RF.  Radio frequency.  Rhodopsin.  The photosensitive pigments that initiate the  visual response in the rods of the eye.  Rigging.  The final adjustment and alignment of an aircraft  and its flight control system that provides the proper  aerodynamic characteristics.  Rigidity.  The characteristic of a gyroscope that prevents its  axis of rotation tilting as the Earth rotates.  Rigidity in space.  The principle that a wheel with a heavily  weighted rim spinning rapidly will remain in a fixed position  in the plane in which it is spinning.  Rime ice.  Rough, milky, opaque ice formed by the  instantaneous freezing of small supercooled water droplets.  Risk.  The future impact of a hazard that is not eliminated  or controlled.  Risk elements.  There are four fundamental risk elements  in aviation: the pilot, the aircraft, the environment, and the  type of operation that comprise any given aviation situation.  Risk management.  The part of the decision-making  process which relies on situational awareness, problem  recognition, and good judgment to reduce risks associated  with each flight.  RMI.  See radio magnetic indicator.  RNAV.  See area navigation.  RNP.  See required navigation performance.  RT.  See receiver-transmitter.  Rudder.  The movable primary control surface mounted on  the trailing edge of the vertical fin of an airplane. Movement  of the rudder rotates the airplane about its vertical axis.  Ruddervator.  A pair of control surfaces on the tail of an  aircraft arranged in the form of a V. These surfaces, when  moved together by the control wheel, serve as elevators,  and when moved differentially by the rudder pedals, serve  as a rudder.  Runway centerline lights.  Runway lighting which consists  of flush centerline lights spaced at 50-foot intervals beginning  75 feet from the landing threshold.  G-26  Sectional aeronautical charts.  Designed for visual  navigation of slow- or medium-speed aircraft. Topographic  information on these charts features the portrayal of relief,  and a judicious selection of visual check points for VFR  flight. Aeronautical information includes visual and radio  aids to navigation, airports, controlled airspace, restricted  areas, obstructions and related data.  Scan.  The first fundamental skill of instrument flight,  also known as “cross-check;” the continuous and logical  observation of instruments for attitude and performance  information.  Sea level engine.  A reciprocating aircraft engine having a  rated takeoff power that is producible only at sea level.  Sea breeze.  A coastal breeze blowing from sea to land  caused by the temperature difference when the land surface  is warmer than the sea surface. The sea breeze usually occurs  during the day and alternates with the land breeze that blows  in the opposite direction at night.  Satellite ephemeris data.  Data broadcast by the GPS  satellite containing very accurate orbital data for that satellite,  atmospheric propagation data, and satellite clock error data.  St. Elmo’s Fire.  A corona discharge which lights up the  aircraft surface areas where maximum static discharge occurs.  SA.  See selective availability.  RVV.  See runway visibility value.  RVR.  See runway visual range.  Runway visibility value (RVV).  The visibility determined  for a particular runway by a transmissometer.  Runway visual range (RVR).  The instrumentally derived  horizontal distance a pilot should be able to see down the  runway from the approach end, based on either the sighting  of high-intensity runway lights, or the visual contrast of  other objects.  Runway end identifier lights (REIL).  A pair of synchronized  flashing lights, located laterally on each side of the runway  threshold, providing rapid and positive identification of the  approach end of a runway.  Runway edge lights.  A component of the runway lighting  system that is used to outline the edges of runways at night  or during low visibility conditions. These lights are classified  according to the intensity they are capable of producing.  S 