the Aviation Dictionary

is set, the resulting decrease in centrifugal force exerted
by the governor flyweights permits the speeder spring
to lower the pilot valve, thereby opening the
propeller-governor metering port. Oil is then directed
to the prop in the proper direction to flatten the pitch of
the propeller and correct the underspeed condition.
understanding . unloading valve
United States - Geographically, the States, the District
of Columbia, Puerto Rico, and the possessions,
including the territorial waters, and the airspace of
those areas.
United States air carrier - A citizen of the United
States who undertakes directly by lease, or other
understanding - Level of learning. Ability to arrangement, to engage in air transportation.
comprehend something learned.
universal chuck - The three clamping jaws of a drill
understanding - A basic level of learning where a
student comprehends or grasps the nature or meaning
of something.
used to hold drill bits and other drill accessories.
universal joint - A joint coupling that allows one shaft
to drive another shaft at angles to each other.
undervoltage relay - An electrical circuit protection
universal motor - A series-wound motor that operates
device that opens the circuit when voltage drops below
a predetermined value.
on either alternating or direct current.
universal propeller protractor - A precision
underwing fueling - A method of fueling an aircraft
measuring device used to measure the amount of blade
from a single-point pressure fueling port located under
angle.
the wing.
universal time (UT)- In the most common civil usage,
Underwriter's Laboratories (UL)- An independent,
not-for-profit product-safety testing and certification
UT refers to a time scale called "Coordinated Universal
organization.
Time" (abbreviated UTC), which is the basis for the
worldwide system of civil time. This time scale is kept
undrainable fuel - The amount of fuel that remains in by time laboratories around the world and is
the system after draining. Undrainable fuel is determined using highly precise atomic clocks. The
considered a part of the empty weight of the aircraft. International Bureau of Weights and Measures makes
use of data from the timing laboratories to provide the
undrainable oil - The oil remaining after draining the
international standard UTC, which is accurate to
oil from an engine. Undrainable oil is considered a part
approximately a nanosecond (billionth of a second) per
of the empty weight.
dav. The length ., of a UTC second is defined in terms of
unfeather - The action of turning the propeller blades to an atomic transition of the element cesium under
an angle not parallel with the airplane's line of flight. specific conditions, and is not directly related to any
unfeathered - Low blade angle condition of a propeller.
astronomical phenomena. The times of various events,
particularly astronomical and weather phenomena, are
UNICOM - A nongovernment communication facility &en given in "Universal Time" (abbreviated UT),
that can provide airport information at certain airports. which is sometimes referred to, now colloquially, as
Locations and frequencies of UNICOMs are shown on "Greenwich Mean Time" (GMT). The two terms are
aeronautical charts and publications.
often used loosely to refer to time kept on the
Greenwich meridian, five hours ahead of Eastern
unidirectional - In composites, a fabric, tape, or
Standard Time. Times given in UT are almost always
laminate with all the major fibers running in one
given in terms of a 24-hour clock. Thus, 14:42 (often
direction, giving strength in that direction.
written simply 1442) is 2:42 p.m., and 21 : 17 (21 17) is
unidirectional current - A flow of electrons in one 9:17 p.m. Sometimes a Z is appended to a time to
direction throughout a circuit. See also direct current. indicate UT, as in 09352.
uniform acceleration - Increasing the speed of an
object at an unvaried rate.
uniform surface corrosion - Unvaried corrosion across
a surface. No pits or localized damage have formed.
unijunction transistor -A transistor that allows voltage
to flow between its two bases when an appropriate
voltage is applied to its emitter.
unleaded gasoline - Engine fuel that has not been
treated with lead or lead components.
unlimited ceiling - A clear sky or a sky cover that does
not constitute a ceiling.
unloaded rotor - In helicopters, the state of a rotor
when rotor force has been removed, or when the rotor
is operating under a low or negative G condition.
union - Connectors or fittings that attach one item, such unloading valve - A Pressure control valve used in
as tubing, to another. aircraft hydraulic systems to prevent excessive
pressures from bursting lines and blowing out seals.
unit - A single piece of equipment.
When the pressure reaches a predetermined value, the
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