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 flightdeck. The flightdeck 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 disk 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 ºC or 59 ºF; 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 fuel-air 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 ºC (-69.7 ºF). 

