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amplitude modulated (AM) radio transmission. The bandwidth of a transmitted radio signal includes the
carrier frequency (the frequency produced by the oscillator in the transmitter) and the frequencies of both
the lower and the upper sidebands.
The frequency of the lower sideband is the carrier frequency minus the highest audio frequency
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 525
used to modulate the carrier. The frequency of the upper sideband is the carrier frequency plus the highest
audio frequency used to modulate the carrier.
side slip (airplane flight maneuver). A flight maneuver in which the airplane is banked while the rudder
keeps it on a straight heading. This crossed-control condition causes the airplane to move sideways toward
the low wing.
sidestep maneuver (air traffic control). A visual maneuver accomplished by a pilot at the completion of
an instrument approach to permit a straight-in landing on a parallel runway not more than 1,200 feet to
either side of the runway to which the instrument approach was conducted.
sidestick controller. A cockpit flight control used on some of the fly-by-wire equipped airplanes. The stick
is mounted rigidly on the side console of the cockpit, and pressures exerted on the stick by the pilot
produce electrical signals that are sent to the computer that flies the airplane.
sidetone (radio transmission). The audio-frequency signal fed back from the microphone of a radio
transmitter into the earphones connected to the radio receiver. Sidetones allow a person to hear the signal as
it is being transmitted.
siemens. A unit of electrical conductance. The siemens has replaced the older measurement called a mho.
One siemens is the amount of electrical conductance that allows an electromotive force of one volt to move
a flow of electrons of one ampere (one coulomb per second). One siemens is the reciprocal of one ohm.
sight glass (air conditioning system component). A small window in the high side of a vapor-cycle
cooling system. Liquid refrigerant flows past the sight glass, and if the charge of refrigerant is low, bubbles
will be seen. There are no bubbles in the refrigerant of a fully charged system.
sight glass (liquid-level indicator). A transparent tube on the outside of a reservoir or tank connected to
the bottom of the container. The level of the liquid in the sight glass is the same as that in the container, and
it gives a visual indication of the level of liquid in the reservoir.
sight line (sheet-metal layout). A line, drawn on a sheet-metal layout, one bend radius from the bend
tangent line. The sight line is placed so it is directly below the nose of the radius bar in a leaf brake. When
the metal is clamped in this position, the bend tangent line is at the correct position for the beginning of the
bend.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
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SIGMET (significant meteorological information). A weather advisory issued concerning weather
significant to the safety of all aircraft. SIGMET advisories cover severe and extreme turbulence, severe
icing, and widespread dust or sandstorms that reduce visibility to less than three miles.
signal generator (electronic test equipment). A piece of electronic test equipment that produces an
electrical signal having a specified voltage, frequency, and waveform. Signal generators are used for
troubleshooting and calibrating electrical and electronic equipment.
signal-strength meter (radio receiver component). A meter connected into the automatic volume control
(AVC) circuit in a radio receiver. A signal-strength meter is calibrated in decibels (dB) or in “S” units and
is used to provide an indication of the strength of the signal being picked up by the receiver.
signal tracer (electronic test equipment). A piece of electronic test equipment used to trace a signal
through each stage of a piece of electronic equipment. Troubleshooting of malfunctioning equipment is
simplified by comparing the waveform of the signal at each stage with the waveform that should be at that
stage.
significant digits. The digits of a decimal number, beginning at the first nonzero digit and extending to the
right to include all of the digits needed to give the required accuracy. One-half expressed as a decimal
fraction with one significant digit is 0.5. If greater accuracy is required, this same value can be expressed
with three significant digits as 0.500.
significant meteorological information. See SIGMET.
silica. Silicon dioxide (SiO2), as it occurs in nature.
silica gel. A form of extremely porous silica used as a desiccant. Silica gel, packaged in porous paper bags,
is stored inside delicate instruments and electronic equipment that could be damaged by moisture. Moisture
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航空术语词典Dictionary of Aeronautical Terms 下(49)
