necessary to avoid it. This may involve following  another aircraft or keeping it in sight until it is no  longer a factor.  (See SEE AND AVOID.)  (Refer to 14 CFR Part 91.)  VLF- (See VERY LOW FREQUENCY.)  VMC- (See VISUAL METEOROLOGICAL  CONDITIONS.)  VOICE SWITCHING AND CONTROL SYSTEM  (VSCS)- A computer controlled switching system  that provides air traffic controllers with all voice  circuits (air to ground and ground to ground)  necessary for air traffic control.  (Refer to AIM.)  VOR- A ground-based electronic navigation aid  transmitting very high frequency navigation signals,  360 degrees in azimuth, oriented from magnetic  north. Used as the basis for navigation in the National  Airspace System. The VOR periodically identifies  itself by Morse Code and may have an additional  voice identification feature. Voice features may be  used by ATC or FSS for transmitting instructions/in- formation to pilots.  (See NAVIGATIONAL AID.)  (Refer to AIM.)  VOR TEST SIGNAL- (See VOT.)  VORTAC- A navigation aid providing VOR  azimuth, TACAN azimuth, and TACAN distance  measuring equipment (DME) at one site.  (See DISTANCE MEASURING EQUIPMENT.)  (See NAVIGATIONAL AID.)  (See TACAN.)  (See VOR.)  (Refer to AIM.)  VTOL AIRCRAFT- (See VERTICAL TAKEOFF AND LANDING  AIRCRAFT.)  VTA- (See VERTEX TIME OF ARRIVAL.)  VSCS- (See VOICE SWITCHING AND CONTROL  SYSTEM.)  VR- (See VFR MILITARY TRAINING ROUTES.)  VOT- A ground facility which emits a test signal to  check VOR receiver accuracy. Some VOTs are  available to the user while airborne, and others are  limited to ground use only.  (See CHART SUPPLEMENT U.S.)  (Refer to 14 CFR Part 91.)  (Refer to AIM.)  (See AIRCRAFT CLASSES.)  (See WAKE TURBULENCE.)  (Refer to AIM.)  VORTICES- Circular patterns of air created by the  movement of an airfoil through the air when  generating lift. As an airfoil moves through the  atmosphere in sustained flight, an area of area of low  pressure is created above it. The air flowing from the  high pressure area to the low pressure area around and  about the tips of the airfoil tends to roll up into two  rapidly rotating vortices, cylindrical in shape. These  vortices are the most predominant parts of aircraft  wake turbulence and their rotational force is  dependent upon the wing loading, gross weight, and  speed of the generating aircraft. The vortices from  medium to super aircraft can be of extremely high  velocity and hazardous to smaller aircraft.  8/15/19 1/30/20  Pilot/Controller Glossary Pilot/Controller Glossary  PCG V-4 