Tower En Route Control (TEC).  The control of IFR en route  traffic within delegated airspace between two or more adjacent  approach control facilities, designed to expedite traffic and  reduce control and pilot communication requirements. TPP.  See United States Terminal Procedures Publication. Tracking.  Flying a heading that will maintain the desired track  to or from the station regardless of crosswind conditions. Traffic Alert Collision Avoidance System (TCAS).  An airborne system developed by the FAA that operates  independently from the ground-based Air Traffic Control  system. Designed to increase flight deck awareness of  proximate aircraft and to serve as a “last line of defense” for  the prevention of mid-air collisions. Traffic information service (TIS).  A ground-based service  providing information to the flight deck via data link using  the S-mode transponder and altitude encoder to improve the  safety and efficiency of “see and avoid” flight through an  automatic display that informs the pilot of nearby traffic. Transcribed Weather Broadcast (TWEB).  Meteorological  and aeronautical data recorded on tapes and broadcast over  selected NAVAIDs. Generally, the broadcast contains route- oriented data with specially prepared NWS forecasts, inflight  advisories, and winds aloft. It also includes selected current  information such as weather reports (METAR/SPECI),  NOTAMs, and special notices. Transponder. The airborne portion of the ATC radar  beacon system. Transponder code.  One of 4,096 four-digit discrete codes  ATC assigns to distinguish between aircraft. Trend. Immediate indication of the direction of aircraft  movement, as shown on instruments. Trim. Adjusting the aerodynamic forces on the control  surfaces so that the aircraft maintains the set attitude without  any control input. TWEB.  See Transcribed Weather Broadcast. True airspeed.  Actual airspeed, determined by applying a  correction for pressure altitude and temperature to the CAS. UHF.  See ultra-high frequency. Ultra-high frequency (UHF).  The range of electromagnetic  frequencies between 962 MHz and 1213 MHz. VFR-on-top.  ATC authorization for an IFR aircraft to operate  in VFR conditions at any appropriate VFR altitude. VFR.  See visual flight rules. Vestibule.  The central cavity of the bony labyrinth of the ear,  or the parts of the membranous labyrinth that it contains. Very-high frequency omnidirectional range (VOR). Electronic navigation equipment in which the flight deck  instrument identifies the radial or line from the VOR station,  measured in degrees clockwise from magnetic north, along  which the aircraft is located. Very-high frequency (VHF).  A band of radio frequencies  falling between 30 and 300 MHz. Vertical speed indicator (VSI).  A rate-of-pressure change  instrument that gives an indication of any deviation from a  constant pressure level. Venturi tube.  A specially shaped tube attached to the outside  of an aircraft to produce suction to allow proper operation  of gyro instruments. Vectoring.  Navigational guidance by assigning headings. VDP.  See visual descent point. VASI.  See visual approach slope indicator. Variation. Compass error caused by the difference in  the physical locations of the magnetic north pole and the  geographic north pole. User-defined waypoints.  Waypoint location and other data  which may be input by the user, this is the only GPS database  information that may be altered (edited) by the user. Unusual attitude. An unintentional, unanticipated, or  extreme aircraft attitude. United States Terminal Procedures Publication (TPP). Booklets published in regional format by the NACO that  include DPs, STARs, IAPs, and other information pertinent  to IFR flight. Underpower.  Using less power than required for the purpose  of achieving a faster rate of airspeed change. Uncaging.  Unlocking the gimbals of a gyroscopic instrument,  making it susceptible to damage by abrupt flight maneuvers  or rough handling. G-19