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proper direction to decrease the pitch of the propeller blades. When the blade pitch decreases, the RPM
increases, the governor flyweights spin out to an on-speed condition, and oil is trapped in the propeller to
hold the pitch.
undervoltage relay (electrical circuit protection device). A type of electrical circuit protection device
that opens the alternator field circuit when the output circuit voltage drops below a specified low value.
underwing fueling (aircraft fuel system). The method of fueling an aircraft in which fuel is pumped into
the aircraft tanks through a single-point fueling port located below the wing. Underwing fueling differs
from overwing fueling, in which the fuel is pumped directly into the tanks through openings in the tops of
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Page 605
the tanks.
Underwriters’ Laboratories. Independent laboratories recognized in the electrical industry as setting
safety standards for electrical equipment and components. Before a piece of electrical equipment is granted
the right to carry the UL symbol, it must pass the tests prescribed by the laboratories for that type of
equipment.
undrainable oil (aircraft specification). Oil that cannot be drained from an engine lubricating system
when the aircraft is in the normal ground attitude and the drain valve is left open. The weight of the
undrainable oil is included in the empty weight of the aircraft.
unfeather (propeller operation). To change the blade pitch of a propeller from the feathered high pitch,
which prevents air loads windmilling it, to a lower pitch. When a propeller is unfeathered in flight,
aerodynamic forces produced by air flowing over the blades cause the propeller to turn the engine.
UNICOM (radio communications). A privately owned radio station located on an airport. Pilots of
aircraft operating from the airport can talk with the operator of the UNICOM station to order service or to
have the operator ready to service their aircraft when they land. UNICOM can be used to give advisory
information about the airport, but it cannot be used to control traffic around the airport.
unidirectional current (electrical current). Another name for direct current. Unidirectional current flows
in one direction only. It does not reverse its direction as alternating current does. It is always either positive
or negative, with respect to a reference value.
unidirectional fabric. Fabric in which all of the threads run in the same direction. These threads are often
bound with a few fibers which cross them at right angles, just enough to hold the yarns together and
prevent their bunching.
uniform acceleration. The speeding up of an object in which the velocity increases at a uniform rate. It
changes the same amount each second it moves.
uniform surface corrosion. A mild form of corrosion in which the surface of a piece of metal is covered
by the salts of corrosion, but there are no localized pits on the surface.
unijunction transistor (UJT). A three-terminal semiconductor device. A pulse of energy on one terminal
controls the flow of electrons between its other two terminals. When a pulse of electrical energy of the
correct polarity is applied between the emitter and one of the bases, a negative resistance is produced
between the two bases, and electrons flow through the UJT between the two bases.
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union (threaded pipe fitting). A three-piece threaded pipe fitting used to join two pipes or to connect a
pipe to a component. A union allows the pipes to be connected without either of the pipes having to be
turned.
United States. 14 CFR Part 1: “In a geographical sense means: (1) the States, the district of Columbia,
Puerto Rico, and the possessions, including the territorial waters, and (2) the airspace of those areas.”
United States air carrier. 14 CFR Part 1: “A citizen of the United States who undertakes directly by lease,
or other arrangement, to engage in air transportation.”
unit magnetic pole. A magnetic pole that will repel a like pole of equal strength, at a distance of one
centimeter, in a vacuum, with a force of one dyne.
universal chuck (machine tool component). A type of clamp used to hold material being turned on a lathe.
The three jaws of a universal chuck are moved in and out at the same time by turning a chuck key. A
universal chuck holds the work centered as the jaws are clamped tightly on the work.
universal joint. A type of shaft coupling that allows one shaft to drive the other, even if the two shafts are
not in perfect alignment. A universal joint has a double hinge, with the axes of the two hinges at right
　
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