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sublimation. The procedure in which a solid material changes directly into a vapor or a vapor changes into
a solid without becoming a liquid in the process.
submerged-arc welding. A form of electric arc welding in which a bare rod is used as the electrode. The
rod is covered with granulated flux that melts in the arc and flows out ahead of the weld to prevent the
formation of oxides in the bead.
subrefraction (radar technology). Less than normal bending of a radar beam, resulting from abnormal
vertical gradients of temperature and/or water vapor.
subscript. A number or letter written to the right of, and partially below, an abbreviation or a number. In
gas turbine engine operation, the symbol PAm identifies ambient pressure. “P” stands for pressure, and the
subscript “Am” tells that this pressure is ambient pressure. The chemical formula for water is H2O. The
subscript “2” indicates that two atoms of hydrogen unite with one atom of oxygen to form one molecule of
water.
subsidence (meteorology). A descending motion of the air in the atmosphere over a rather broad area.
subsonic flight (aircraft flight). Flight of an aircraft in which all the air passing over the aircraft structure
is moving at a speed slower than the speed of sound.
substandard. The quality of an object that is less than can be considered normal or acceptable.
substitute. To use one material or object to take the place of something else. Plans and specifications often
call for a certain type of material to be used. But if this particular material is not available, another material
may be used in its place. The material used in place of the one specified is a substitute material.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 562
substrate. The wafer of semiconductor material on which the components of integrated circuits are
diffused and deposited.
subsystem. A small system inside a larger system. Each subsystem has a job it must perform for the main
system to operate properly.
subtraction (mathematics). The mathematical process in which the value of one number is taken from the
value of another.
subtrahend (mathematics). The number in a subtraction problem that is to be subtracted.
suction. A term commonly used for a negative pressure, or a pressure below atmospheric pressure. The
terms suction and vacuum are both used to mean a negative pressure.
suction relief valve (pneumatic system component). A component that holds a constant and steady
suction, a pressure lower than that of the atmosphere, on a system. Some gyroscopic instruments used in an
aircraft are operated by air pulled into the instrument case by a suction produced by an engine-driven
vacuum pump.
A suction relief valve in the line between the pump and the instrument case has a spring-loaded
poppet that remains closed until the pressure becomes low enough to allow outside air pressure to force the
poppet off its seat, then outside air is taken into the pump through the valve, rather than through the
instrument case. This air entering the system holds the pressure inside the instrument case at the value for
which the suction relief valve is set.
sudden stoppage (engine operating condition). A condition likely to cause internal damage to an aircraft
engine. An engine is said to have a sudden stoppage if it comes to a complete stop within one revolution of
the crankshaft.
sulfating (battery condition). A condition in a lead-acid storage battery in which both the positive and
negative plates are covered with a hard deposit of lead sulfate. When a lead-acid battery discharges, the
active material on both its plates changes into lead sulfate, which can be removed by recharging the battery.
But if the battery is allowed to stand in a discharged state for a long period of time, the sulfate hardens and
is difficult, if not impossible, to remove by normal recharging.
sulfur. A pale yellow, nonmetallic chemical element. Sulfur’s symbol is S, its atomic number is 16, and its
atomic weight is 32.064. Sulfur, also spelled sulphur, is extremely important in the manufacture of
commercial and industrial compounds and is found in its free state in underground beds along the Gulf
coast of Texas and Louisiana.
sum (mathematics). The answer to an addition problem when two or more numbers are added together. In
the problem 2 + 4 = 6, the number 6 is the sum.
summation principle (meteorology). The principle which states that the sky cover assigned to a layer is
equal to the summation of the cover of the lowest layer, plus the additional coverage at all successively
higher layers, up to and including the layer in question. No layer can be assigned a sky cover less than a
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