VORs and VORTACs are classed according to operational  use. There are three classes:  Basically, these checks consist of verifying that the VOR  radials the aircraft equipment receives are aligned with the  radials the station transmits. There are not specific tolerances  in VOR checks required for VFR flight. But as a guide to  assure acceptable accuracy, the required IFR tolerances can  be used—±4° for ground checks and ±6° for airborne checks.  These checks can be performed by the pilot.  The normal useful range for the various classes is shown in  the following table:  The VOR transmitting station can be positively identified  by its Morse code identification or by a recorded voice  identification that states the name of the station followed by  “VOR.” Many FSSs transmit voice messages on the same  frequency that the VOR operates. Voice transmissions should  not be relied upon to identify stations because many FSSs  remotely transmit over several omniranges that have names  different from that of the transmitting FSS. If the VOR is  out of service for maintenance, the coded identification is  removed and not transmitted. This serves to alert pilots that  this station should not be used for navigation. VOR receivers  are designed with an alarm flag to indicate when signal  strength is inadequate to operate the navigational equipment.  This happens if the aircraft is too far from the VOR or the  aircraft is too low and, therefore, is out of the line of sight  of the transmitting signals.  The useful range of certain facilities may be less than 50  miles. For further information concerning these restrictions,  refer to the Communication/NAVAID Remarks in the Chart  Supplement U.S.  The accuracy of course alignment of VOR radials is  considered to be excellent. It is generally within plus or minus  1°. However, certain parts of the VOR receiver equipment  deteriorate, affecting its accuracy. This is particularly true  at great distances from the VOR station. The best assurance  of maintaining an accurate VOR receiver is periodic checks  and calibrations. VOR accuracy checks are not a regulatory  requirement for VFR flight. However, to assure accuracy of  the equipment, these checks should be accomplished quite  frequently and a complete calibration should be performed  each year. The following means are provided for pilots to  check VOR accuracy:  If an aircraft has two VOR receivers installed, a dual VOR  receiver check can be made. To accomplish the dual receiver  check, a pilot must tune both VOR receivers to the same VOR  ground facility. The maximum permissible variation between  the two indicated bearings is 4°. A list of the airborne and  ground checkpoints is published in the Chart Supplement U.S.  When the course selector is rotated, it moves the CDI or  needle to indicate the position of the radial relative to the  aircraft. If the course selector is rotated until the deviation  The course selector is an azimuth dial that can be rotated to  select a desired radial or to determine the radial over which  the aircraft is flying. In addition, the magnetic course “TO”  or “FROM” the station can be determined.  The CDI is found in most training aircraft. It consists of an  omnibearing selector (OBS) sometimes referred to as the  course selector, a CDI needle (left-right needle), and a TO/  FROM indicator.  In review, for VOR radio navigation, there are two  components required: ground transmitter and aircraft  receiving equipment. The ground transmitter is located at a  specific position on the ground and transmits on an assigned  frequency. The aircraft equipment includes a receiver with  a tuning device and a VOR or omninavigation instrument.  The navigation instrument could be a course deviation  indicator (CDI), horizontal situation indicator (HSI), or a  radio magnetic indicator (RMI). Each of these instruments  indicates the course to the tuned VOR.  Course Deviation Indicator (CDI)  Using the VOR  VOR/VORTAC NAVAIDS  Normal Usable Altitudes and Radius Distances • T (Terminal)  • L (Low altitude)  • H (High altitude)  • FAA VOR test facility (VOT)  • Certified airborne checkpoints  • Certified ground checkpoints located on airport  surfaces  16-23  Distance  Class  Altitudes  ( M i l e s )  T  12,000' and below  25  L  Below 18,000'  40  H  Below 14,500'  40  H  Within the conterminous 48 states  only, between 14,500 and 17,999'  100  H  18,000'—FL 450  130  H  FL 450—60,000'  100 