Wednesday, February 13, 2008


Normal Usable Altitudes and Radius Distances

Distance Class Altitudes (Miles)
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 FL450 – 60,000' 100

The useful range of certain facilities may be less than 50 miles. For further information concerning these restrictions, refer to the Comm/NAVAID Remarks in the Airport/Facility Directory. 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, and this affects 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 along with a complete calibration each year. The following means are provided for pilots to check VOR accuracy:
  • FAAVOR test facility (VOT);
  • certified airborne checkpoints;
  • certified ground checkpoints located on airport surfaces.

If dual VOR is installed in the airplane and tuned to the same VOR ground facility, the maximum permissible variation between the two indicated bearings is 4°. A list of these checkpoints is published in the Airport/Facility Directory.

Basically, these checks consist of verifying that the VOR radials the airplane 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 tolerance can be used which are ±4° for ground checks and ±6° for airborne checks.

The pilot can perform these checks. The VOR transmitting station can be positively identified by its Morse code identification or by a recorded voice identification which states the name of the station followed by the word "VOR." Many Flight Service Stations 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, which have different names than 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 airplane is too far from the VOR or the airplane is too low and therefore, is out of the line-of-sight of the transmitting signals.

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