VFR over-the-top.  A VFR operation in which an aircraft  operates in VFR conditions on top of an undercast. Victor airways.  Airways based on a centerline that extends  from one VOR or VORTAC navigation aid or intersection,  to another navigation aid (or through several navigation aids  or intersections); used to establish a known route for en route  procedures between terminal areas. Visual approach slope indicator (VASI).  A visual aid of  lights arranged to provide descent guidance information  during the approach to the runway. A pilot on the correct  glide slope will see red lights over white lights. Visual descent point (VDP).  A defined point on the final  approach course of a nonprecision straight-in approach  procedure from which normal descent from the MDA to the  runway touchdown point may be commenced, provided the  runway environment is clearly visible to the pilot. Visual flight rules (VFR). Flight rules adopted by the  FAA governing aircraft flight using visual references. VFR  operations specify the amount of ceiling and the visibility the  pilot must have in order to operate according to these rules.  When the weather conditions are such that the pilot can not  operate according to VFR, he or she must use instrument  flight rules (IFR). Visual meteorological conditions (VMC).  Meteorological  conditions expressed in terms of visibility, distance from  cloud, and ceiling meeting or exceeding the minimums  specified for VFR. VMC.  See visual meteorological conditions. VOR.  See very-high frequency omnidirectional range. VORTAC.  A facility consisting of two components, VOR  and TACAN, which provides three individual services: VOR  azimuth, TACAN azimuth, and TACAN distance (DME) at  one site. VOR test facility (VOT).  A ground facility which emits a  test signal to check VOR receiver accuracy. Some VOTs are  available to the user while airborne, while others are limited  to ground use only. VOT.  See VOR test facility. VSI.  See vertical speed indicator. WAAS.  See wide area augmentation system. Warning area.  An area containing hazards to any aircraft  not participating in the activities being conducted in the  area. Warning areas may contain intensive military training,  gunnery exercises, or special weapons testing. Waypoint.  A designated geographical location used for route  definition or progress-reporting purposes and is defined in  terms of latitude/longitude coordinates. WCA.  See wind correction angle. Weather and radar processor (WARP).  A device that  provides real-time, accurate, predictive and strategic weather  information presented in an integrated manner in the National  Airspace System (NAS). Weight.  The force exerted by an aircraft from the pull of  gravity. Wide area augmentation system (WAAS).  A differential  global positioning system (DGPS) that improves the accuracy  of the system by determining position error from the GPS  satellites, then transmitting the error, or corrective factors,  to the airborne GPS receiver. Wind correction angle (WCA). The angle between the  desired track and the heading of the aircraft necessary to  keep the aircraft tracking over the desired track. Wind shear.  A change in wind speed and/or wind direction  in a short distance. Work.  A measurement of force used to produce movement. Zone of confusion.  Volume of space above the station where  a lack of adequate navigation signal directly above the VOR  station causes the needle to deviate.  G-20