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structure reaches a speed of Mach one, the speed of sound.
local oscillator (radio receiver circuit). An electronic oscillator in a superheterodyne radio receiver. A
superheterodyne receiver is highly selective because it selects an extremely narrow band of frequencies and
rejects all frequencies on either side.
A radio-frequency (RF) signal is picked up by the receiver and mixed with a signal produced by
the local oscillator, which is tuned with the same tuning control used to select the RF signal. Since both the
local oscillator and the RF signal are tuned at the same time, the frequency of the local oscillator is always
the same number of kilohertz or megahertz different from the RF, regardless of the frequency of the RF.
The difference frequency between the received RF and the frequency produced by the local
oscillator is filtered and amplified through the remaining stages of the receiver. This frequency is called the
intermediate frequency, the IF.
local traffic (air traffic control). Aircraft operating in the traffic pattern or within sight of the tower,
aircraft known to be departing or arriving from flight in local practice areas, or aircraft executing practice
instrument approaches at the airport.
locator (ICAO). An LF/MF nondirectional beacon used as an aid to final approach.
locked-rotor current (electric motor specification). The steady-state current flowing through the
windings of an electric motor when the rotor is held in a locked position with the rated voltage applied to it.
Locked-rotor current is much higher than the running current, because as soon as the rotor begins
to turn, it generates a counterelectromotive force that opposes the source voltage and decreases the running
current.
lockout debooster (brake system component). A component in an aircraft power brake system, used to
reduce the pressure between the brake control valve and the wheel cylinder. A lock-out debooster shuts off
the flow of fluid to the wheel cylinder if the line should break. See debooster.
lock ring (safety device). A retaining ring used to prevent a shaft moving lengthwise out of a hole. A
groove is cut around the outside of the shaft, and the spring steel lock ring grips the groove and holds the
shaft in place.
A lock ring is made in the shape of an open circle, with tabs at both of its ends so a pair of special
lock ring pliers can be used to expand the ring enough for it to slip over the shaft. When the ring is over the
groove, the pliers are released, and the spring action of the lock ring holds it tight in the groove. Lock rings
are made for both external and internal grooves.
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 358
lockstitch (aircraft fabric covering). A type of stitch used to prevent an entire hand-sewed seam
loosening if the thread should break at any point. When making a sewed-in patch to aircraft fabric, the
patch is sewed in using a baseball stitch. At every eight to ten stitches, the seam is locked with a modified
seine knot.
lock tab (mechanical locking device). A type of mechanical lock, used to prevent a nut from loosening on
a shaft. A locking hole or keyway is cut in the shaft and the internal tab on the lock ring is slipped into it.
The nut is screwed down onto the shaft and tightened to the proper torque; then one or more of the external
locking tabs are bent up against the flats of the nut to keep it from backing off and becoming loose.
lockwire. A term used for safety wire. Lockwire is soft brass, stainless steel, or galvanized low-carbon
steel wire, used to safety fasteners so they will not loosen by vibration. Lockwire is passed through holes in
the fasteners and twisted, then secured to a hole in an adjacent fastener or to a hole in the structure. Twisted
lockwire should always pull on the fastener in the direction that tries to tighten it.
lodestone. A natural magnetic iron oxide. Lodestones were discovered in ancient times and were the first
devices to be used as a magnetic compass. The term lodestone comes from “leading stone.”
log (record). A record kept of activities, with entries made as they occur. In the field of aviation, a log is
kept by pilots to record flight time. A log is kept on every airframe, engine, propeller, and rotor, to show
the amount of time in service and to record all the maintenance that has been done on each device.
logarithm. The exponent used to indicate the power to which a base number must be raised to obtain a
given number. In the expression 64 = 1,296, the base number is 6, and 4 is the power to which 6 must be
raised to get 1,296. Four is the logarithm of 1,296 to the base 6.
logarithmic curve. A mathematical curve plotted in such a way that the coordinate of any point varies as
　
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