Sea-level boosted engine. A reciprocating engine that has had its sea-level rated horsepower increased by supercharging. This is the same as a ground-boosted engine. 

Secondary winding. The winding in a magneto or ignition coil that connects to the distributor rotor. The secondary winding is normally made of very small diameter wire and has a large number of turns, typically about 20,000. 

Self-accelerating speed.  The speed attained by a gas turbine engine during start-up that allows it to accelerate to its normal idling speed without assistance from the starter. 

Semiconductor transducer.  A piezoelectric crystal that converts input energy of one form, such as pressure, into output energy of another, such as an electrical signal. 

Series-wound motor.  An electric motor with field coils connected in series with the armature. 

Serviceable limits. Limits included in a reciprocating engine overhaul manual. If a part measures outside of the new-parts limits, but within the serviceable limits, it will not likely wear to the point of causing engine failure within the next TBO interval. 

Servo system. A type of automatic control system in which part of the output is fed back into the input. 

Shaft horsepower.  The horsepower actually available at a rotating shaft. 

Shielding.  The electrically conductive covering placed around an electrical component to intercept and conduct to ground any electromagnetic energy radiated from the device. 

Short circuit.  A low-resistance connection between two points in an electric circuit. 

Shower of Sparks ignition system.  A patented ignition system for reciprocating engines. An induction civrator sends pulsating DC into a set of retard breaker points on one of the magnetos. This provides a hot and retarded spark for starting the engine. 

Single-shaft turbine engine.  A turboprop engine in which the propeller reduction gears are driven by the same shaft that drives the compressor for the gas generator. 

Single-spool gas-turbine engine.  A type of axial-flow- compressor gas turbine engine that has only one rotating element. 

Skin radiator.  A type of radiator used on some early liquid- cooled racing airplanes. The radiator was made of two thin sheets of brass, slightly separated so the heated coolant could flow between them. Skin radiators were mounted on the surface of the wing, on the sides of the fuselage, or on the floats of seaplanes. Air flowing over the smooth surface of the radiator removed heat from the coolant. 

Slip (propeller specification).  The difference between the geometric and effective pitch of a propeller. 

Slip ring.  A smooth, continuous ring of brass or copper mounted on the rotor shaft of an electrical generator or alternator. Brushes riding on the smooth surface of the slip ring carry current into and out of the rotor coil. 

Slow-blow ruse. A special type of electrical circuit protection device that allows a momentary flow of excess current, but opens the circuit if the excessive flow is sustained. 

Sludge.  A heavy contaminant that forms in an aircraft engine lubricating oil because of oxidation and chemical decomposition of the oil. 

Sludge plugs.  Spool-shaped sheet metal plugs installed in the hollow throws of some engine crankshafts. 

Slug.  The unit of mass equal to that which experiences an acceleration of one foot per second, per second when a force of one pound acts on it. It is equal to 32.174 pounds, or 14.5939 kilograms, of mass. Also called a G-pound. 

SOAP.  Spectrometric oil analysis program. An oil analysis program in which a sample of oil is burned in an electric arc and an analysis is made of the wavelength composition of the resulting light. Each chemical element in the oil, when burned, produces light containing a unique band of frequencies. A computer analyzes the amount of each band of frequencies and prints out the number of parts of the element per million parts of the entire sample. SOAP can predict engine problems by warning the engine operator of an uncharacteristic increase of any elements in the oil. 

Sound suppressor. The airframe component that replaces the turbine engine tail pipe. It reduces the distance the sounds made by the exhaust gases propagate by converting low- frequency vibrations. 

Specific gravity. The ratio of the density of a material to the density of pure water. 

Specific weight. The ratio of the weight of an aircraft engine to the brake horsepower it develops. 

