SDF.  See simplified directional facility. 

Selective availability (SA).  A satellite technology permitting the Department of Defense (DOD) to create, in the interest of national security, a significant clock and ephemeris error in the satellites, resulting in a navigation error. 

Semicircular canal. An inner ear organ that detects angular acceleration of the body. 

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. 

Sensitive altimeter.  A form of multipointer pneumatic altimeter with an adjustable barometric scale that allows the reference pressure to be set to any desired level. 

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. 

SIDS. See standard instrument departure procedures. 

SIGMET. The acronym for Significant Meteorological information. A weather advisory in abbreviated plain language concerning the occurrence or expected occurrence of potentially hazardous en route weather phenomena that may affect the safety of aircraft operations. SIGMET is warning information, hence it is of highest priority among other types of meteorological information provided to the aviation users. 

Signal-to-noise ratio.  An indication of signal strength received compared to background noise, which is a measure of the adequacy of the received signal. 

Significant weather prognostic.  Presents four panels showing forecast significant weather. 

Simplex.  Transmission and reception on the same frequency. 

Simplified directional facility (SDF).  A NAVAID used for nonprecision instrument approaches. The final approach course is similar to that of an ILS localizer; however, the SDF course may be offset from the runway, generally not more than 3°, and the course may be wider than the localizer, resulting in a lower degree of accuracy. 

Single-pilot resource management (SRM). The ability for a pilot to manage all resources effectively to ensure the outcome of the flight is successful. 

Situational awareness.  Pilot knowledge of where the aircraft is in regard to location, air traffic control, weather, regulations, aircraft status, and other factors that may affect flight. 

Skidding turn.  An uncoordinated turn in which the rate of turn is too great for the angle of bank, pulling the aircraft to the outside of the turn. 

Skills and procedures. The procedural, psychomotor, and perceptual skills used to control a specific aircraft or its systems. They are the airmanship abilities that are gained through conventional training, are perfected, and become almost automatic through experience. 

Skin friction drag. Drag generated between air molecules and the solid surface of the aircraft. 

Slant range.  The horizontal distance from the aircraft antenna to the ground station, due to line-of-sight transmission of the DME signal. 

Slaved compass.  A system whereby the heading gyro is “slaved to,” or continuously corrected to bring its direction readings into agreement with a remotely located magnetic direction sensing device (usually a flux valve or flux gate compass). 

Slipping turn.  An uncoordinated turn in which the aircraft is banked too much for the rate of turn, so the horizontal lift component is greater than the centrifugal force, pulling the aircraft toward the inside of the turn. 

Small airplane. An airplane of 12,500 pounds or less maximum certificated takeoff weight. 

Somatogravic illusion.  The misperception of being in a nose-up or nose-down attitude, caused by a rapid acceleration or deceleration while in flight situations that lack visual reference. 

