Relative wind.  Direction of the airflow produced by an object moving through the air. The relative wind for an airplane in flight flows in a direction parallel with and opposite to the direction of flight; therefore, the actual flight path of the airplane determines the direction of the relative wind. 

Remote communications outlet (RCO). An unmanned communications facility that is remotely controlled by air traffic personnel. 

Required navigation performance (RNP). A specified level of accuracy defined by a lateral area of confined airspace in which an RNP-certified aircraft operates. 

Restricted area. Airspace designated under 14 CFR part 73 within which the flight of aircraft, while not wholly prohibited, is subject to restriction. 

Reverse sensing.  The VOR needle appearing to indicate the reverse of normal operation. 

RF.  Radio frequency. 

Rhodopsin. The photosensitive pigments that initiate the visual response in the rods of the eye. 

Rigidity.  The characteristic of a gyroscope that prevents its axis of rotation tilting as the Earth rotates. 

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. 

RMI.  See radio magnetic indicator. 

RNAV.  See area navigation. 

RNP. See required navigation performance. 

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 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. 

RVR.  See runway visual range. 

RVV.  See runway visibility value. 

St. Elmo’s Fire.  A corona discharge which lights up the aircraft surface areas where maximum static discharge occurs. 

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. 

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. 

SDF.  See simplified directional facility. 

Selective availability.  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. 

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. 

SIDS. See standard instrument departure procedures. 

SIGMET. The acronym for Significant Meteorological information. A weather advisory issued concerning weather significant to the safety of all aircraft. 

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

