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sector gear. A piece of a gear wheel that contains the hub and a portion of the rim with the teeth. A sector
gear, which is used where only a small amount of angular movement is needed, usually meshes with and
drives a small pinion gear. The shaft of the pinion gear can turn several revolutions, while the shaft of the
sector gear is turning through only a few degrees.
sector visibility (meteorology). Meteorological visibility within a specified sector of the horizon circle.
sediment. Contaminants that settle to the bottom of a reservoir or container.
seesaw rotor (helicopter rotor). Another name for a semirigid rotor. A semirigid rotor is a helicopter rotor
whose blades can rotate about their feathering axis, but there are no separate flapping hinges. The rotor
pivots on the mast in much the same way a seesaw pivots on its fulcrum.
segment (part of a circle). The part of a circle formed by a chord (a line passing across the circle and
touching the circumference at two points) and the arc between the two points.
segmented circle. A visual indicator around a wind sock or tetrahedron designed to show the traffic pattern
for each runway. Segmented circles are used at airports without operating control towers, and which have
some runways with right-hand traffic patterns and others with left-hand patterns.
segmented-rotor brake (multiple-disk brake). A heavy-duty, multiple-disk brake used on large,
high-speed aircraft. Stators, surfaced with a material that retains its friction characteristics at high
temperatures, are keyed to the axle. Rotors, keyed into the wheels, rotate between each pair of stators. The
rotor disks are made in segments to aid in dissipation of heat and to prevent their warping.
seize (equipment failure). A type of failure in which parts stick or jam because of friction, pressure, or
excessive temperature.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
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Selcal system. Selective calling system. Each aircraft operated by an airline is assigned a particular four
tone audio combination for identification purposes. A ground station keys the signal whenever contact with
that particular aircraft is desired. The signal is decoded by the airborne selcal decoder and the crew alerted
by the selcal warning system.
selective plating. A method of electroplating only a portion of a metal part. The portion not to be plated is
masked with a nonconductive material, and only the unmasked area accepts the electroplating.
selectivity (radio reception characteristic). The ability of a radio receiver to separate the signals it
receives. Selectivity is a very important characteristic of a communications receiver because of the large
number of transmissions in a narrow band of frequencies. The better the selectivity of a receiver, the
narrower the band of frequencies it amplifies.
selector switch. A multi-pole switch used to connect a single conductor to any one of several other
conductors. See rotary switch.
selector valve (fluid power system component). A flow control valve used in hydraulic systems to direct
pressurized fluid into one side of an actuator, and at the same time direct return fluid from the other side of
the actuator back to the reservoir.
There are two basic types of selector valves: open-center valves and closed-center valves. The
four-port closed-center valve is the most frequently used type. See closed-center selector valve and
open-center selector valve.
selenium. A steel-gray, highly toxic, nonmetallic chemical element. Selenium’s symbol is Se, its atomic
number is 34, and its atomic weight is 78.96. Selenium, which is in the sulfur family, has the useful
characteristic of changing its resistance as light falls on it. In the dark, selenium’s resistance is high, but in
the bright light, its resistance is low.
selenium rectifier. A type of rectifier using a cadmium and selenium junction to control the flow of
electrons. When a layer of selenium is deposited on an aluminum base, and a cadmium alloy plate is
pressed against the selenium, a barrier is formed between the selenium and the cadmium. Electrons can
easily pass through this barrier from the cadmium, through the selenium to the aluminum, but they cannot
pass through the barrier in the opposite direction.
self-aligning bearing. A type of bearing used on a control rod end that functions properly, even though the
control rod does not move at right angles to the surface to which it is attached. One of the most commonly
used self-aligning bearings is the spherical bearing. The bearing surface has the shape of a sphere with a
hole drilled through it. The two sides of the sphere perpendicular to the hole are ground flat so it can be
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