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increase the internal resistance of the cell.
depression (meteorology). An area of low pressure, also called a low, or a trough. The term depression is
usually applied to a certain stage in the development of a tropical cyclone, but it also applies to migratory
lows and troughs and to upper-level lows and troughs that are only weakly developed.
depth gage. An instrument used to measure the depth of a hole or groove. The measuring scale protrudes
from the center of a flat blade. The blade is laid across the top of the hole or groove, and the measuring
scale is pushed to the bottom. The amount the scale sticks out of the blade is the depth of the hole or
groove.
Depth gages are used to measure the depth of the tread on aircraft tires.
depth micrometer. A precision measuring instrument which uses a standard micrometer caliper head
mounted in a flat-ground bar. The distance the spindle sticks out of the bar is read in thousandths of an inch.
Depth micrometers are used to measure the depth of machined grooves.
derate. To reduce the power output allowed for an engine or the voltage or current allowed for an electrical
component. Derating is done to extend the life or reliability of a device. Newly designed aircraft engines
are quite often placed in service with a lower power rating than they are designed to produce — they are
derated. As operational experience is gained, the rated power is increased.
derichment (fuel metering system operation). A condition in the fuel metering system of a large
reciprocating engine in which the fuel-air mixture is leaned out, or deriched, when antidetonation injection
(ADI) fluid is injected into the engine. If the mixture were not leaned, the addition of the ADI fluid would
enrich the mixture so much the engine could not produce its maximum power.
descent (aircraft flight condition). A decrease in altitude.
descent speed adjustments (air traffic control). Speed decrease calculations made to determine an
accurate vertex time of arrival (VTA). These calculations start at the transition point and use arrival speed
segments to the vertex.
desiccant. A material, such as silica gel, that is able to absorb moisture from the air. When precision
equipment is stored, bags of silica gel are packed with it. The silica gel absorbs moisture from the air and
prevents the moisture damaging the equipment.
In pneumatic power systems, the high-pressure compressed air passes through a desiccant which
absorbs all the moisture in the air. If moisture were left in the air, it would freeze when the air expands and
its temperature drops.
design load. The maximum load a component is designed to carry. The design load is made up of the
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 184
actual load the component is expected to encounter, plus a factor of safety.
destructive testing. A method of testing that results in the destruction of the part being tested.
Destructive testing is normally an engineering procedure, used to determine the ultimate strength
of a device or part, and to identify its weakest point.
detachable part. A part of a component which can be removed without damaging the component.
detail drawing (aircraft mechanical drawing). A drawing that contains enough information to allow a
component to be fabricated.
detector (electronic component). A rectifier in an electronic circuit. The term detector is normally used
for a rectifier that changes modulated radio-frequency alternating current into pulsating direct current
whose amplitude varies in the same way as the modulation on the original AC.
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 fuel selector valves to provide a positive means of identifying the
position in which the valve is fully on and fully off.
detergent. A synthetic cleaning material. A detergent is similar to soap in its ability to emulsify oil and
hold dirt and other solid or semi-solid contaminants. Unlike soap, a detergent is composed of man-made
chemicals, rather than organic matter such as animal or vegetable fat.
detergent oil. A type of mineral oil to which metallic-ash-forming detergents have been added. Detergent
oil is not generally used in aircraft engines because of its tendency to loosen carbon deposits from the
engine parts. These loosened deposits can plug oil passages inside the engine.
detonation (reciprocating engine condition). An explosion or uncontrolled burning inside the cylinder of
a reciprocating engine. Detonation occurs when the pressure and temperature inside the cylinder become
higher than the critical pressure and temperature of the fuel, and may be caused by using fuel that has a
　
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