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Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 193
discrete code is any one of the 4096 selectable Mode 3/A aircraft transponder codes except those ending in
zero zero. Examples of discrete codes are 0010, 1201, 2317, and 7777.
Nondiscrete codes, such as 0100, 1200, and 7700, are normally reserved for radar facilities that are
not equipped with discrete decoding capability and for other purposes such as emergencies (7700) and VFR
aircraft (1200).
discrete component (electrical components). A device such as an individual resistor or capacitor.
Discrete components have been replaced in many modern circuits with integrated circuits in which a
number of individual components are built into a single integrated circuit (IC) chip.
discriminator (frequency modulated radio circuit). A circuit in a frequency modulated (FM) radio
receiver that changes deviations in the frequency of the carrier into amplitude variations of the
audio-frequency output.
The audio-frequency output, used to operate a speaker, is a duplicate of the modulating signal in
the transmitter.
disengage. To disconnect, or release one device or component from another.
dish antenna (radio equipment antenna). A shallow, concave, circular reflector used with a radio antenna,
actually with the shape of a parabola.
Signals from a transmitter are received in the dish and are reflected in such a way that they
concentrate at a point from which they are picked up and carried into the receiver. Signals picked up by a
dish are highly directional.
disk area (helicopter parameter). The total area swept by the main rotor of a helicopter.
disk brake. A popular type of nonservo brake used on aircraft. See multiple disk brake and single disk
brake.
disk loading (helicopter parameter). The ratio found by dividing the gross weight of a helicopter by the
total area of its rotor disk.
disk sander (power tool). A power tool consisting of a rotating disk covered with an abrasive material.
The part to be sanded, or abraded, is held against the disk, and the abrasive wears it away.
dispersant. A material or substance used to keep something dispersed, or suspended, in something else.
Ashless Dispersant (AD) oil contains a dispersant that keeps all the contaminants picked up by the
oil dispersed until it passes through filters where the contaminants are trapped. If the oil did not have the
dispersant, the contaminants would settle out and plug passages inside the engine.
displaced threshold. The beginning of the usable portion of a runway that is located at a point other than
the designated beginning of the runway.
display tube (electrical instruments). A cathode-ray tube used to display information. Cathode-ray
oscilloscopes and computer video monitors use display tubes.
dissimilar-metal corrosion. Corrosion that forms where two different types of metals are in contact with
each other. The severity of the corrosion is determined by the relative location of the metals in the
electrochemical series. See electrochemical series.
dissipate. To scatter, or disperse. Smoke from a smokestack blown by the wind is dissipated, or scattered,
into the atmosphere.
dissipate (power). To use something up. When electrons flow through a resistor, power is dissipated, and
the resistor gets hot. The resistor dissipates power by converting it into heat.
dissolve. To become a liquid and form a mixture with another liquid. When grains of solid sugar are put
into a glass of water, the sugar dissolves. It becomes a liquid and mixes with the water.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 194
dissolved acetylene gas. Acetylene gas which has been dissolved, or absorbed, in liquid acetone.
Acetylene gas becomes unstable when it is held under pressure of more than about 15 psi, but if it is
dissolved in acetone, it can be safely held under a pressure of more than 250 psi.
dissymmetry of lift (helicopter flight condition). The unequal lift produced across the rotor disk of a
helicopter moving through the air. The advancing blade, the blade whose tip is moving in the same
direction the helicopter is flying, has a speed equal to its own speed plus the speed of the helicopter. The
retreating blade, the blade whose tip is moving in the opposite direction, is traveling at a speed equal to its
own speed less the speed of the helicopter. Since aerodynamic lift is determined by the speed of the airfoil
through the air, the advancing blade produces more lift than the retreating blade.
Dissymmetry of lift is compensated by allowing the blades to flap. As the lift produced by the
advancing blade increases, it flaps upward, decreasing its angle of attack, and thus its lift. As the lift
　
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