Type certification.  Official recognition that the design and  operating limitations of an aircraft, engine, or propeller meet  the airworthiness standards prescribed by the Code of Federal  Regulations for that particular category or type of aircraft,  engine, or propeller. Virga.  Precipitation that falls from a cloud and evaporates  before reaching the ground. Volume.  The total amount of air or gas (expressed in cubic  feet) contained in a balloon envelope. Warm front.  In weather, the leading edge of a warm air mass  displacing a colder air mass. Warp.  The threads in a piece of fabric that run the length  of the fabric. Uncontrolled lift.  Lift that occurs without specific action by  the pilot. Often referred to as false lift. Useful lift (load). The potential weight of the pilot,  passengers, equipment and fuel. It is the basic empty weight  of the aircraft subtracted from the maximum allowable gross  weight.  Weigh-off.  Determine neutral buoyancy of a gas balloon or  airship by taking weight off at launch. Wind direction.  The direction the wind is coming from. VAD winds.  In weather; velocity azimuth display winds are  derived from the output of the 160 or more WRS-88 radar  sites located throughout the United States. The WRS-88 is  configured to produce radar returns off of dust and other  particulate matter in the air, and in turn, those returns can  be used to indicate wind direction and speed at different  altitudes. Generally reported in 1000 foot increments. Wind shear.  A strong and sudden shift in wind speed or  direction, which may be either vertical or horizontal. Wind  shear should not be confused with normal wind change, which  is gentler. Wind shear is often associated with the passage of  a weather front, or a strong temperature inversion. WINGS Program. A program that encourages general  aviation pilots to continue training and provides an opportunity  to practice selected maneuvers in a minimum of instruction  time. Participation in the WINGS program relieves a pilot  from compliance with flight review requirements, provided  all WINGS requirements are met. Previously governed by  Advisory Circular 61-91, the WINGS program is available  as on online education program at www.faasafety.gov,  effective mid-2007. Variometer.  See Vertical Speed Indicator. Vent. (1) The action of opening the vent to cool the air in the  envelope. (2) An envelope opening that will automatically  close. Vertical Speed Indicator (VSI). An instrument that  continuously records the rate at which an aircraft climbs or  descends. Usually measured in FPM. A required instrument  in a balloon. Vent line.  The line that activates the cooling vent. Visual Flight Rules (VFR).  Flight rules governing aircraft  flight when the pilot has visual reference to the ground at  all times. G-9