Navigation

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