Wednesday, September 26, 2007

VOR orientation READ THIS FIRST, many diagrams explained too :)

A total cut and paste entry but it is my only one thus far. The best way to read this is with a sectional in hand and try to follow along. The above sites are VOR simulators, not beautiful but may help you with VORs and again it is free.

AOPA article on VOR Needle Mania

Needle Mania!
How To Use The VOR
By Robert I. Snow

Although it is being eclipsed by the satellite-based Global Positioning System (GPS), the Very-high-frequency Omni-directional Range (VOR) is still the backbone of the U.S. airway system. Because the VOR is the primary form of radio navigation for visual flight rules (VFR) and instrument flight rules (IFR) operations, understanding how it works and how to use it effectively is an essential skill. Once understood, VOR navigation and orientation is not difficult. The key phrase, however, is "once understood." VOR signals are omni-directional, meaning they radiate from the station in all directions. A VOR receiver determines the aircraft's position relative to the station by comparing the phase difference between two components of VOR's radio signal.
How It Works
A lighthouse with two lights explains how the VOR signal is processed. One light is white, visible from all directions, and flashes once a minute. The other is a directional green light that rotates at 1 rpm. The white light is timed so that it flashes only when the rotating light passes through magnetic north.
Your bearing from the lighthouse is determined by the time between the two lights. If you're north of the lighthouse, you'll see both lights at the same time. If you see the green light 30 seconds after the white one (the time it takes for the rotating light to turn 180 degrees), you're south of the station.
While the VOR receiver's electronic circuitry is a bit more complex, the concept of timing the difference between the radio beams is the same. Although bearings can theoretically be determined to a small angle, the VOR signal is resolved to the nearest degree. In other words, it radiates 360 radials, one for each degree on the compass, and all VOR radials are aligned to magnetic north.
VORs transmit continuously in the VHF (Very High Frequency) range, using frequencies from 108.0-111.8 mHz that are 200 kHz apart (108.2, 108.4, etc.), and from 112.0-117.9 mHz with 100 kHz spacing (112.1, 112.2, etc.).
The VHF carrier signal transmits a Morse code station identifier along with its navigational components. Many VORs also have a voice feature that can be used for communications. Flight Service Stations (FSSs) use selected VORs as remote transmitters to increase their voice communications coverage. The FSS transmits on the VOR frequency, and pilots respond on a designated communication frequency.
You should check the Morse code identifier every time you use a VOR. If no identifier is heard, don't use the station for navigation, even though it appears that the VOR indicator is showing the correct bearing. The FAA may be aligning the transmitter, and if used, it may lead you astray. If you hear Morse code spell "TEST" (- . ... -), this also means that the signal should not be used for navigation.
How To Use It
Using a VOR is easy. Find the desired station and its frequency on a sectional, terminal, world aeronautic, or IFR enroute chart. Tune the frequency into the navigation receiver, turn up the volume, select the nav receiver on your audio panel (if so equipped), and listen for the Morse code identifier (the dots and dashes for which are given on the charts). Depending on your receiver, you may also have to activate a knob marked Identify or ID to clearly hear the identifier.
Once you've identified the station, you navigate by the VOR display--the OmniBearing Selector (OBS--Figure 1). This dial features a full or partial (depending on the manufacturer) compass rose that is adjusted with a knob. You select a radial with this knob, putting the radial on the compass rose under the indicator at top center.
The needle in the center is the Course Deviation Indicator (CDI), which tells if you're on or off the selected radial by moving left and right. The flag indicates whether you're flying TO or FROM the station and selected radial; OFF means you're not receiving a VOR signal and shouldn't use the VOR for navigation. Some displays replace the flag with a small triangle that points up for TO and down for FROM; a separate flag denotes OFF.
To navigate by VOR, you must know which radial to select. The one you need will be determined by what you want to do. There are several navigational options available, but before discussing them, there are a few basics that must be addressed. Figure 1 displays a typical OBS. Notice that the needle is two dots to the right of center. The farther the needle gets from the center, the farther you are from the selected radial, in this case, the 360° radial.
As you fly farther off the selected radial, the CDI will eventually show a full-scale deflection, meaning you're at least 10 degrees off the radial (it's 12 or 15 degrees in some older OBSs). Since the OBS has five dots on either side of center, it makes sense that each dot represents 2°, and Figure 1 is 4° to the right of course.
VOR Relativity
When using a VOR, it's important to remember that the indication is determined by the aircraft's position relative to the station, not the aircraft's heading. Although you control the aircraft's heading intercept and track a given radial, aircraft heading has nothing to do with what the VOR displays.
The thick red arrow in Figure 2 corresponds with the 360° radial. Radials always emanate from the station, and the arrow's tail points to the reciprocal of the selected radial. Look at each of the OBS indications in the diagram and note that the CDI is centered anywhere along the selected radial or its reciprocal.
