Rack-and-pinion actuator.  A form of rotary actuator where  the fluid acts on a piston on which a rack of gear teeth is cut.  As the piston moves, it rotates a pinion gear which is mated  with the teeth cut in the rack. Radial.  A directional line radiating outward from a radio  facility, usually a VOR. When an aircraft is flying outbound  on the 330º from the station. Radius dimpling.  A process of preparing a hole in sheet  metal for flush riveting. A cone-shaped male die forces the  edges of the rivet hole into the depression in a female die.  Radius dimpling forms a round-edged depression into which  the rivet head fits. Range markings.  Colored marks on an instrument dial  that identify certain ranges of operation as specified in  the aircraft maintenance or flight manual and listed in the  appropriate aircraft Type Certificate Data Sheets or Aircraft  Specifications. Color coding directs attention to approaching  operating difficulties. Airspeed indicators and most pressure  and temperature indicators are marked to show the various  ranges of operation. These ranges and colors are the most  generally used: Red radial line, do not exceed. Green arc,  normal operating range. Yellow arc, caution range. Blue  radial line, used on airspeed indicators to show best single- engine rate of climb speed. White arc, used on airspeed  indicators to show flap operating range. RDF.  Radio direction finding. Rebreather oxygen mask.  A type of oxygen mask used  with a continuous flow oxygen system. Oxygen continuously  flows into the bottom of the loose-fitting rebreather bag on  the mask. The wearer of the mask exhales into the top of  the bag. The first air exhaled contains some oxygen, and  this air goes into the bag first. The last air to leave the lungs  contains little oxygen, and it is forced out of the bag as the  bag is filled with fresh oxygen. Each time the wearer of the  mask inhales, the air first exhaled, along with fresh oxygen,  is taken into the lungs. Receiver-dryer.  The component in a vapor-cycle cooling  system that serves as a reservoir for the liquid refrigerant.  The receiver-dryer contains a desiccant that absorbs any  moisture that may be in the system. Rectangle.  A plane surface with four sides whose opposite  sides are parallel and whose angles are all right angles. Rectification (arc welding condition).  A condition in AC- electric arc welding in which oxides on the surface of the  metal act as a rectifier and prevent electrons flowing from  the metal to the electrode during the half cycle when the  electrode is positive. Reducing flame.  See carburizing flame. Reed valve.  A thin, leaf-type valve mounted in the valve  plate of an air conditioning compressor to control the flow  of refrigerant gases into and out of the compressor cylinders. Reinforcing tape.  A narrow strip of woven fabric material  placed over the fabric as it is being attached to the aircraft  structure with rib lacing cord. This tape carries a large amount  of the load and prevents the fabric tearing at the stitches. Rejuvenator.  A finishing material used to restore resilience  to an old dope film. Rejuvenator contains strong solvents to  open the dried-out film and plasticizers to restore resilience  to the old dope. Relative wind.  The direction the wind strikes an airfoil. Relay.  An electrical component which uses a small amount  of current flowing through a coil to produce a magnetic pull  to close a set of contacts through which a large amount of  current can flow. The core in a relay coil is fixed. Relief hole.  A hole drilled at the point at which two bend  lines meet in a piece of sheet metal. This hole spreads the  stresses caused by the bends and prevents the metal cracking. Relief valve.  A pressure-control valve that relieves any  pressure over the amount for which it is set. They are damage- preventing units used in both hydraulic and pneumatic  systems. In an aircraft hydraulic system, pressure relief valves  prevent damaging high pressures that could be caused by a  malfunctioning pressure regulator, or by thermal expansion  of fluid trapped in portions of the system. Repair.  A maintenance procedure in which a damaged  component is restored to its original condition, or at least to  a condition that allows it to fulfill its design function. Restrictor.  A fluid power system component that controls  the rate of actuator movement by restricting the flow of fluid  into or out of the actuator. Retard breaker points.  A set of breaker points in certain  aircraft magnetos that are used to provide a late (retarded)  spark for starting the engine.  G-28