Static.  Still, not moving. 

Static air pressure.  Pressure of the ambient air surrounding the aircraft. Static pressure does not take into consideration any air movement. 

Static dischargers.  Devices connected to the trailing edges of control surfaces to discharge static electricity harmlessly into the air. They discharge the static charges before they can build up high enough to cause radio receiver interference. 

Static stability.  The characteristic of an aircraft that causes it to return to straight and level flight after it has been disturbed from that condition. 

Stoddard solvent.  A petroleum product, similar to naphtha, used as a solvent and a cleaning fluid. 

STOL.  Short takeoff and landing. 

Stop drilling. A method of stopping the growth of a crack in a piece of metal or transparent plastic by drilling a small hole at the end of the crack. The stresses are spread out all around the circumference of the hole rather than concentrated at the end of the crack. 

Straight polarity welding.  DC-electric arc welding in which the electrode is negative with respect to the work. 

Strain. A deformation or physical change in a material caused by a stress. 

Stress.  A force set up within an object that tries to prevent an outside force from changing its shape. 

Stressed skin structure.  A type of aircraft structure in which all or most of the stresses are carried in the outside skin. A stressed skin structure has a minimum of internal structure. 

Stress riser.  A location where the cross-sectional area of the part changes abruptly. Stresses concentrate at such a location and failure is likely. A scratch, gouge, or tool mark in the surface of a highly stressed part can change the area enough to concentrate the stresses and become a stress riser. 

Stringer. A part of an aircraft structure used to give the fuselage its shape and, in some types of structure, to provide a small part of fuselage strength. Formers give the fuselage its cross-sectional shape and stringers fill in the shape between the formers. 

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. 

Sublimation. A process in which a solid material changes directly into a vapor without passing through the liquid stage. 

Subsonic flight.  Flight at an airspeed in which all air flowing over the aircraft is moving at a speed below the speed of sound. 

Summerwood.  The less porous, usually harder portion of an annual ring that forms in the latter part of the growing season, the summer of the year. 

Sump. A low point in an aircraft fuel tank in which water and other contaminants can collect and be held until they can be drained out. 

Supercooled water.  Water in its liquid form at a temperature well below its natural freezing temperature. When supercooled water is disturbed, it immediately freezes. 

Superheat.  Heat energy that is added to a refrigerant after it changes from a liquid to a vapor. 

Super heterodyne circuit. A sensitive radio receiver circuit in which a local oscillator produces a frequency that is a specific difference from the received signal frequency. The desired signal and the output from the oscillator are mixed, and they produce a single, constant intermediate frequency. This IF is amplified, demodulated, and detected to produce the audio frequency that is used to drive the speaker. 

Supersonic flight.  Flight at an airspeed in which all air flowing over the aircraft is moving at a speed greater than the speed of sound. 

Supplemental Type Certificate (STC).  An approval issued by the FAA for a modification to a type certificated airframe, engine, or component. More than one STC can be issued for the same basic alteration, but each holder must prove to the FAA that the alteration meets all the requirements of the original type certificate. 

Surface tape.  Strips of aircraft fabric that are doped over all seams and places where the fabric is stitched to the aircraft structure. Surface tape is also doped over the wing leading edges where abrasive wear occurs. The edges of surface tape are pink, or notched, to keep them from raveling before the dope is applied. 

