SAFETY (SQUAT) SWITCH—An electrical switch mounted on one of the landing gear struts. It is used to sense when the weight of the aircraft is on the wheels.

SCAN—A procedure used by the pilot to visually identify all resources of information in flight.

SEA LEVEL—A reference height used to determine standard atmospheric conditions and altitude measurements.

SEGMENTED CIRCLE—A visual ground based structure to provide traffic pattern information.

SERVICE CEILING— The maximum density altitude where the best rate-of-climb airspeed will produce a 100 feet-per-minute climb at maximum weight while in a clean configuration with maximum continuous power.

SERVO TAB—An auxiliary control mounted on a primary control surface, which automatically moves in the direction opposite the primary control to provide an aerodynamic assist in the movement of the control.

SHAFT HORSE POWER (SHP)— Turboshaft engines are rated in shaft horsepower and calculated by use of a dynamometer device. Shaft horsepower is exhaust thrust converted to a rotating shaft.

SHOCK WAVES—A compression wave formed when a body moves through the air at a speed greater than the speed of sound.

SIDESLIP—A slip in which the airplane’s longitudinal axis remains parallel to the original flightpath, but the airplane no longer flies straight ahead. Instead, the horizontal component of wing lift forces the airplane to move sideways toward the low wing.

SINGLE ENGINE ABSOLUTE CEILING—The altitude that a twin-engine airplane can no longer climb with one engine inoperative.

SINGLE ENGINE SERVICE CEILING—The altitude that a twin-engine airplane can no longer climb at a rate greater then 50 f.p.m. with one engine inoperative.

SKID—A condition where the tail of the airplane follows a path outside the path of the nose during a turn.

SLIP—An intentional maneuver to decrease airspeed or increase rate of descent, and to compensate for a crosswind on landing. A slip can also be unintentional when the pilot fails to maintain the aircraft in coordinated flight.

SPECIFIC FUEL CONSUMPTION— Number of pounds of fuel consumed in 1 hour to produce 1 HP.

SPEED—The distance traveled in a given time.

SPEED BRAKES—A control system that extends from the airplane structure into the airstream to produce drag and slow the airplane.

SPEED INSTABILITY— A condition in the region of reverse command where a disturbance that causes the airspeed to decrease causes total drag to increase, which in turn, causes the airspeed to decrease further.

SPEED SENSE—The ability to sense instantly and react to any reasonable variation of airspeed.

SPIN—An aggravated stall that results in what is termed an “autorotation” wherein the airplane follows a downward corkscrew path. As the airplane rotates around the vertical axis, the rising wing is less stalled than the descending wing creating a rolling, yawing, and pitching motion.

SPIRAL INSTABILITY— A condition that exists when the static directional stability of the airplane is very strong as compared to the effect of its dihedral in maintaining lateral equilibrium.

SPIRALING SLIPSTREAM—The slipstream of a propeller-driven airplane rotates around the airplane. This slipstream strikes the left side of the vertical fin, causing the airplane to yaw slightly. Vertical stabilizer offset is sometimes used by aircraft designers to counteract this tendency.

SPLIT SHAFT TURBINE ENGINE—See FREE POWER TURBINE ENGINE.

SPOILERS—High-drag devices that can be raised into the air flowing over an airfoil, reducing lift and increasing drag. Spoilers are used for roll control on some aircraft. Deploying spoilers on both wings at the same time allows the aircraft to descend without gaining speed. Spoilers are also used to shorten the ground roll after landing.

SPOOL—A shaft in a turbine engine which drives one or more compressors with the power derived from one or more turbines.

STABILATOR—A single-piece horizontal tail surface on an airplane that pivots around a central hinge point. A stabilator serves the purposes of both the horizontal stabilizer and the elevator.

STABILITY—The inherent quality of an airplane to correct for conditions that may disturb its equilibrium, and to return or to continue on the original flightpath. It is primarily an airplane design characteristic.

STABILIZED APPROACH—A landing approach in which the pilot establishes and maintains a constant angle glidepath towards a predetermined point on the landing runway. It is based on the pilot’s judgment of certain visual cues, and depends on the maintenance of a constant final descent airspeed and configuration.

STALL—A rapid decrease in lift caused by the separation of airflow from the wing’s surface brought on by exceeding the critical angle of attack. A stall can occur at any pitch attitude or airspeed.

STALL STRIPS—A spoiler attached to the inboard leading edge of some wings to cause the center section of the wing to stall before the tips. This assures lateral control throughout the stall.

STANDARD ATMOSPHERE— At sea level, the standard atmosphere consists of a barometric pressure of 29.92 inches of mercury (in. Hg.) or 1013.2 millibars, and a temperature of 15°C (59°F). Pressure and temperature normally decrease as altitude increases. The standard lapse rate in the lower atmosphere for each 1,000 feet of altitude is approximately 1 in. Hg. and 2°C (3.5°F). For example, the standard pressure and temperature at 3,000 feet mean sea level (MSL) is 26.92 in. Hg. (29.92 - 3) and 9°C (15°C - 6°C).

STANDARD DAY— See STANDARD ATMOSPHERE.

STANDARD EMPTY WEIGHT (GAMA)—This weight consists of the airframe, engines, and all items of operating equipment that have fixed locations and are permanently installed in the airplane; including fixed ballast, hydraulic fluid, unusable fuel, and full engine oil.

STANDARD WEIGHTS—These have been established for numerous items involved in weight and balance computations. These weights should not be used if actual weights are available.

STANDARD-RATE TURN—A turn at the rate of 3º per second which enables the airplane to complete a 360º turn in 2 minutes.

STARTER/GENERATOR— A combined unit used on turbine engines. The device acts as a starter for rotating the engine, and after running, internal circuits are shifted to convert the device into a generator.

STATIC STABILITY—The initial tendency an aircraft displays when disturbed from a state of equilibrium.

STATION—A location in the airplane that is identified by a number designating its distance in inches from the datum. The datum is, therefore, identified as station zero. An item located at station +50 would have an arm of 50 inches.

STICK PULLER—A device that applies aft pressure on the control column when the airplane is approaching the maximum operating speed.

STICK PUSHER—A device that applies an abrupt and large forward force on the control column when the airplane is nearing an angle of attack where a stall could occur.

STICK SHAKER—An artificial stall warning device that vibrates the control column.

STRESS RISERS— A scratch, groove, rivet hole, forging defect or other structural discontinuity that causes a concentration of stress.

SUBSONIC—Speed below the speed of sound.

SUPERCHARGER—An engine- or exhaust-driven air compressor used to provide additional pressure to the induction air so the engine can produce additional power.

SUPERSONIC—Speed above the speed of sound.

SUPPLEMENTAL TYPE CERTIFICATE (STC)— A certificate authorizing an alteration to an airframe, engine, or component that has been granted an Approved Type Certificate.

SWEPT WING—A wing planform in which the tips of the wing are farther back than the wing root.