Schematic diagram.  A diagram of an electrical system in which the system components are represented by symbols rather than drawings or pictures of the actual devices. 

Schrader valve.  A type of service valve used in an air conditioning system. This is a spring-loaded valve much like the valve used to put air into a tire. 

Scissors. A name commonly used for torque links. See torque links. 

Scrim cloth. Scrim cloth can be used in repair applications or for reinforcement of other types of materials including fiberglass, concrete and some plastics. When fully cured, the scrim cloth will add reinforcement and mimic the expansion and contraction of the surrounding substrate. 

Scupper.  A recess around the filler neck of an aircraft fuel tank. Any fuel spilled when the tank is being serviced collects in the scupper and drains to the ground through a drain line rather than flowing into the aircraft structure. 

Sea level engine.  A reciprocating engine whose rated takeoff power can be produced only at sea level. 

Sector gear.  A part of a gear wheel containing the hub and a portion of the rim with teeth. 

Series circuit.  A method of connecting electrical components in such a way that all the current flows through each of the components. There is only one path for current to flow. 

Series-parallel circuit.  An electrical circuit in which some of the components are connected in parallel and others are connected in series. 

Selcal system.  Selective calling system. Each aircraft operated by an airline is assigned a particular four-tone audio combination for identification purposes. A ground station keys the signal whenever contact with that particular aircraft is desired. The signal is decoded by the airborne selcal decoder and the crew alerted by the selcal warning system. 

Selsyn system.  A DC synchro system used in remote indicating instruments. The rotor in the indicator is a permanent magnet and the stator is a tapped toroidal coil. The transmitter is a circular potentiometer with DC power fed into its wiper which is moved by the object being monitored. The transmitter is connected to the indicator in such a way that rotation of the transmitter shaft varies the current in the sections of the indicator toroidal coil. The magnet in the indicator on which the pointer is mounted locks with the magnetic field produced by the coils and follows the rotation of the transmitter shaft. 

Segmented-rotor brake. A heavy-duty, multiple-disk brake used on large, high-speed aircraft. Stators that are surfaced with a material that retains its friction characteristics at high temperatures are keyed to the axle. Rotors which are keyed into the wheels mesh with the stators. The rotors are made in segments to allow for cooling and for their large amounts of expansion. 

Selector valve.  A flow control valve used in hydraulic systems that directs pressurized fluid into one side of an actuator, and at the same time directs return fluid from the other side of the actuator back to the reservoir. There are two basic types of selector valves: open-center valves and closed-center valves. The four-port closed-center valve is the most frequently used type. See closed-center selector valve and open-center selector valve. 

Selvage edge.  The woven edge of fabric used to prevent the material unraveling during normal handling. The selvage edge, which runs the length of the fabric parallel to the warp threads, is usually removed from materials used in composite construction. 

Semiconductor diode.  A two-element electrical component that allows current to pass through it in one direction, but blocks its passage in the opposite direction. A diode acts in an electrical system in the same way a check valve acts in a hydraulic system. 

Semimonocoque structure. A form of aircraft stressed skin structure. Most of the strength of a semimonocoque structure is in the skin, but the skin is supported on a substructure of formers and stringers that give the skin its shape and increase its rigidity. 

Sensible heat.  Heat that is added to a liquid causing a change in its temperature but not its physical state. 

Sensitivity.  A measure of the signal strength needed to produce a distortion-free output in a radio receiver. 

Sequence valve. A valve in a hydraulic system that requires a certain action to be completed before another action can begin. Sequence valves are used to assure that the hydraulically actuated wheel-well doors are completely open before pressure is directed to the landing gear to lower it. 

Servo. An electrical or hydraulic actuator connected into a flight control system. A small force on the flight deck control is amplified by the servo and provides a large force to move the control surface. 

