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 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 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 ºC (-69.7 ºF).

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.