Square.  A four-sided plane figure whose sides are all the  same length, whose opposite sides are parallel, and whose  angles are all right angles. Spike knot.  A knot that runs through the depth of a beam  perpendicular to the annual rings. Spike knots appear most  frequently in quartersawed wood. Spin.  A flight maneuver in which an airplane descends in  a corkscrew fashion. One wing is stalled and the other is  producing lift. Squat switch.  An electrical switch actuated by the landing  gear scissors on the oleo strut. When no weight is on the  landing gear, the oleo piston is extended and the switch is  in one position, but when weight is on the gear, the oleo  strut compresses and the switch changes its position. Squat  switches are used in antiskid brake systems, landing gear  safety circuits, and cabin pressurization systems. Spirit level.  A curved glass tube partially filled with a liquid,  but with a bubble in it. When the device in which the tube is  mounted is level, the bubble will be in the center of the tube. Splayed patch (wood structure repair).  A type of patch  made in an aircraft plywood structure in which the edges  of the patch are tapered for approximately five times  the thickness of the plywood. A splayed patch is not  recommended for use on plywood less than  1 ⁄ 10 inch thick. Squib.  An explosive device in the discharge valve of a  high-rate-discharge container of fire-extinguishing agent.  The squib drives a cutter into the seal in the container to  discharge the agent. SRM.  Structural Repair Manual. Split bus.  A type of electrical bus that allows all of the  voltage-sensitive avionic equipment to be isolated from the  rest of the aircraft electrical system when the engine is being  started or when the ground-power unit is connected. Split-rocker switch.  An electrical switch whose operating  rocker is split so one half of the switch can be opened without  affecting the other half. Split-rocker switches are used as  aircraft master switches. The battery can be turned on without  turning on the alternator, but the alternator cannot be turned  on without also turning on the battery. The alternator can  be turned off without turning off the battery, but the battery  cannot be turned off without also turning off the alternator. Stabilator.  A flight control on the empennage of an airplane  that acts as both a stabilizer and an elevator. The entire  horizontal tail surface pivots and is moved as a unit. Stability.  The characteristic of an aircraft that causes it to  return to its original flight condition after it has been disturbed. Stabilons.  Small wing-like horizontal surfaces mounted on  the aft fuselage to improve longitudinal stability of airplanes  that have an exceptionally wide center of gravity range. Stagnation point.  The point on the leading edge of a wing  at which the airflow separates, with some flowing over the  top of the wing and the rest below the wing. Split (wood defect).  A longitudinal crack in a piece of wood  caused by externally induced stress. Spoilers.  Flight controls that are raised up from the upper  surface of a wing to destroy, or spoil, lift. Flight spoilers are  used in conjunction with the ailerons to decrease lift and  increase drag on the descending wing. Ground spoilers are  used to produce a great amount of drag to slow the airplane  on its landing roll. Stall.  A flight condition in which an angle of attack is reached  at which the air ceases to flow smoothly over the upper  surface of an airfoil. The air becomes turbulent and lift is lost. Stall strip.  A fixed device employed on the leading edge  of fixed-wing aircraft to initiate flow separation at chosen  locations on the wing during high-angle of attack flight,  so as to improve the controllability of the aircraft when it  enters stall. Standpipe.  A pipe sticking up in a tank or reservoir that  allows part of the tank to be used as a reserve, or standby,  source of fluid. Spongy brakes.  Hydraulic brakes whose pedal has a spongy  feel because of air trapped in the fluid. Spontaneous combustion.  Self-ignition of a material caused  by heat produced in the material as it combines with oxygen  from the air. Springwood.  The portion of an annual ring in a piece of  wood formed principally during the first part of the growing  season, the spring of the year. Springwood is softer, more  porous, and lighter than the summerwood. Starter-generator.  A single-component starter and generator  used on many of the smaller gas-turbine engines. It is used  as a starter, and when the engine is running, its circuitry is  shifted so that it acts as a generator. G-33