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.  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.  SDF.  See simplified directional facility.  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.  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.  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.  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.  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.  Skin friction drag.  Drag generated between air molecules  and the solid surface of the aircraft.  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.  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).  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.  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.  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.  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.  Significant weather prognostic.  Presents four panels  showing forecast significant weather.  Simplex.  Transmission and reception on the same frequency.  G-27 