the Aviation Dictionary

springback St. Elmo's Fire
springback - The amount that metal springs back after square root - The factor of a number that, when
it has been bent to a specific angle. This must be squared, will give the number. For example, the square
allowed for when making bends in sheet metal. root of 25 is * 5.
spring-loaded - A condition where a spring holds one
part in a specific relationship with another. Spring
loading usually allows for some movement but returns
the parts to their original relationship.
springwood - The light portion of wood that represents
fast-growing wood from the spring of the year. Slower
growing wood of the summer is darker. The two colors
create the rings seen in the cross section of a tree.
sprue - The filler hole in a mold. The resulting
extension (the shape of the filler hole) of the part is
also referred to as the sprue.
spur and pinion reduction gear system - A gear
system used in reduction gearing. In a prop reduction
gearing system, the spur and pinion gears consist of a
large driving gear, or sun gear, splined to the shaft.
There is also a large stationary gear, called a bell gear,
and a set of small spur planetary pinion gears mounted
on a carrier ring. When the engine is operating, the sun
gear rotates. Because the planetary gears are meshed
with this ring, they rotate around the sun gear. The ring
in which the planetary gears are mounted rotates the
prop shaft in the same direction as the crankshaft but at
a reduced speed.
spur gear - An external toothed gear.
squall - A sudden increase in wind speed by at least 15
knots to a peak of 20 knots or more and lasting for at
least one minute. Essential difference between a gust
and a squall is the duration of the peak speed.
squall line - A line of thunderstorms often located along
or ahead of a vigorous cold front. Squall lines can
contain severe thunderstorms. The term is also used to
describe a line of heavy precipitation with an abrupt
wind shift but no thunderstorms, as sometimes occurs
in association with fronts.
square - 1. A plane geometric shape having four equal
sides with all four angles being right angles. 2. The
mathematical process of multiplying a number by
itself.
square engine - An engine whose bore and stroke are
equal.
square file - A double-cut file with a square cross
section, tapered lengthwise. Used for filing slots for
keyways.
square knot - A knot made up of opposite loops, each
one enclosing the parallel sides of the other.
square mil - An area equivalent to a square having sides
1 mil (0.001 in.) in length.
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square wave - The waveform of a multi-vibrator
oscillator where the leading edge and the trailing edge
of the wave are both vertical.
squaring shears - A large, floor-mounted sheet metal
tool used to make square cuts across sheets of metal.
squat switch - An electrical switch mounted on one of
the landing gear struts. It is used to sense when the
weight of the aircraft is on the wheels.
squawk (mode, code, function) - Activate specific
modes/codes/functions on the aircraft transponder; e.g.,
"Squawk threelalpha, two one zero five, low." See also
transponder.
squealing brakes - A noise made by glazed brakes
chattering at such a high frequency that the sound
resembles a squeal rather than a hammering.
squeegee - A long-handled rubber scraper blade used to
remove liquid from a surface.
squeeler tip - A tip of reduced thickness at the outer
end of rotor blades. This section is designed to wear
away rather than damage the shroud ring if tip loading
forces cause contact.
squeeze bottle - A soft plastic bottle that, when
squeezed, forces the bottle contents out through its top.
squeeze riveter - A pneumatic or hydraulic riveting gun
in which sets for both the manufactured head and the
upset head are mounted in the jaws of the large clamp.
When the squeeze-gun is actuated, the jaws come
together just enough to form the proper size upset head.
Also referred to as a rivet squeezer.
squelch - A circuit in a communications receiver
holding the output volume down until a signal is
received.
squelch circuit - See squelch.
squib - A small electric device used to ignite a charge.
Found on aircraft where an action needs to be
controlled remotely, such as a fire extinguisher bottle
in an engine nacelle.
squirrel-cage induction motor - An induction motor
whose rotor resembles a squirrel cage.
SRM (structural repair manual) - A manual
developed by the manufacturer that covers all items not
listed as minor maintenance, including instructions for
structural repair, major component removal,
installation, and adjustment, setup, etc. Contains
manufacturer-approved data for major repairs and
replacement.
St. Elmo's Fire - A luminous brush discharge of
electricity from protruding objects, such as masts and