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soldered to a component, or where the wire is spliced. Modern spaghetti is often made of some form of
polyvinyl chloride tubing, and it can be either transparent or color coded. The name spaghetti comes from
the hollow tube of pasta the tubing resembles.
spalling. A type of damage in which chips are broken from the surface of a case-hardened material such as
a bearing race. Spalling takes place when a bearing race is put under a load great enough to distort the
softer inner part of the metal and cause the hard, brittle surface to crack. Once a crack forms in the surface,
chips break out.
span loading (aerodynamic measurement). An aerodynamic measurement found by dividing the span of
an airfoil by the total weight the airfoil supports.
spanner. The name generally used by the British for a wrench. In American usage, a spanner is a special
type of wrench with a hook-shaped arm and a hole, pin, or projection in its hooked end. A spanner is used
to turn ring-shaped nuts.
spanner nut. A special type of round nut with holes, notches, or projections around its periphery. Spanner
Printed from Summit Aviation's Computerized Aviation Reference Library, 2/7/2007
Page 542
nuts are turned with a hook spanner or pin spanner.
span of an airfoil. The length of an airfoil measured between its tips.
spar (airplane wing component). The main spanwise, load-carrying structural member in an airplane
wing.
spark. An electrical discharge between two electrodes or points within a circuit having a difference of
electrical potential. A spark normally has a very short duration and produces intense heat, light, and
electromagnetic radiation that cover a wide band of frequencies. A spark is used inside the cylinder of an
internal combustion engine to ignite the fuel-air mixture. A discharge of lightning is an extremely large
spark.
spark coil (reciprocating engine ignition system component). A step-up transformer used to produce a
high voltage. This high voltage causes a spark to jump across the electrodes of a spark plug inside the
cylinder of a reciprocating engine. The primary winding of the spark coil consists of comparatively few
turns of heavy copper wire, wound over a soft iron core. When primary current flows through this winding
and through the breaker points, a magnetic field is formed that extends out beyond the secondary winding.
The secondary winding consists of many hundreds of turns of very fine copper wire wound on the
outside of the primary winding. One end of the secondary winding connects to the primary, and the other
end goes to the rotor of the distributor, a high-voltage selector switch that directs the high voltage to the
proper spark plug.
When the breaker points open, primary current stops flowing, and its magnetic field instantly
collapses. As it collapses, the lines of flux cut across the turns of wire in the secondary winding and induce
a high voltage in it. This high voltage produces the spark in the spark plug.
spark-ignition engine (reciprocating engine). A form of reciprocating engine in which the fuel-air
mixture inside the cylinder is ignited by an electric spark. The spark is timed to occur when the piston is in
the correct position on the compression stroke.
spark plug (reciprocating engine ignition system component). A device in the ignition system of a
reciprocating engine that produces the spark which ignites the fuel-air mixture inside the cylinder. A spark
plug has a steel shell and an insulated center electrode. Accurately spaced gaps are formed between the
ground electrodes on the shell and the insulated center electrode.
The high-voltage lead from the ignition distributor is connected to the insulated terminal of the
spark plug and, at the correct time, a pulse of high-voltage electrical energy is sent to the spark plug. As
this electrical energy passes to ground, it produces an intense spark between the two electrodes.
spark plug bushing (aircraft reciprocating engine component). A threaded bronze or steel bushing
screwed and shrunk into an opening in the cast aluminum cylinder head of an aircraft engine. The spark
plugs screw into the bushings which protect the soft aluminum from damage when the spark plugs are
changed.
spark plug resistor. A resistor installed in a shielded spark plug between the ignition lead terminal and the
center electrode. The capacitance in the shielded ignition lead stores enough energy to keep the spark
flowing across the gap long enough to erode the electrodes. The resistor decreases the duration of the spark
and thus minimizes electron erosion.
spark suppressor. A device placed across a set of electrical contacts to suppress the spark that forms when
the contacts open. Spark suppressors normally consist of a capacitor and resistor and sometimes a diode
　
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