Aeronautical Terms beginning with S
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.
Sea level engine
A reciprocating aircraft engine having a rated takeoff power that is producible only at sea level.
Semimonocoque
A fuselage design that includes a substructure of bulkheads and/or formers, along with stringers, to support flight loads and stresses imposed on the fuselage.
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
A motor or other form of actuator which receives a small signal from the control device and exerts a large force to accomplish the desired work.
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.
Skin friction drag
Drag generated between air molecules and the solid surface of the aircraft.
Small airplane
An airplane of 12,500 pounds or less maximum certificated takeoff weight.
Special fuel consumption
The amount of fuel in pounds per hour consumed or required by an engine per brake horsepower or per pound of thrust.
Specific Fuel Consumption
Number of pounds of fuel consumed in 1 hour to produce 1 HP.
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.
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.
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.
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 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
Weights established for numerous items involved in weight and balance computations. These weights should not be used if actual weights are available.
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 Load
The load imposed on an aircraft structure due to the weight of the aircraft and its contents.
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.
Stress Risers
A scratch, groove, rivet hole, forging defect or other structural discontinuity that causes a concentration of stress.
Structural Station
This is a location in the aircraft, such as a bulkhead, which is identified by a number designating its distance in inches or percent MAC from the datum. The datum is, therefore, identified as station zero. The stations and arms are identical. An item located at station +50 would have an arm of 50 inches.
Subsonic
Speed below the speed of sound.
Suction relief valve
A relief valve in an instrument vacuum system required to maintain the correct low pressure inside the instrument case for the proper operation of the gyros.
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.
Synchro
A device used to transmit indications of angular movement or position from one location to another.
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