- Radar frequency Operating bands of airborne radars are given according to frequency. That part of the electromagnetic spectrum appropriate to above-surface short-range communication and radar (but not OTH) used in aviation is given in the adjacent table with an approximate cross-reference to previously used wavelength bands.
- radius The approximate distance an aircraft can fly from base and return without intermediate landing.
- RAI Registro Aeronautico Italiano (Italian civil aviation authority).
- RAM Radar absorbent material.
- ramp weight Maximum weight at start of flight (MTOW plus taxi/run-up fuel).
- range Too many definitions to list, but essentially the distance an aircraft can fly (or is permitted to fly) with specified load and usually while making allowance for specified additional manoeuvres (diversions, standoff, go-around and so on).
- RAS Rectified airspeed, IAS corrected for position error.
- raster Generation of large-area display, for example TV screen, by close-spaced horizontal lines scanned either alternately or in sequence.
- RAT Ram air turbine.
- rating Any of several values of thrust of shaft power which an engine is qualified (usually also guaranteed) to develop under specified conditions.
- RCS Radar cross-section; apparent size of echo.
- redundant Provided with spare capacity or data channels and thus able to survive failures.
- reversion Ability to switch to manual control following failure of a powered system.
- RFAS Russian Federation and Associated States (CIS).
- RFP Request(s) for proposals.
- rigid rotor See bearingless rotor.
- RMI Radio magnetic indicator; combines compass and navaid bearings.
- R/nav Calculates position, distance and time from groups of airways beacons.
- RON Research octane number of fuel.
- roving Multiple strands of fibre, as in a rope (but usually not twisted).
- rpm Revolutions per minute.
- RPV Remotely piloted vehicle (pilot in other aircraft or on ground); contrast UAV.
- RSA Réseau du Sport de l'Air.
- ruddervators Flying control surfaces, usually a V tail, that control both yaw and pitch attitude.
- RVSM Reduced vertical separation minimum. Halved (1,000 ft) air traffic control separation between FL290 and FL410.
- RWR Radar warning receiver.