When you're south of the station, the flag indicates that you're flying TO the station. But you are not flying on the 360° radial, you're flying TO it. Because you're south of the station, you are on the 180° radial, the reciprocal of the 360° radial. If you're north of the station, the flag indicates that you're flying away FROM the station on the selected (360°) radial.
Notice that the CDI points to the selected radial when you're east or west of the station. The flag also seems to show whether you are TO or FROM the station. Unfortunately, it's not this simple, because the aircraft's heading has nothing to do with what the VOR displays.
Figure 3 is exactly the same as Figure 2 except for the aircraft headings. Imagine you're flying Aircraft A; if the CDI points to the radial, you should turn right to intercept the radial, correct? Obviously, a right turn will take you away from the radial. This situation is called reverse sensing, and the only way to avoid it is to know where you are, where you want to go, and to select a radial that corresponds with the direction being flown.
The north/south line through the station is defined by the 360° radial and the 180° radial. If you're trying to navigate toward the south, select the 180° radial, the thick red line in Figure 4. Notice that your CDI now shows a left turn to intercept the selected radial and that the flag shows you flying FROM the station.
At first glance, the OBS indications in Figure 4 seem reversed, but they really are not. When you selected the 180° radial, you essentially rotated Figure 3 180 degrees. Upon close examination of Figures 3 and 4, you will see that the indications relative to the selected radial have not changed.
Radial Reciprocals
Any VOR navigational chore allows you to choose either a radial or its reciprocal. The ability to use either with equal facility comes with experience and practice, but to make the process as foolproof as possible, choose the radial that corresponds to your direction of flight.
If, for some reason, this selection is not possible, mentally turn the aircraft so that it's heading in the same direction as the radial selected. For example: You are flying IFR on an airway that is heading to the northwest. There is an intersection, FLIGG, which is located at the point where the Boondocks 082° radial crosses (intersects with) the airway on which you are flying (Figure 5A).
Because you are not in radar contact, air traffic control (ATC) asks if you have passed FLIGG yet. You tune and identify Boondocks on your second VOR receiver, select the 082° radial, and get the OBS indication shown. Are you approaching FLIGG or have you just passed it?
To orient yourself so you can answer the question, mentally turn the plane (don't actually turn without ATC clearance) and imagine that you're flying FROM the Boondocks VOR on its 082° radial (Figure 5B). Now you can think of the CDI as pointing to the radial. For the 082° radial to be to the right of the aircraft, you must be north of the radial, thereby past FLIGG.
Whenever you are confused about VOR indications, mentally turn the plane so that it is headed in the same direction as the radial. Upon doing this, you may think of the CDI as pointing the way to the radial and the flag indicating whether you are flying TO or FROM the station. Remember, this is for orientation only!
VOR Intercepts
Once you've selected a radial, you have to get to and on it. Assume you're trying to intercept the Boondocks 270° radial from your position in the previous example. You select the radial and get a CDI displacement to the left. Does this mean you blindly turn left? Only if you want to make endless (or until you run out of gas) left 360s.
The left CDI is really telling you to select a heading to the left of the selected radial in order to intercept it. If you fly a heading of 225°, you will eventually intercept the 270° radial at a 45-degree angle.
There are several things you must consider when selecting an intercept angle. Among them are wind direction and speed, aircraft speed, your distance from the station, and how quickly you must get on the radial. If you're facing a strong north wind and there's no pressure to quickly intercept the radial, you could fly a heading of 270° and let the wind blow you gradually toward the radial.
If the wind is from the south and you must intercept the radial quickly, you might fly a heading of 210°, which will give you a 60-degree intercept angle. When in doubt, a 45-degree intercept angle is a good compromise between intercepting rapidly and making good progress toward the eventual destination.
Most VOR displays have markers at 45 degrees to either side of the top of the compass rose (Figure 1). These simplify the math needed to determine the intercept heading. Instead of adding or subtracting 45 degrees, simply fly the heading under the appropriate marker.
With this knowledge, you can easily intercept any radial with a simple step-by-step procedure: Tune and identify the station. Select the radial you want to intercept, remembering to select one that's going the way you want to go. Note the CDI indication. Then fly the heading under the 45-degree marker that corresponds to a left or right CDI indication. If the CDI is centered, the radial selected is the heading you want. As you fly your intercept heading, double check to make sure your heading makes sense. Getting unlost is an article unto itself.
VOR In & Out
Always remember that radials emanate FROM the station
. This is especially important when a controller tells you to fly inbound or outbound on a given radial. You are the one who selects the radial that results in a TO or FROM flag. While an instruction to fly a particular radial outbound (away from the station) is usually clear enough, pilots sometimes get confused when asked to track a radial inbound.
ATC has instructed the pilot of the plane in Figure 6A to fly inbound to the VOR station on the 090° radial. He selected the 090° radial with a TO flag, meaning he's flying TO the station. This pilot has confused the assigned radial with the bearing he must fly in order to comply with the instructions. He's flying TO the 090° radial, which is on the other side of the station. He's not flying inbound TO the station on the 090° radial.
