Pressure demand oxygen system.  A demand oxygen system  that supplies 100 percent oxygen at sufficient pressure above  the altitude where normal breathing is adequate. Also referred  to as a pressure breathing system.  Prevailing visibility.  The greatest horizontal visibility  equaled or exceeded throughout at least half the horizon  circle (which is not necessarily continuous).  Preventive maintenance.  Simple or minor preservative  operations and the replacement of small standard parts  not involving complex assembly operation as listed in 14  CFR part 43, appendix A. Certificated pilots may perform  preventive maintenance on any aircraft that is owned or  operated by them provided that the aircraft is not used in air  carrier service.  Primary and supporting.  A method of attitude instrument  flying using the instrument that provides the most direct  indication of attitude and performance.  Primary flight display (PFD).  A display that provides  increased situational awareness to the pilot by replacing the  traditional six instruments used for instrument flight with  an easy-to-scan display that provides the horizon, airspeed,  altitude, vertical speed, trend, trim, and rate of turn among  other key relevant indications.  PRM.  See precision runway monitor.  Procedure turn.  A maneuver prescribed when it is necessary  to reverse direction to establish an aircraft on the intermediate  approach segment or final approach course.  Profile view.  Side view of an IAP chart illustrating the vertical  approach path altitudes, headings, distances, and fixes.  Prohibited area.  Designated airspace within which flight  of aircraft is prohibited.  Propeller.  A device for propelling an aircraft that, when  rotated, produces by its action on the air, a thrust approximately  perpendicular to its plane of rotation. It includes the control  components normally supplied by its manufacturer.  Propeller/rotor modulation error.  Certain propeller  rpm settings or helicopter rotor speeds can cause the VOR  course deviation indicator (CDI) to fluctuate as much as  ±6°. Slight changes to the rpm setting will normally smooth  out this roughness.  Radio or radar altimeter.  An electronic altimeter that  determines the height of an aircraft above the terrain by  measuring the time needed for a pulse of radio-frequency  energy to travel from the aircraft to the ground and return.  Radio frequency (RF).  A term that refers to alternating  current (AC) having characteristics such that, if the current is  input to antenna, an electromagnetic (EM) field is generated  suitable for wireless broadcasting and/or communications.  Radials.  The courses oriented from a station.  Radar weather report (SD).  A report issued by radar  stations at 35 minutes after the hour, and special reports  as needed. Provides information on the type, intensity, and  location of the echo tops of the precipitation.  Radar summary chart.  A weather product derived from the  national radar network that graphically displays a summary  of radar weather reports.  Radar services.  Radar is a method whereby radio waves are  transmitted into the air and are then received when they have  been reflected by an object in the path of the beam. Range is  determined by measuring the time it takes (at the speed of light)  for the radio wave to go out to the object and then return to the  receiving antenna. The direction of a detected object from a  radar site is determined by the position of the rotating antenna  when the reflected portion of the radio wave is received.  Radar approach.  The controller provides vectors while  monitoring the progress of the flight with radar, guiding  the pilot through the descent to the airport/heliport or to a  specific runway.  Radar.  A system that uses electromagnetic waves to identify  the range, altitude, direction, or speed of both moving and  fixed objects such as aircraft, weather formations, and terrain.  The term RADAR was coined in 1941 as an acronym for  Radio Detection and Ranging. The term has since entered  the English language as a standard word, radar, losing the  capitalization in the process.  Rabbit, the.  High-intensity flasher system installed at many  large airports. The flashers consist of a series of brilliant  blue-white bursts of light flashing in sequence along the  approach lights, giving the effect of a ball of light traveling  toward the runway.  R  G-24 