Twist drill.  A metal cutting tool turned in a drill press or  handheld drill motor. A twist drill has a straight shank and  spiraled flutes. The cutting edge is ground on the end of the  spiraled flutes. Twist rope.  A stripe of paint on flexible hose that runs the  length of the hose. If this stripe spirals around the hose after  it is installed, it indicates the hose was twisted when it was  installed. Twist stripes are also called lay lines. Two-terminal spot-type fire detection system.  A fire  detection system that uses individual thermoswitches  installed around the inside of the area to be protected. These  thermoswitches are wired in parallel between two separate  circuits. A short or an open circuit can exist in either circuit  without causing a fire warning. Type Certificate Data Sheets (TCDS).  The official  specifications of an aircraft, engine, or propeller issued by the  Federal Aviation Administration. The TCDS lists pertinent  specifications for the device, and it is the responsibility of  the mechanic and/or inspector to ensure, on each inspection,  that the device meets these specifications. UHF.  Ultrahigh frequency. Ultimate tensile strength.  The tensile strength required to  cause a material to break or to continue to deform under a  decreasing load. Ultraviolet-blocking dope.  Dope that contains aluminum  powder or some other pigment that blocks the passage of  ultraviolet rays of the sun. The coat of dope protects the organic  fabrics and clear dope from deterioration by these rays. Undamped oscillation.  Oscillation that continues with an  unchanging amplitude once it has started. Underslung rotor.  A helicopter rotor whose center of gravity  is below the point at which it is attached to the mast. Unidirectional fabric.  Fabric in which all the threads run  in the same direction. These threads are often bound with a  few fibers run at right angles, just enough to hold the yarns  together and prevent their bunching. Unloading valve.  This is another name for system pressure  regulator. See system pressure regulator. Utility finish.  The finish of an aircraft that gives the necessary  tautness and fill to the fabric and the necessary protection  to the metal, but does not have the glossy appearance of a  show-type finish. Vapor lock.  A condition in which vapors form in the fuel  lines and block the flow of fuel to the carburetor. Vapor pressure.  The pressure of the vapor above a liquid  needed to prevent the liquid evaporating. Vapor pressure is  always specified at a specific temperature. Variable displacement pump.  A fluid pump whose output  is controlled by the demands of the system. These pumps  normally have a built-in system pressure regulator. When the  demands of the system are low, the pump moves very little  fluid, but when the demands are high, the pump moves a lot  of fluid. Most variable displacement pumps used in aircraft  hydraulic systems are piston-type pumps. Varnish (aircraft finishing material).  A material used to  produce an attractive and protective coating on wood or  metal. Varnish is made of a resin dissolved in a solvent and  thinned until it has the proper viscosity to spray or brush. The  varnish is spread evenly over the surface to be coated, and  when the solvents evaporate, a tough film is left. Varsol.  A petroleum product similar to naphtha used as a  solvent and cleaning fluid. Veneer.  Thin sheets of wood “peeled” from a log. A wide- blade knife held against the surface of the log peels away the  veneer as the log is rotated in the cutter. Veneer is used for  making plywood. Several sheets of veneer are glued together,  with the grain of each sheet placed at 45° or 90° to the grain  of the sheets next to it. Vertical axis.  An imaginary line, passing vertically through  the center of gravity of an airplane. Vertical fin.  The fixed vertical surface in the empennage of  an airplane. The vertical fin acts as a weathervane to give  the airplane directional stability. VFR.  Visual flight rules. VHF.  Very high frequency. Vibrator-type voltage regulator.  A type of voltage regulator  used with a generator or alternator that intermittently places  a resistance in the field circuit to control the voltage. A set  of vibrating contacts puts the resistor in the circuit and takes  it out several times a second. Viscosity.  The resistance of a fluid to flow. Viscosity refers  to the “stiffness” of the fluid, or its internal friction.  G-38