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dry adiabatic lapse rate (meteorology). The rate of decrease of temperature with altitude when
unsaturated air is lifted adiabatically — that is, lifted without the addition to, or loss of, any heat energy.
The dry adiabatic lapse rate is 3°C (5.4°F) per thousand feet.
dry air (standard atmosphere). Air that contains no water vapor. Dry air is used as an atmospheric
standard. Dry air weighs 0.07651 pound per cubic foot under standard sealevel conditions of 59°F (15°C)
and a barometric pressure of 14.69 pounds per square inch or 29.92 inches of mercury.
dry air pump. See dry-type air pump.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 206
dry-bulb temperature. Temperature measured by a thermometer not affected by the evaporation of water.
Relative humidity is measured by finding the difference between wet-bulb and dry-bulb temperature.
The thermometer that measures wet-bulb temperature has a wick around its sensitive end. This
wick is saturated with water, and air blown across it evaporates the water. The evaporation causes the
wet-bulb temperature to be lower than the dry-bulb temperature, and the amount lower is determined by the
humidity, or the amount of water vapor in the air.
dry-cell battery. A common term used for a single cell of a carbon-zinc battery. A dry cell is not totally
dry. Its electrolyte is a moist paste, and it is sealed to retain its moisture.
dry-charged battery. A lead-acid storage battery that has been fully charged by the manufacturer. Before
the battery is shipped, all the electrolyte is drained out of it, and the cells are all sealed.
Before the battery is placed in service, it is filled with electrolyte having the proper specific
gravity, and the battery is given a freshening charge (a charge that brings its voltage up to the proper value).
dry-chemical fire extinguisher. A type of hand-held fire extinguisher which uses a dry chemical, such as
sodium bicarbonate, as the extinguishing agent. The dry chemical is normally forced out of the extinguisher
by compressed nitrogen.
dry ice. Solidified carbon dioxide. Dry ice sublimates, or changes from a solid directly into a gas, at a
temperature of –110°F (–78.5°C). without passing through the liquid state. Dry ice is used as a refrigerant
and as a means of chilling mechanical parts when they are being assembled with an interference fit.
drying oil. An oil used in paints and varnishes. The oil, usually cottonseed or linseed oil, is mixed in the
paint or varnish. When it is exposed to air, the oil oxidizes and forms a hard, dry film.
dry rot. A type of decay found in seasoned wood. Dry rot is caused by fungi in the wood which change the
normal fibers into a soft material that becomes a powder. Wood with dry rot has no structural strength.
dry-sump engine. An engine in which the lubricating oil supply is carried in a reservoir, or tank, that is not
part of the engine. Both gas turbine and reciprocating engines can have dry-sump lubricating systems.
dry-type air pump (engine accessory). An engine-driven air pump that uses carbon vanes, rather than
steel vanes.
Dry-type pumps require no lubrication, but the vanes are extremely susceptible to damage from
solid airborne particles. These pumps must therefore be operated with filters in their inlet.
dual-beam oscilloscope. A cathode-ray oscilloscope with two electron beams which produce two traces on
the screen at the same time. Dual-beam oscilloscopes may be used to observe the signals on the input and
the output of a portion of a circuit at the same time.
dual controls (aircraft flight controls). Two sets of fully functioning flight controls in an aircraft. Dual
controls allow the aircraft to be flown from either the left or right seat. All aircraft whose certification
requires a pilot and copilot must be equipped with fully functioning dual controls.
dual ignition. An ignition system of an aircraft reciprocating engine that has two of every critical unit,
including two spark plugs in each cylinder.
Dual ignition provides safety in the event of one system malfunctioning, but more important,
igniting the fuel-air mixture inside the cylinder at two locations provides more efficient expansion of the air
in the cylinder.
dual indicator (aircraft instrument). An aircraft instrument in which two instrument mechanisms move
pointers across a single dial.
Many engine instruments installed in multiengine aircraft are dual indicators. For example, the oil
pressure for both engines may be indicated on a single instrument. There are two separate indicator
mechanisms and two pointers, but only one instrument case and one dial.
dual magneto. A magneto having a single rotating magnet and cam, but two sets of breaker points, two
　
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