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. 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. 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. Sensitivity.  A measure of the signal strength needed to  produce a distortion-free output in a radio receiver. Sensible heat.  Heat that is added to a liquid causing a change  in its temperature but not its physical state. 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. 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. 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. 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. 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.  G-30