اااVOR

اااااA. GENERAL


The airway system is based primarily on the very high frequency omnidirectional range (VOR). This extensive system consists of several hundred ground stations that transmit navigation track guidance signals used by aircraft in flight.

The VOR navigational system has many advantages for the IFR pilot. The VOR transmits in the very high frequency range of 108.1 through 117.95 MegaHertz (MHz); therefore, it is relatively free from precipitation static and annoying interference caused by storms or other weather phenomena. Accuracy is another advantage: a track accuracy of plus or minus 1° is possible when flying a VOR radial. Wind drift is compensated for by flying to center the track bar indicator.

VOR signals are transmitted on line-of-sight. Any obstacles (buildings, mountains or other terrain features, including the curvature of the earth) block VOR signals and restrict the distance over which they are received at a given altitude. This can result in a sudden scalloping fluctuation of the cockpit indicators -normally for short time intervals. Certain terrain features may produce areas where VOR navigation signals are unusable, so every instrument pilot making an "off airways" flight should be aware of the restrictions along the route.

Because of greater reception distances at higher altitudes, it is possible for an aircraft to receive erroneous indications due to the reception of two VOR stations operating on the same frequency. Stations on the same frequency are spaced as far apart as possible, but there are, nevertheless, more VORs than the 160 frequencies available. The solution has been to design and classify VORs according to the usable cylindrical service volume.

This is the system by which VOR frequencies are assigned to stations far enough apart to prevent overlapping, confusing signals. As long as pilots use the proper chart, they are protected from interference between two VORs. The pilot uses low-altitude charts below 18,000 ft and the high-altitude charts at and above 18,000 ft.

B. VOR ACCURACY

1. REQUIREMENTS FOR CHECKS: The Air Regulations and good judgement dictate that the VOR equipment of aircraft flying under instrument flight rules be within specified tolerances. Airborne VOR equipment used on IFR flights must be maintained, checked and inspected under an approved procedure. The pilot normally makes these operational checks. In preparing for an IFR flight the pilot should check the aircraft instrument, then make a physical check on the appropriate VOT (test) frequency to determine whether the VOR equipment meets accuracy requirements.

2. VOR TEST FACILITY- The first method is to use a VOR test facility signal (VOT). This is an approved test signal, located on an airport, that enables the pilot to check the receivers conveniently and accurately. First, the pilot should tune the VOR receiver to the VOT frequency. These frequencies are coded with a series of Morse code dots or a continuous 1020-cycle tone. When the pilot sets the course selector to 0°, the track bar (TB) indicator should center and the TO-FROM indicator should read FROM. Then the pilot sets the selector to 180°. The TO-FROM indicator should read TO and the TB should be centered.

The pilot determines the exact error in the receiver by turning the track selector until the TB is centered, and noting the degrees difference between 180° or 0°. The maximum Permissible bearing error with this system check is plus or minus 4°. Apparent errors greater than 4° indicate that the aircraft receiver is beyond acceptable tolerance. In such circumstances the pilot should determine the cause of the error and have it corrected before attempting IFR flight.

The airports with VOT facilities are listed in the Airport/ Facility Directory. Because the VOT signal is only a special test signal, it may be received and used regardless of the aircraft's position on the airport.

3. VOR CHECK POINT SIGNS: A number of aerodromes have VOR check point signs located beside taxiways. These signs indicate a point on the aerodrome where there is sufficient signal strength from a VOR to check aircraft VOR equipment against the radial designated on the sign. Frequently a DME distance will also be indicated for check purposes. The maximum permissible difference between aircraft equipment and the designated radial is 4° and 0.5 NM of the posted distance.

4. DUAL VOR CHECK.- If neither a test signal (VOT) nor a designated check point on the surface is available and an aircraft is equipped with dual VORs (units independent of each other except for the antenna), the equipment may be checked against each other by tuning both sets of the same VOR facility and noting the indicated bearings to that station. A difference greater than 4° between the aircraft's two VOR receivers indicate that one of the aircraft's receivers may be beyond acceptable tolerance. In such circumstances, the cause of the error should be investigated and, if necessary, corrected before the equipment is used for an IFR flight.http://www.allstar.fiu.edu/aero/VOR.htm