VOR Navigation VOR Navigation

   The location, transmitter frequency, and identification code of VORs and VORTACs are shown on aeronautical charts. Along with a magnetic compass rose at each station to depict the radials, designated airways which emanate from the station are also shown with their outbound magnetic courses (radials) specified.

   To fly from one VOR directly to another VOR the pilot should draw a course line (or use the printed centerline of the airway) between the two stations to determine the magnetic course or radial that will be followed (Fig. 12-8). After takeoff the omnireceiver should be tuned to the assigned frequency of the VOR from which the flight will start and the identification of the station verified. Then the omnibearing selector should be set to the planned magnetic course from that station. The TO/FROM indicator will show FROM and the course deviation indicator will be deflected to the right, to the left, or centered, depending on where the airplane is located in relation to the selected course. The airplane should then be turned so that its heading is the same as that set on the omnibearing indicator. The airplane will now be flying parallel to the radial. This will help the pilot determine which way to turn to intercept the radial if not exactly on the radial. If off the radial a considerable distance, a turn should be made to a heading that will intercept the planned course at about a 30 to 45 degree angle.

   If the course deviation needle was originally deflected to one side or the other, it will gradually move to the center as the airplane approaches and intercepts the selected radial or course. When the needle is centered, indicating that the airplane is now on course, a turn should be made to the preplanned heading. However, while following the radial the airplane may be affected by crosswinds. Unless a correction is made for the crosswind as the flight continues, it will be noted that the course deviation indicator moves to the upwind side of center. This indicates that the airplane is drifting off course. Instead of making a sharp turn to get back on the desired radial, a turn of 10 to 20 degrees should be made to reintercept the radial. If a turn of more than 20 degrees is made when the needle is off center only slightly, chances are the airplane will overshoot the radial, particularly when close to the station, and the flight will then become a series of time wasting zigzags. When the deviation indicator needle is again centered, showing that the airplane is back on course, approximately a 5 degree drift correction should be made, and the flight continued making only slight heading changes as necessary to keep the needle centered.

   Arithmetical computations for crosswind corrections when flying cross-country using VOR are actually unnecessary. Flying whatever heading that results in keeping the course deviation indicator needle centered automatically "crabs" the airplane into the wind just the right amount to fly a straight line course to (or from) the VOR station. As long as the course deviation indicator needle shows the airplane to be on a radial, the airplane is on it. The airplane may be crabbed any number of degrees, and the only concern is to keep the course deviation indicator needle centered.

   Upon reaching the approximate halfway point between the departure VOR and the next VOR on the planned course, the omnireceiver should be tuned to the next VOR - but it is essential to positively identify it. The TO/FROM indicator will now show TO. Since the outbound radial of the first VOR usually coincides with the inbound course to the second VOR (Fig. 12-8), it is normally not necessary to reset the bearing selector to a new course. When it is reset for the radial of the approaching VOR, it must be remembered that the radial shown on the chart is the outbound course. To follow it inbound, the reciprocal of the shown radial must be set on the omnibearing selector. Otherwise the indicator would provide "reverse sensing" - that is, the course deviation indicator would be on the wrong side and it would be necessary to turn away from the needle. That could result in confusion. After setting the OBS to the proper course, the pilot is again set for another period of navigation by simply correcting toward the needle.

   Upon nearing the second VOR, the deviation indicator needle will begin to deviate or fluctuate rather rapidly. This means that the airplane is close to the station. Under this condition, rather than make heading corrections for each fluctuation, the compass heading that has been keeping the airplane on the course to the station should be maintained, and the needle fluctuations ignored.

   Within a few seconds the airplane will be over the station. The deviation indicator needle will swing sharply back and forth, and the TO/FROM indicator will change from TO to FROM. This means the airplane is now heading away from the station. If there is a slight difference between the previous inbound course and the desired outbound course from this VOR, the bearing selector to the new outbound course should be set and again the heading should be changed to keep the deviation indicator centered.

   At certain times a red flag alarm or OFF indication will appear adjacent to the course deviation needle, and the needle itself will swing from side to side as though "hunting." The TO/FROM needle may settle in a neutral or center position and may not move out of the neutral indication, even though the receiver is tuned to the proper frequency. Such visual warnings indicate that the received signal is too weak to give reliable indications.

   These warnings appear when obstacles or mountainous terrain interfere with the line of sight transmission, or the airplane is too low or too far away from the station to obtain steady, reliable signals. The alarm provides a good safeguard against using erroneous or unreliable bearing readings. In a case like this, the pilot should tune to the next VOR on the course and maintain a constant heading toward the station until the flag alarm disappears and the course deviation needle again comes to rest at or near the center position.

   The need for identifying the VOR station cannot be overemphasized. Receivers with hand tunable oscillators are frequently found to be off calibration. Merely tuning to the indicated frequency on the dial is no guarantee of receiving the desired station. Also, frequencies of ground stations are sometimes changed and unless the pilot has current information at hand, the wrong station could be tuned in.

   One of the main advantages of VOR is that the pilot can quickly and easily locate the airplane's exact position by taking bearings from two VOR stations. The airplane's position (fix) is where the two bearings cross. For the most accurate fix, the cross bearings should be 90 degrees to each other or as close to a right angle as possible. If several stations are available for a bearing check, it is best to use the one which will give the closest to a right angle crossbearing. Position checks or fixes along a course are most convenient if the pilot has two omnireceivers and keeps one set to maintain the course being flown while using the other to take the crossbearing on the off course VOR. For uniformity and to avoid confusion, a bearing to establish a fix is best made by using degrees FROM the off course station rather than degrees TO the station. In this manner the bearings or lines of position can be drawn from the station since the station's location is known and the airplane's location is still unknown.

   To determine the airplane's position in relation to intersections of airways while tracking on a radial, the pilot can use the same crossbearing or fix techniques used to pinpoint the airplane's position. Since the bearing of the intersection from the off course VOR is known, this should be set in advance. As the intersection is approached, the course deviation needle will move towards center from full deflection. When the needle is in the center position, the airplane is at the intersection. Again, this procedure is even simpler if there are two omnireceivers - one for enroute navigation; the other for locating the intersection.