Surfactant.  A surface active agent, or partially soluble  contaminant, which is a by-product of fuel processing or of  fuel additives. Surfactants adhere to other contaminants and  cause them to drop out of the fuel and settle to the bottom  of the fuel tank as sludge. Surveyor’s transit.  An instrument consisting of a telescope  mounted on a flat, graduated, circular plate on a tripod.  The plate can be adjusted so it is level, and its graduations  oriented to magnetic north. When an object is viewed through  the telescope, its azimuth and elevation may be determined. Swashplate.  The component in a helicopter control system  that consists basically of two bearing races with ball bearings  between them. The lower, or nonrotating, race is tilted by the  cyclic control, and the upper, or rotating, race has arms which  connect to the control horns on the rotor blades. Movement  of the cyclic pitch control is transmitted to the rotor blades  through the swashplate. Movement of the collective pitch  control raises or lowers the entire swashplate assembly to  change the pitch of all the blades at the same time. Synchro system.  A remote instrument indicating system.  A synchro transmitter is actuated by the device whose  movement is to be measured, and it is connected electrically  with wires to a synchro indicator whose pointer follows the  movement of the shaft of the transmitter. Symmetrical airfoil.  An airfoil that has the same shape on  both sides of its chord line, or center line. Symmetry check.  A check of an airframe to determine that  the wings and tail are symmetrical about the longitudinal axis. System-pressure regulator (hydraulic system component).  A type of hydraulic system-pressure control valve. When the  system pressure is low, as it is when some unit is actuated,  the output of the constant-delivery pump is directed into the  system. When the actuation is completed and the pressure  builds up to a specified kick-out pressure, the pressure  regulator shifts. A check valve seals the system off and the  pressure is maintained by the accumulator. The pump is  unloaded and its output is directed back into the reservoir  with very little opposition. The pump output pressure drops,  but the volume of flow remains the same. When the system  pressure drops to the specified kick-in pressure, the regulator  again shifts and directs fluid into the system. Spool-type  and balanced-pressure-type system pressure regulators  are completely automatic in their operation and require no  attention on the part of the flight crew. Tetraethyl lead (TEL).  A heavy, oily, poisonous liquid,  Pb(C$_{2}$H$_{5}$)$^{4}$ , that is mixed into aviation gasoline to increase  its critical pressure and temperature. Terminal VOR.  A low-powered VOR that is normally  located on an airport. Terminal strips.  A group of threaded studs mounted in a  strip of insulating plastic. Electrical wires with crimped-on  terminals are placed over the studs and secured with nuts. Tempered glass.  Glass that has been heat-treated to increase  its strength. Tempered glass is used in bird-proof, heated  windshields for high-speed aircraft. Teflon.  The registered trade name for a fluorocarbon resin used  to make hydraulic and pneumatic seals, hoses, and backup rings. TCAS.  Traffic Alert Collision Avoidance System. Tang.  A tapered shank sticking out from the blade of a knife  or a file. The handle of a knife or file is mounted on the tang. Takeoff warning system.  An aural warning system that  provides audio warning signals when the thrust levers are  advanced for takeoff if the stabilizer, flaps, or speed brakes  are in an unsafe condition for takeoff. Tailets.  Small vertical surfaces mounted underside of  the horizontal stabilizer of some airplanes to increase the  directional stability. Tacky.  Slightly sticky to the touch. Tack weld.  A method of holding parts together before they  are permanently welded. The parts are assembled, and small  spots of weld are placed at strategic locations to hold them  in position. Tack rag.  A clean, lintless rag, slightly damp with thinner.  A tack rag is used to wipe a surface to prepare it to receive  a coat of finishing material. Tack coat.  A coat of finishing material sprayed on the surface  and allowed to dry until the solvents evaporate. As soon as  the solvents evaporate, a wet full-bodied coat of material is  sprayed over it. TACAN (Tactical Air Navigation).  A radio navigation  facility used by military aircraft for both direction and  distance information. Civilian aircraft receive distance  information from a TACAN on their DME. G-35