Figure 6B shows the correct orientation. Remember that controllers think about TO or FROM flags. Radials start at the station and extend outward in only one direction. The pilot in 6B is complying with the instruction, and he selected the 270° radial to avoid reverse sensing. If this pilot is not in radar contact, when the controller asks him on which radial he's flying, the pilot would say, "I'm inbound on 090."
In reality, ATC seldom assigns radials without giving a heading that will intercept it. Having an initial intercept heading makes it much easier to get the orientation right. If you're left on your own to determine the intercept heading, however, remember to use the reciprocal of the radial assigned whenever the controller says "inbound."
Tracking directly to the station is another common VOR operation. To do this, rotate the OBS until the CDI centers with a TO indication. If the FROM flag shows, you have the wrong radial. Once the needle is centered with a TO flag, note the radial selected and turn to that heading. If you're close to the station, the needle may be out of center by the time you complete your turn. Just recenter the needle using the OBS and turn to the new heading.
If you need to fix your position without actually flying over a VOR, center the needle with a FROM flag and note the radial you have selected. If you draw a line on your chart that corresponds to the selected radial, you're on it someplace. If you repeat the procedure using another VOR, your position is where the two lines intersect.
VOR Tracking
Once the CDI is centered, flying along the selected radial in a no-wind situation is a matter of flying the same magnetic heading. But how often have you flown in no-wind conditions? When you see that the wind is blowing you off course, you'll have to correct by turning toward the radial.
How large your correction should be is determined by the wind and how quickly you need to get back on course. If in doubt, start with a 20-degree turn toward the radial. If the CDI doesn't start moving toward the center after about a minute, increase the correction.
If the CDI starts to center with the initial correction, hold the heading until the CDI centers. Remember that any heading that takes you to the radial will also take you through it, so don't forget to turn back to the radial when the needle centers.
Obviously, if you don't adjust for the crosswind, you'll have to repeat the procedure. Therefore, head 10 degrees into the wind and observe the CDI. If you're blown off the radial again, increase the correction; do the opposite if you fly through the radial. You may have to make several adjustments before you find the heading that keeps the needle centered.
You don't have to wait until you're blown off the radial to take corrective action. If you're aware of the wind, as all pilots should be, correct for it when you intercept the radial. If you intercept the 090° radial and know that the wind is from the north, make your initial heading 085° or 080°, and then fine-tune your correction by watching the CDI.
Needle Mania
The CDI will become increasingly sensitive or twitchy as you approach the station; don't try to correct for it. Just fly the heading that held the needle centered to this point. When the flag changes from TO to FROM, rotate the OBS to the radial you want to track and intercept it when the CDI settles down.
CDI sensitivity can sometimes lead to confusing VOR indications. The pilot in Figure 7 has been tracking the 180° radial inbound (and he's set the OBS to the 360° radial to avoid reverse sensing). Instead of correcting for the wind, he's drifted east until he has a full-scale left CDI deflection. The red lines are 10 degrees to either side of the radial; beyond them is full scale CDI deflection.
The pilot starts to correct for the wind at Position A. This correction will eventually bring him back to the course line, but it will happen beyond the station. The CDI remains pegged because the 10-degree line of full-scale deflection is converging on the station as fast as the aircraft is approaching the radial. Concerned that the CDI hasn't moved, the pilot increases his correction at Point B, not realizing his proximity to the station. His new heading brings him to the radial at, or just past, the station.
In the cockpit, the flag flips from TO to FROM. The CDI slams from full-scale left deflection to full-scale right. And the pilot banks steeply in an attempt to catch the needle. Fortunately, the problem tends to resolve itself once the plane is far enough from the station for the CDI to get less sensitive. The pilot, however, is probably still wondering what happened. (His passengers are most likely thinking the same thing.)
Had the pilot been situationally aware, he would have either not corrected at all while south of the station, or held his initial correction and smoothly intercepted his course north of the station. Keeping the CDI perfectly centered within a couple of miles of the station is not essential. This isn't an excuse for sloppy flying; the message is simply to take it easy when near the station.
The real key to VOR navigation is the same for any skill--practice! Practice your intercepts and tracking. Keep an eye on the ground and note your position relative to the station as you get certain VOR indications. That will help you form a good mental picture of what is really going on when you can't see the ground.
And if you are not instrument-rated, try it again under the hood with an instructor. Always visualize where you are. Moving map displays may one day be in every cockpit, but until that day arrives, you will have created a moving map in your mind. (Even if you have a moving map on the panel, you should still have one in your head, because displays fail.) When you do that, you will never again have serious trouble with VOR navigation.