Trim tab.  A small control tab mounted on the trailing edge  of a movable control surface. The tab may be adjusted to  provide an aerodynamic force to hold the surface on which it  is mounted deflected in order to trim the airplane for hands- off flight at a specified airspeed. TR unit.  A transformer-rectifier unit. A TR unit reduces the  voltage of AC and changes it into DC. Tractor powerplant.  An airplane powerplant in which the  propeller is mounted in the front, and its thrust pulls the  airplane rather than pushes it. Trimmed flight.  A flight condition in which the aerodynamic  forces acting on the control surfaces are balanced and the  aircraft is able to fly straight and level with no control input. Trip-free circuit breaker.  A circuit breaker that opens a circuit  any time an excessive amount of current flows, regardless of  the position of the circuit breaker’s operating handle. Troubleshooting.  A procedure used in aircraft maintenance  in which the operation of a malfunctioning system is analyzed  to find the reason for the malfunction and to find a method  for returning the system to its condition of normal operation. Trammel (verb).  To square up the Pratt truss used in an  airplane wing. Trammel points are set on the trammel bar  so they measure the distance between the center of the front  spar, at the inboard compression strut, and at the center of  the rear spar at the next compression strut outboard. The drag  and antidrug wires are adjusted until the distance between the  center of the rear spar at the inboard compression strut and the  center of the front spar at the next outboard compression strut  is exactly the same as that between the first points measured. True airspeed (TAS).  Airspeed shown on the airspeed  indicator (indicated airspeed) corrected for position error  and nonstandard air temperature and pressure. Trammel bar.  A wood or metal bar on which trammel points  are mounted to compare distances. Trammel points.  A set of sharp-pointed pins that protrude  from the sides of a trammel bar. Trunnion.  Projections from the cylinder of a retractable  landing gear strut about which the strut pivots retract. Transducer.  A device that changes energy from one form  to another. Commonly used transducers change mechanical  movement or pressures into electrical signals. Transformer rectifier.  A component in a large aircraft  electrical system used to reduce the AC voltage and change  it into DC for charging the battery and for operating DC  equipment in the aircraft. Truss-type structure.  A type of structure made up of  longitudinal beams and cross braces. Compression loads  between the main beams are carried by rigid cross braces.  Tension loads are carried by stays, or wires, that go from one  main beam to the other and cross between the cross braces. Turbine.  A rotary device actuated by impulse or reaction  of a fluid flowing through vanes or blades that are arranges  around a central shaft. Translational lift.  The additional lift produced by a  helicopter rotor as the helicopter changes from hovering to  forward flight. Transonic flight.  Flight at an airspeed in which some air  flowing over the aircraft is moving at a speed below the  speed of sound, and other air is moving at a speed greater  than the speed of sound. Turn and slip indicator.  A rate gyroscopic flight instrument  that gives the pilot an indication of the rate of rotation of the  aircraft about its vertical axis. A ball in a curved glass tube  shows the pilot the relationship between the centrifugal force  and the force of gravity. This indicates whether or not the  angle of bank is proper for the rate of turn. The turn and slip  indicator shows the trim condition of the aircraft and serves  as an emergency source of bank information in case the  attitude gyro fails. Turn and slip indicators were formerly  called needle and ball and turn and bank indicators. Transverse pitch.  See gauge. Triangle.  A three-sided, closed plane figure. The sum of the  three angles in a triangle is always equal to 180°. Turnbuckle.  A component in an aircraft control system  used to adjust cable tension. A turnbuckle consists of a brass  tubular barrel with right-hand threads in one end and left-hand  in the other end. Control cable terminals screw into the two  ends of the barrel, and turning the barrel pulls the terminals  together, shortening the cable. Tricresyl phosphate (TCP).  A chemical compound,  (CH$_{3}$C$_{6}$H$_{4}$O)$^{3}$PO, used in aviation gasoline to assist in  scavenging the lead deposits left from the tetraethyl lead. G-37