of deflecting-beam torque wrench that uses
a dial indicator to measure the amount the beam deflects. The dial of the indicator is graduated in
foot-pounds, inch-pounds, or meter-kilograms of torque, rather than units of beam deflection.
dial indicator. A precision measuring instrument that uses a compound lever arrangement to amplify the
movement of a stylus. The amount the stylus moves, in thousandths of an inch, is read on a circular dial.
diamagnetic material. A material which has a magnetic permeability of less than one, less than that of a
vacuum. A diamagnetic material is repelled by a magnet and will try to align itself across the lines of
magnetic flux, rather than along them.
diameter (dimension of a circle). The length of a line that touches the circumference of a circle in two
places and passes through its center. The diameter of a circle is twice its radius.
diamond. An extremely hard, almost colorless form of pure carbon. Some diamonds reflect light so well
they are used as gems, and low-grade diamonds are hard enough to be used as industrial cutting tools.
diamond chisel (metal-cutting tool). A metal-cutting chisel whose cutting edge is in the shape of a
diamond. A diamond chisel is used to cut V-shaped grooves in metal.
diamond-point cutting tool (machine tool). A cutting tool used for machining hard metal. The cutting
edge of the tool is an industrial diamond.
diamond-point dressing tool (machine tool). A tool used to smooth the surface of an abrasive grinding
wheel. An industrial diamond is mounted in the tool, which is clamped in the tool rest in such a way that
the diamond can be moved across the face of the abrasive wheel. The diamond wears away any roughness
on the wheel and produces a smooth surface.
dibromodifluoromethane (fire extinguishing agent). A colorless, heavy liquid fire extinguishing agent.
Dibromodifluoromethane is also known as Halon 1202.
dichromate solution. A solution of potassium or sodium dichromate used to form a hard, airtight film on
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the surface of magnesium parts to prevent corrosion.
dichromate treatment (corrosion protection). A corrosion-preventive treatment for magnesium alloys. A
tight oxide film is formed on the surface of a magnesium alloy when it is treated with a solution of
potassium or sodium dichromate.
die (metal-cutting tool). A cutting tool used to cut threads on the outside of a piece of round stock.
die (permanent mold). A permanent mold used to cast parts from metal or plastic resin. Material in its
plastic state is forced into the die under pressure.
die casting. A method of forming metal parts by forcing metal in its plastic state into a high-strength die,
using a large amount of force. Die-cast parts have more strength than parts cast by pouring the molten
metal into a mold.
dielectric. An insulating material that can store electrical energy in an electrostatic field.
The valence electrons in a dielectric material can be pulled away from their atom only with a
strong electrical force, but the orbits of these electrons can be distorted by an electric field. Electrical
energy is stored in this distorted field. Glass and ceramics are examples of good dielectrics.
　
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