Underpower.  Using less power than required for the purpose  of achieving a faster rate of airspeed change.  Turbocharger.  An air compressor driven by exhaust gases,  which increases the pressure of the air going into the engine  through the carburetor or fuel injection system.  Turbofan engine.  A fanlike turbojet engine designed to  create additional thrust by diverting a secondary airflow  around the combustion chamber.  Booklets published in regional format by FAA Aeronautical  Navigation Products (AeroNav Products) that include DPs,  STARs, IAPs, and other information pertinent to IFR flight.  United States Terminal Procedures Publication (TPP).  V  U  G-32  VEF.  Calibrated airspeed at which the critical engine of a  multi-engine aircraft is assumed to fail.  Vectoring.  Navigational guidance by assigning headings.  Vector.  A force vector is a graphic representation of a force  and shows both the magnitude and direction of the force.  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.  Ulitimate load factor.  In stress analysis, the load that  causes physical breakdown in an aircraft or aircraft  component during a strength test, or the load that according  to computations, should cause such a breakdown.  Uncaging.  Unlocking the gimbals of a gyroscopic instrument,  making it susceptible to damage by abrupt flight maneuvers  or rough handling.  Uncontrolled airspace.  Class G airspace that has not been  designated as Class A, B, C, D, or E. It is airspace in which  air traffic control has no authority or responsibility to control  air traffic; however, pilots should remember there are VFR  minimums which apply to this airspace.  Ultra-high frequency (UHF).  The range of electromagnetic  frequencies between 300 MHz and 3,000 MHz.  Vapor lock.  A problem that mostly affects gasoline-fuelled  internal combustion engines. It occurs when the liquid fuel  changes state from liquid to gas while still in the fuel delivery  system. This disrupts the operation of the fuel pump, causing  loss of feed pressure to the carburetor or fuel injection system,  resulting in transient loss of power or complete stalling.  Restarting the engine from this state may be difficult. The  fuel can vaporize due to being heated by the engine, by the  local climate or due to a lower boiling point at high altitude.  V$_{A}$.  See maneuvering speed.  V$_{2}$.  See takeoff safety speed.  Turn coordinator.  A rate gyro that senses both roll and  yaw due to the gimbal being canted. Has largely replaced  the turn-and-slip indicator in modern aircraft.  TWEB.  See Transcribed Weather Broadcast.  UHF.  See ultra-high frequency.  Turn-and-slip indicator.  A flight instrument consisting  of a rate gyro to indicate the rate of yaw and a curved glass  inclinometer to indicate the relationship between gravity and  centrifugal force. The turn-and-slip indicator indicates the  relationship between angle of bank and rate of yaw. Also  called a turn-and-bank indicator.  V$_{1}$.  See takeoff decision speed.  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.  Turboshaft engine.  A gas turbine engine that delivers power  through a shaft to operate something other than a propeller.  Turboprop engine.  A turbine engine which drives a  propeller through a reduction gearing arrangement. Most of  the energy in the exhaust gases is converted into torque, rather  than using its acceleration to drive the aircraft.  Useful load.  The weight of the pilot, copilot, passengers,  baggage, usable fuel, and drainable oil. It is the basic empty  weight subtracted from the maximum allowable gross weight.  This term applies to general aviation aircraft only.  Unusual attitude.  An unintentional, unanticipated, or  extreme aircraft attitude.  Turbojet engine.  A turbine engine which produces its thrust  entirely by accelerating the air through the engine. 