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the fraction below the line. The numerator shows the number of parts of the denominator that are used. In
the fraction 9/16, nine is the numerator. The whole unit is divided into 16 equal parts (the denominator),
and we are using only nine of these parts.
numerical control. A digital control system. Numerical control (digital control) is used with production
machine tools to machine complex parts with more precision than can be done with an analog control
system.
numerical weather prediction (meteorology). Forecasting by digital computers solving mathematical
equations. Numerical weather prediction is used extensively in weather services throughout the world.
nut (threaded fastener). A small metal collar, usually having a hexagonal (six-sided) shape. The hole in
the nut is threaded so that it can be screwed onto a bolt. Bolts and nuts are used to fasten two or more
pieces of material together. The bolt is installed in holes drilled through the pieces, and a nut is screwed
onto the bolt to clamp the pieces together.
nutation. The slow back-and-forth rocking of the axis of a spinning body. A gyroscope, a spinning top, and
even the earth, as it spins, have a slight nutation.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 409
nutplate (aircraft fastener). A special form of nut that can be riveted to an aircraft structure. Screws can
be driven into a nutplate without having to hold the nutplate with a wrench.
nylon. The name of a family of synthetic materials that are part of the polymer chain. Nylon is used for
making cloth and rope and for such molded parts as wheels, gears, and pulleys. Nylon is strong, lightweight,
and resistant to most chemicals.
O
OALT (operational acceptable level of traffic) (air traffic control). An air traffic activity level
associated with the designed capacity for a sector or airport. The OALT considers dynamic changes in
staffing, personnel experience levels, equipment outages, operational configurations, weather, traffic
complexity, aircraft performance mixtures, transitioning flights, adjacent airspace, hand-off/point-out
responsibilities, and other factors that may affect an air traffic operational position or system element.
The OALT is normally considered to be the total number of aircraft that any traffic functional
position can accommodate for a defined period of time under a given set of circumstances.
oblique angle. An angle that is neither a right angle (a 90° angle) nor a straight line (a 180° angle). An
oblique angle can be either an acute angle (an angle of less than 90°) or an obtuse angle (an angle of more
than 90°).
oblique photography. A type of aerial photography in which photographs are taken with the camera
pointed at an angle, rather than straight down, as is done in aerial mapping.
oblique shock wave (aerodynamics). A shock wave that forms on a sharp-pointed object moving through
the air at a speed greater than the speed of sound. Air passing through an oblique shock wave is slowed
down, but not to a subsonic speed, and its static pressure is increased. See illustration for shock wave.
oblique triangle. A closed three-sided, plane (flat) figure that does not contain any right angles (90°
angles).
oblong shape. A shape that is neither square nor round, but can be either an elongated (stretched) square or
circle. An oblong circle is called an ellipse, and an oblong square is called a rectangle.
obscuration (meteorology). A term meaning that the sky is hidden by surface-based obscuring phenomena
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
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and vertical visibility is restricted overhead.
obscuring phenomena (meteorology). Any dry particles or particles of liquid water other than clouds.
Obscuring phenomena may be either surface-based or aloft.
observation aircraft. Aircraft used by the military to fly behind enemy lines and observe the movement of
troops or the effects of artillery fire.
obsolete. Out of date or no longer in use because of being replaced with something newer.
obstacle (air traffic control). An existing object, object of natural growth, or terrain at a fixed
geographical location or which may be expected at a fixed location within a prescribed area, with reference
to which vertical clearance is or must be provided during flight operation.
obstacle free zone. See OFZ.
obstruction light (airport lighting). A light or one of a group of lights, usually red or white, frequently
mounted on a surface structure or natural terrain to warn pilots of the presence of an obstruction.
obtuse angle. An angle of more than 90°, yet less than 180°. An obtuse angle may be called an open angle.
　
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