Aviation Maintenance Terms beginning with S
Safety gap
A location in a magneto that allows a spark to jump to ground from the secondary circuit before the voltage rises high enough to damage the secondary insulation.
Sand casting
A method of molding metal parts in a mold made of sand. A pattern that duplicates the part to be molded is made of wood and is covered with a special casting sand that contains a resin to bind it. The mold is separated along a special parting line, and the pattern is removed. The mold is put back together, and molten metal is poured into the cavity. When the metal cools, the sand is broken away from the molded part. Sand casting is less expensive than permanent-mold casting.
Saybolt Seconds Universal (SSU) viscosity
A measurement of viscosity (resistance to flow)of a lubricating oil. The number of seconds needed for 60 milliliters of oil at a specified temperature to flow through a calibrated orifice. The viscosity number used for commercial aviation engine lubricating oil relates closely to the SSU viscosity of the oil at 210°F.
Scavenge subsystem
The subsystem in the lubrication system of a gas turbine engine that collects oil after it has lubricated the bearings and gears and returns it to the oil tank.
Scimitar shape
The shape of the blades of the propellers mounted on UHB engines. The name is derived from the shape of a curved Asian sword that has its edge on the convex side. See UHB engine.
Scramjet
Supersonic combustion ramjet. A special type of ramjet engine whose fuel can be ignited while the vehicle is mobbing at a supersonic speed.
Scuffing
Severe damage to moving parts caused when one metal part moves across another without sufficient lubricant between them. Enough heat is generated by friction to cause the high points of the surfaces to weld together; continued movement tears, or scuffs, the metal.
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 vibrator 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 fuse
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.
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.
Spline
Parallel slots cut in the periphery of a shaft, parallel to its length. Matching slots, cut into the hub or wheel that fits on the shaft, lock the shaft into the device to transmit torque.
Sprag clutch
A freewheeling, nonreversible clutch that allows torque to be applied to a driven unit in one direction only.
Springback
A condition in the rigging of an aircraft engine control in which the stop at the engine is reached before the stop in the flight deck. The flight deck control moves slightly after the stop in the engine is reached, and when it is released, it springs back slightly.
Spur-gear pump
A form of constant-displacement fluid pump that uses two meshing spur-gears mounted in a close fitting housing. Fluid is taken into the housing where it fills the space between the teeth of the gears and is carried around the housing as the gears rotate. On the discharge side of the pump, the teeth of the two gears mesh, and the fluid is forced out of the pump.
Squat switch
An electrical switch actuated by the landing gear scissors on the oleo strut. When no weight is on the landing gear, the oleo piston is extended and the switch is in one position; but when weight is on the gear, the oleo strut compresses and the switch changes its position.
Squealer tip (compressor blade tip)
See profile tip.
Squeeze film bearings
Another name for oil-damped bearings. See oil-damped bearings.
Stage length
The distance between landing points in airline operation.
Stage of a compressor
One disc of rotor blades and the following set of stator vanes in an axial-flow compressor.
Staggered timing
Ignition timing that causes the spark plug nearest the exhaust valve to fire a few degrees of crankshaft rotation before the spark plug nearest the intake valve.
Standard day conditions
Conditions that have been decided upon by the ICAO for comparing all aircraft and engine performance. The most basic standard day conditions are: temperature, 15°C or 59°F; altitude, mean sea level; pressure, 29.92 inches of mercury.
Standard J-1
A World War I training airplane powered by a Curtiss OX-5 engine.
Standpipe
A pipe which protrudes upward from the base of an oil tank and through which oil used for normal engine lubrication is drawn. In the event of a catastrophic leak when all oil available to the engine-driven pump is lost overboard, enough oil is available from an outlet below the standpipe to feather the propeller.
Starter-generator
A single-component starter and generator used on many smaller gas-turbine engines. It is used to start the engine, and when the engine is running, its circuitry is shifted so that it acts as a generator.
Static pressure
The pressure of an unmoving fluid.
Static rpm
The number of revolutions per minute an aircraft engine can produce when the aircraft is not moving.
Steam cooling
A method of liquid cooling in which the coolant, normally water, is allowed to absorb enough heat that it boils. The steam gives up its heat when it condenses back into a liquid.
Stellite
A nonferrous alloy of cobalt, chromium, and tungsten. Stellite is hard, water resistant, and corrosion resistant, and it does not soften until its temperature is extremely high. Stellite is welded to the faces of many reciprocating engine exhaust valves that operate at very high temperatures.
Stepping motor
A precision electric motor whose output shaft position is changed in steps by pulses from the control device. Stepping motors can make high-torque changes in small angular increments to their output shaft.
Stoichiometric mixture
The air/fuel mixture ratio that, when burned, leaves no uncombined oxygen nor any free carbon. It releases the maximum amount of heat, and therefore produces the highest exhaust gas temperature. A stoichiometric mixture of gasoline and air contains 15 pounds of air for 1 pound of gasoline.
Straight-through combustor
A combustor in a gas turbine engine through which the air from the compressor to the turbine flows in an essentially straight line.
Stratosphere
The upper part of the Earth’s atmosphere. The stratosphere extends upward from the tropopause, which is approximately 36,000 feet above the surface of the Earth, to approximately 85,000 feet. The temperature of the air in the stratosphere remains constant at -56.5°C (-69.7°F).
Stress
A force within an object that tries to prevent an outside force from changing its shape.
Stroboscopic tachometer
A tachometer used to measure the speed of any rotating device without physical contact. A highly accurate variable-frequency oscillator triggers a high-intensity strobe light. When the lamp is flashing at the same frequency the device is rotating, the device appears to stand still.
Stroke
The distance the piston moves inside the cylinder.
Sump (aircraft engine component)
A low point in an aircraft engine in which lubricating oil collects and is stored or transferred to an external oil tank. A removable sump attached to the bottom of the crankcase of a reciprocating engine is often called an oil pan.
Sump (fuel tank component)
A low point in an aircraft fuel tank in which water and other contaminants collect and are held until they can be drained out.
Supercharged engine
A reciprocating engine that uses a mechanically driven compressor to increase the air pressure before it enters the engine cylinders.
Supercharger
An air compressor used to increase the pressure of the air being taken into the cylinders of a reciprocating engine.
Surface roughness
The condition of the surface of a reciprocating engine cylinder wall that has been honed to make it hold lubricating oil. Surface roughness is measured in micro-inches rms.
Surge
A condition of unstable airflow, through the compressor of a gas turbine engine, in which the compressor blades have an excessive angle of attack. Surge usually affects an entire stage of compression.
Synthetic oil
Oil made by chemical synthesis of a mineral, animal, or vegetable base. Synthetic oils have appropriate additives that give them such characteristics as low volatility, low pour point, high viscosity index, good lubricating qualities, low coke and lacquer formation, and low foaming.