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exactly meshes with it. The number marked on the gage that fits the threads shows the number of threads
per inch in the nut or on the bolt.
scriber. A sharp-pointed tool used to scribe (mark) lines on metal.
scud (meteorology). A commonly used term for small patches of low clouds that often form below a heavy
overcast.
scuffing. A severe type of damage to moving parts caused when one metal part moves across another
without sufficient lubricant between them. Enough heat is generated by friction to cause the high points of
the surfaces to weld together, and continued movement tears, or scuffs, the metal.
scupper (fuel tank component). A recess around the filler neck of an aircraft fuel tank. Any fuel spilled
when the tank is being filled collects in the scupper and drains to the ground through a drain line, rather
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than flowing into the aircraft structure.
sea anchor. An open-bottom canvas bucket towed behind a boat or seaplane in the water. A sea anchor
causes a large amount of drag and is used to reduce the drift and to aid in maneuvering the vessel.
sea breeze (meteorology). A coastal breeze blowing from sea to land. Sea breezes are caused by the
temperature difference when the land surface is warmer than the sea surface.
sea fog (meteorology). A type of advection fog that forms when air that has been lying over a warm land
surface moves over a colder water surface.
seal. A component used to prevent fluid leaking between two surfaces. If there is relative movement
between the surfaces, the seal is called a packing. If there is no movement between the surfaces, it is called
a gasket.
sealant. A heavy liquid or paste spread between two surfaces to form a seal. Sealant may be used with a
gasket or, in some instances, instead of a gasket.
sea level. The average level of the surface of the sea. Sea level is the mid position between the level of the
water at low tide and the level at high tide.
sea-level-boosted engine. A reciprocating engine that has had its sea-level rated horsepower increased by
supercharging. This is the same as a ground-boosted engine.
sea-level engine. 14 CFR Part 1: “A reciprocating aircraft engine having a rated takeoff power that is
producible only at sea level.”
sea-level pressure (meteorology). The atmospheric pressure that exists at mean (average) sea level.
Sea-level pressure can be measured directly, by stations located at sea level, or determined from charts
showing the station pressure and temperature taken at stations not at sea level.
Sea-level pressure is used as a common reference for analysis of surface pressure patterns.
seam welding (electrical resistance welding). A form of electrical resistance welding in which small
metal wheels are used as the electrodes. These wheels roll over the metal, and the current is conducted
through the metal from the wheel on one side to the wheel on the other side. Heat caused by the current
flowing through the resistance of the metal softens the metal, and pressure between the wheels forces the
softened metal in the two pieces to flow together.
Seam welding is similar to spot welding, except that the weld is a continuous line, rather than
individual spots. Fuel tanks are usually seam welded, rather than spot welded.
seaplane. An airplane that can operate from water, rather than land. Some seaplanes are landplanes with
the wheels removed and floats installed. A flying boat is a type of seaplane whose fuselage is similar to the
hull of a boat. Sponsons (a type of stub wing near the water line) or wing-tip floats keep the flying boat
relatively level while it is in the water.
seasoned lumber. Lumber that has been dried, in a kiln or in some other way, and has had its moisture
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content reduced to a specified low amount.
secant (mathematics). The trigonometric function of a right triangle which is the ratio of the length of the
hypotenuse to the length of the side adjacent to an acute angle.
second (unit of angular measurement). One-sixtieth part of a minute of angular measurement. Three
hundred and sixty degrees, 21,600 minutes, or 1,296,000 seconds makes up a complete circle.
second (unit of time). The basic unit of time in both the English and the metric systems of measurement.
Sixty seconds is equal to one minute, 60 minutes (3,600 seconds) is one hour, and 24 hours (86,400
seconds) is one day, or the amount of time needed for one complete revolution of the earth.
secondary. A condition in which something has a level of importance just below that which is most
important.
secondary air (gas turbine engines). Air which has passed through a gas turbine engine compressor and is
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