Very-high Frequency Omni-range (VOR)
This system is now over 50 years old and will eventually be replaced by the global positioning system (GPS). VOR information only tells you of the VOR radial you are on and which way to turn to get to a specific radial. Situational awareness using the VOR requires considerable training and attention to detail. The most neglected detail is referencing VOR indications to the compass/heading indicator indications.
Your VOR receiver determines position by comparing the phase (time) difference between to different parts of the VOR station signal. One signal is sent out in all directions, line-of-sight much the way an airport beacon sends light. The second signal is used to time the first from north. The TACAN (military) part of a VORTAC is mechanical rather than electronic but nine-times more precise than the civil system. The carrier wave of the signal is coded in a three letter Morse Code identifier. Your knowing Morse Code is a good plus. The Code is at about eight words a minute, which is slow enough for beginners. Some VOR have duplex communication capability. This means that you can talk to a Flight Service Station (FSS) on 122.1 from your radio's com side and hear them talk back to you on the VOR frequency.
At 30 miles from a VOR slight out of tolerance and an OBS off by the allowable 4° you could be 4 nautical miles off the centerline. At best, even with the needle centers from a VOR you are seldom where you think you are. At six miles from a VOR and the needle centered you could be off nearly a mile; at ten miles the error is 1.2 miles. None of these allow the possibility that you mis-set the OBS by a degree. A field that has the VOR centered on it shows 200' error with one dot deflection at the half mile airport boundary. At twenty miles you could be over a mile off the line and still have the needle centered.
You will frequently note that VOR radials, while apparently in a straight line through the VOR, will have differing numbers than would be reciprocal. The lines are using the great circle route that change direction with longitude. Additionally, differences in magnetic variation make for the different numbers.
The term 'reverse sensing' comes from operational problems using the VOR TO/FROM window and the |
OBS radial selected. In general you are either going TO or FROM a VOR radial. The instructional rule of
thumb is to always fly toward the needle.
If you are going TO the VOR radial and the window shows TO you always fly toward the needle when it is not centered as the process for centering the needle.
If you are going FROM the VOR radial and the window shows FROM you always fly toward the needle when it is not centered as the process for centering the needle.
Reverse Sensing
In general you are either going TO or FROM a VOR radial.
If you are going TO the VOR radial and the window shows TO you always fly toward the needle when it is not centered as the process for centering the needle.
If you are going FROM the VOR radial and the window shows FROM you always fly toward the needle when it is not centered as the process for centering the needle.
If you are going TO the VOR radial and the window shows FROM any flying toward the needle will make the needle move even further away from center.
If you are going FROM the VOR radial and the window shows TO any flying toward the needle will make the needle move even further from center.
In the REVERSE SENSING the foregoing rule of thumb will not work because the needle sense of movement is backwards..
The term 'reverse sensing' comes from operational problems using the VOR TO/FROM window and the OBS radial selected..
If you are going TO the VOR radial and the window shows FROM any flying toward the needle will make the needle move even further away from center.
If you are going FROM the VOR radial and the window shows TO any flying toward the needle will make the needle move even further from center.
In five years (2010), with the removal of VORs, the whole problem is moot.
VOR Procedures:
Aside from an initial introduction to the use of the VOR during a flight from Rio Vista to CCR I make a practice to minimize VOR instruction. I have found that reliance on VORs reduces the efforts of a student to pay attention to terrain and area features. In the real world of emergencies the use of a VOR at any appreciable altitude is very unlikely in a radar environment. In this same world, the use of a VOR at low altitude is very marginal and capable of flying you into intervening terrain. It is essential that the VOR user be knowledgeable as to the altitude and line of sight restrictions to a given VOR. In mountain areas, a VOR may be unusable in certain directions beyond 20 miles regardless of altitude. The Airport/Facilities directory gives this information. Look up CCR VOR.
During the cross-country training phase I teach the use of a VOR radial as a backup to a checkpoint. I minimize this and other uses of the VOR and select flights that are "rich" in visual checkpoints. I feel that over dependence on VOR navigation is likely to cause future problems. VORs fail and are sometimes out of service. This is always at the most inopportune times such as during bad weather or reduced visibility. Skill in pilotage can be taught. It improves with use and is the most reliable last resort of navigation. Knowing where you are is a unique kind of brain food. Just note how poorly the brain functions when you are lost. It is unwise to rely on a device that may lose capability at just the wrong time.
Unfortunately, the average VOR user just sets the frequency and the OBS. Good practice recommends that you check the Morse code identifier every time you use the station. Some pilots keep the code on at a low level as an additional precaution. Most pilots seem to not identify. Such a practice will eventually bite you and it will be at the worst possible time. Besides, if you are lost your brain will have stopped working and you will not use the correct and proven procedures. Use of the steps to good VOR navigation requires constant practice. I teach the complete VOR process only during the proficiency phase of instruction.
The essential of good VOR tracking is accuracy of settings and precision of headings. Since the OBS is in divided into spaces of 5 degrees, such as 340 to 345, it is difficult to set 342 and 343. Just centering between the marks is the best choice. It is possible to track to or from a radial without an accurately set Heading Indicator. However, it is much less confusing to judge the wind if the H.I is properly set. The ability to fly and hold headings both VFR and IFR is gradually acquired. Once acquired, it makes flying the VOR or Localizer relatively simple and a no-brainer. It does take practice. Fly the heading, not the needle.
Flying TO a VOR is much like trying to guide a ball with a stick between two very long boards placed at a 20 degree angle. The idea is not to let the ball touch either board while moving it toward the vertex. You may weave some at the very beginning but it is important that as the angle narrows the precision required greatly increases. Inversely, if rolling the ball from the vertex the ever-widening space makes keeping the ball relative near the centerline becomes easier. This analogy very aptly explains flight TO and FROM a VOR. The full deflection of a VOR needle pegs out at 10 degrees to each side. That of the Localizer does so in 2.5 degrees it is four times as sensitive.
The course reversal used in ground reference can be used effectively in practicing VOR tracking. Find an isolated VOR that will allow legal flight below 1000' and track directly into the wind to the VOR and then two minutes from it. Perform a 90/270 course reversal and during the 270 part reverse the OBS and track inbound again. Do this several times and then do it with a 90-degree cross wind several times. A good study of needle action is to fly a rather large circle around the VOR with a few steep 360s every 90 degrees. Don't change the OBS until you get back to the initial point.
Two specific regions of flight near a VOR provide unreliable signals. The most common area is the cone of confusion that projects upward from the VOR. As a cone its zone of confusion becomes greater with altitude so that you can get rather precise TO/FROM reversal as below 1000' but at three thousand feet and higher it may take twenty seconds or longer.
The second region exists to both sides of the VOR in 20-degree fans as determined by the OBS setting. If the OBS is set to 360 and the flight transitions the VOR area on a 360 heading toward but to the left or right of the 180/360 radials you will fly through a region of ambiguity. This region extends from the VOR to the sides in a 20-degree fan. Approaching from the south you will have a FROM reading until reaching either the 260 or 100 radials. Inside these radials the TO/FROM will waver back and forth until it changes to OFF and then to FROM on exceeding the zone fan at the 280/080 radials. This can best be understood by drawing it out over a VOR compass rose.
Continue To Next Page