Reverse thrust.  A condition where jet thrust is directed  forward during landing to increase the rate of deceleration.  Reversing propeller.  A propeller system with a pitch change  mechanism that includes full reversing capability. When the  pilot moves the throttle controls to reverse, the blade angle  changes to a pitch angle and produces a reverse thrust, which  slows the airplane down during a landing.  Runway end identifier lights (REIL).  One component of  the runway lighting system. These lights are installed at many  airfields to provide rapid and positive identification of the  approach end of a particular runway.  Runway incursion.  Any occurrence at an airport involving  an aircraft, vehicle, person, or object on the ground that  creates a collision hazard or results in loss of separation  with an aircraft taking off, intending to takeoff, landing, or  intending to land.  Roll.  The motion of the aircraft about the longitudinal axis.  It is controlled by the ailerons.  Roundout (flare).  A pitch-up during landing approach to  reduce rate of descent and forward speed prior to touchdown.  Runway threshold markings.  Runway threshold markings  come in two configurations. They either consist of eight  longitudinal stripes of uniform dimensions disposed  symmetrically about the runway centerline, or the number of  stripes is related to the runway width. A threshold marking  helps identify the beginning of the runway that is available  for landing. In some instances, the landing threshold may  be displaced.  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.  Safety (SQUAT) switch.  An electrical switch mounted on  one of the landing gear struts. It is used to sense when the  weight of the aircraft is on the wheels.  Scan.  A procedure used by the pilot to visually identify all  resources of information in flight.  Runway centerline lights.  Runway centerline lights are  installed on some precision approach runways to facilitate  landing under adverse visibility conditions. They are located  along the runway centerline and are spaced at 50-foot  intervals. When viewed from the landing threshold, the  runway centerline lights are white until the last 3,000 feet of  the runway. The white lights begin to alternate with red for  the next 2,000 feet, and for the last 1,000 feet of the runway,  all centerline lights are red.  Sea level.  A reference height used to determine standard  atmospheric conditions and altitude measurements.  Segmented circle.  A visual ground based structure to provide  traffic pattern information.  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.  Runway centerline markings.  The runway centerline  identifies the center of the runway and provides alignment  guidance during takeoff and landings. The centerline consists  of a line of uniformly spaced stripes and gaps.  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.  Runway edge lights.  Runway edge lights are used to outline  the edges of runways during periods of darkness or restricted  visibility conditions. These light systems are classified  according to the intensity or brightness they are capable  of producing: they are the High Intensity Runway Lights  (HIRL), Medium Intensity Runway Lights (MIRL), and the  Low Intensity Runway Lights (LIRL). The HIRL and MIRL  systems have variable intensity controls, whereas the LIRLs  normally have one intensity setting.  Shaft horse power (SHP).  Turboshaft engines are rated in  shaft horsepower and calculated by use of a dynamometer  device. Shaft horsepower is exhaust thrust converted to a  rotating shaft.   G-14 S