Servo amplifier.  An electronic amplifier in an autopilot  system that increases the signal from the autopilot enough  that it can operate the servos that move the control surfaces. Servo tab.  A small movable tab built into the trailing edge  of a primary control surface of an airplane. The flight deck  controls move the tab in such a direction that it produces an  aerodynamic force moving the surface on which it is mounted. Setback.  The distance the jaws of a brake must be set back  from the mold line to form a bend. Setback for a 90° bend  is equal to the inside radius of the bend plus the thickness of  the metal being bent. For a bend other than 90°, a K-factor  must be used. See also K-factor. Shake (wood defect).  Longitudinal cracks in a piece of wood,  usually between two annual rings. SHF.  Super-high frequency. Shear section.  A necked-down section of the drive shaft of a  constant-displacement engine-driven fluid pump. If the pump  should seize, the shear section will break and prevent the pump  from being destroyed or the engine from being damaged. Some  pumps use a shear pin rather than a shear section. Shear strength.  The strength of a riveted joint in a sheet  metal structure in which the rivets shear before the metal  tears at the rivet holes. Shelf life.  The length of time a product is good when it  remains in its original unopened container. Shielded wire.  Electrical wire enclosed in a braided metal  jacket. Electromagnetic energy radiated from the wire is  trapped by the braid and is carried to ground. Shimmy.  Abnormal, and often violent, vibration of the  nose wheel of an airplane. Shimmying is usually caused  by looseness of the nose wheel support mechanism or an  unbalanced wheel. Shimmy damper.  A small hydraulic shock absorber installed  between the nose wheel fork and the nose wheel cylinder  attached to the aircraft structure. Shock mounts.  Resilient mounting pads used to protect  electronic equipment by absorbing low-frequency, high  amplitude vibrations. Silicon controlled rectifier (SCR).  A semiconductor  electron control device. An SCR blocks current flow in  both directions until a pulse of positive voltage is applied  to its gate. It then conducts in its forward direction, while  continuing to block current in its reverse direction. Sight line.  A line drawn on a sheet metal layout that is one  bend radius from the bend-tangent line. The sight line is  lined up directly below the nose of the radius bar in a cornice  brake. When the metal is clamped in this position, the bend  tangent line is in the correct position for the start of the bend. Sight glass (air conditioning system component).  A small  window in the high side of a vapor-cycle cooling system.  Liquid refrigerant flows past the sight glass, and if the charge  of refrigerant is low, bubbles will be seen. A fully charged  system has no bubbles in the refrigerant. Sidestick controller.  A flight deck flight control used on  some of the fly-by-wire equipped airplanes. The stick is  mounted rigidly on the side console of the flight deck, and  pressures exerted on the stick by the pilot produce electrical  signals that are sent to the computer that flies the airplane. Shuttle valve.  An automatic selector valve mounted on  critical components such as landing gear actuation cylinders  and brake cylinders. For normal operation, system fluid flows  into the actuator through the shuttle valve, but if normal  system pressure is lost, emergency system pressure forces  the shuttle over and emergency fluid flows into the actuator. Shunt winding.  Field coils in an electric motor or generator  that are connected in parallel with the armature. Show-type finish.  The type of finish put on fabric-covered  aircraft intended for show. This finish is usually made up of  many coats of dope, with much sanding and rubbing of the  surface between coats. Shop head.  The head of a rivet which is formed when the  shank is upset. Shock wave.  A pressure wave formed in the air by a flight  vehicle moving at a speed greater than the speed of sound. As  the vehicle passes through the air, it produces sound waves  that spread out in all directions. But since the vehicle is flying  faster than these waves are moving, they build up and form a  pressure wave at the front and rear of the vehicle. As the air  passes through a shock wave it slows down, its static pressure  increases, and its total energy decreases. G-31