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angles to each other.
universal motor. A form of series-wound electric motor that operates equally well on alternating current or
direct current. A universal motor has a commutator on its armature, and armature current is fed into the
commutator through brushes.
universal propeller protractor. A precision protractor that can be read to one tenth of a degree. Universal
propeller protractors are used to check and set the pitch of propeller blades and to measure the amount of
movement of aircraft control surfaces.
universal time. Solar time at the prime meridian. Solar time is the time based on the rotation of the earth
about the sun, and the prime meridian is the meridian of longitude that passes through the Royal
Observatory at Greenwich, England. Universal time is known also as Greenwich mean time (GMT) and
Zulu time.
unleaded gasoline (reciprocating engine fuel). A fuel for reciprocating engines that does not contain any
tetraethyl lead (TEL). See TEL.
unlimited ceiling (meteorology). A clear sky or a sky cover that does not meet the criteria for a ceiling.
unloading valve (hydraulic system component). A pressure control valve used in an aircraft hydraulic
system to unload the engine-driven pump when the system pressure reaches a predetermined value. The
unloading valve traps the pressure in the pressure manifold, where it is held by the accumulator, and routes
the output of the pump back to the reservoir. When the pressure in the manifold drops to a lower
predetermined value, the unloading valve directs the pump output back into the pressure manifold.
An unloading valve is also called a system pressure regulator. See system pressure regulator.
unscheduled maintenance. Maintenance performed on an aircraft as a result of a problem discovered in
the course of normal operation.
Scheduled maintenance is maintenance performed according to a predetermined plan or schedule.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 607
unstable air (meteorology). A mass of air whose temperature lapse rate, the change in temperature as the
altitude changes, is greater than that of the air surrounding it. When unstable air is forced upward by
flowing up the side of a mountain, for example, it will continue to move upward, increasing its speed as it
goes up. This causes updrafts in the air. Cumulus clouds form at the top of a column of rising unstable air.
unusable fuel (aircraft specification). Fuel in an aircraft fuel system that will not flow to the fuel
metering system when the aircraft is in its normal flight attitude. This fuel is also called residual fuel. The
weight of unusable fuel is included in the empty weight of the aircraft.
updraft (meteorology). A localized upward current of air.
updraft carburetor (reciprocating engine component). A type of carburetor that mounts below the
intake manifold of a reciprocating engine. All of the air flowing into the engine flows upward through the
venturi.
upholstery. The fabric, padding, and other components used to make a soft covering for a seat. Upholstery
makes the seat comfortable to sit in and attractive to look at.
upper deck pressure (fuel metering system pressure). The pressure that exists between the turbocharger
compressor and the throttle plate of a fuel injection system installed on a reciprocating engine. Upper deck
pressure, also called turbocharger discharge pressure, differs from manifold pressure by the pressure drop
across the throttle valve.
upper front (meteorology). A front aloft that does not extend to the earth’s surface.
upset head (rivet head). The head of a rivet formed when the rivet is installed. The upset head is also
called the shop head, to distinguish it from the manufactured head on the rivet before it is driven.
upslope fog (meteorology). Fog formed when air flows upward over rising terrain and is cooled
adiabatically to a temperature at or below its initial dew point.
upwind. The direction from which the wind is blowing.
uranium. A heavy, silvery-white, metallic chemical element in the actinide group. Uranium’s symbol is U,
and its atomic number is 92. An isotope of uranium, U-235, is used as one of the basic atomic fuels. When
U-235 is bombarded with neutrons, it changes into smaller atoms, and in making this change, it releases
neutrons and produces an enormous amount of heat.
useful load (aircraft operation). The difference between the maximum allowable weight of the aircraft
and its empty weight. The useful load of an aircraft includes the weight of the fuel and oil, the crew, the
passengers, all of their baggage, and any cargo carried.
U.S. gallon. A measure of quantity equal to 231 cubic inches, or 0.133 cubic foot.
utility category aircraft. A classification of aircraft approved for limited acrobatics. The maneuvers
　
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