December 2000Features

The ABCs Of VORs

Make The Most Of This Navigation Tool


The VOR (very high frequency omnidirectional radio range) receiver and its corresponding VOR stations form the world's most commonly used ground-based electronic navigational system. Because of its signal range and many uses to pilots, the VOR receiver is standard equipment on nearly every type of aircraft ranging from Cessna 150s to Boeing 747s and Bell JetRanger helicopters. VOR airways, also called Victor airways, can be flown in every corner of the globe as can VOR instrument approach procedures.

Even as pilots depend on VOR navigation and the basic concepts involved, most pilots have difficulty mastering VOR techniques in their early training, and this confusion can linger well into their flying careers. Ironically, the very simple concept of opposites (To/From and Left/Right) creates an often confusing and abstract set of clues for pilots already hard at work flying the aircraft, communicating with air traffic control, and keeping track of the airspace they are using.

The VOR Navigational Station

There are three types of VOR navigational stations: VOR (just the VOR), VOR-DME (VOR plus distance measuring equipment), and vortac (VOR plus the military's tactical air navigation system). Each VOR station can further be classified according to its range - terminal, low altitude, or high altitude. Terminal VORs are designed to be clearly received up to 25 nautical miles from the station at altitudes of 1,000 feet agl through 12,000 feet agl. Low-altitude VORs are meant to be used from 1,000 feet agl through 18,000 feet agl at distances of up to 40 nm from the station. Finally, high altitude VORs have the greatest range - 130 nm - between 18,000 feet agl and 45,000 feet agl, although they can also be effective at shorter ranges of 40 nm or more from 1,000 feet agl all the way through 60,000 feet agl. Remember that all VORs can only be received line-of-sight. So, if there's a mountain between you and the VOR, you will not receive a reliable signal even though you are within the station's range.

All VORs operate within the 108.0 to 117.95 MHz frequency band. VOR-DME stations and vortacs both have distance measuring capabilities. So, with the right equipment in the cockpit, pilots can determine not only their radial relative to the VOR station but also the slant-range distance from their aircraft to the station. Many VORs also have voice capability that can identify the VOR or give the pilot weather information, including HIWAS (hazardous in-flight weather advisories), TWEBs (transcribed weather broadcasts), and instructions from flight service stations.

The VOR Indicator

The VOR indicator in the cockpit features an OBS (omni bearing selector) knob that the pilot uses to select VOR radials by placing them above the course index (on top of the VOR indicator) or below the reciprocal course index (at the bottom of the VOR indicator). A Left/Right needle called a course deviation indicator (CDI) shows the pilot how many degrees and in which direction from the selected radial he is flying. The direction of deflection tells the pilot where he is relative to the radial, while the dots on the instrument's face tell the pilot how many degrees he is off his course. Each dot represents a 2-degree deflection from the desired course. There are 10 degrees of deflection on either side of the center disk, creating 20 degrees total indicating capability. The To/ From flag on the face of the instrument tells the pilot on which side of the selected radial he or she is flying.

VOR Functions - Secret To Success

The VOR indicator can give the pilot many types of information about his or her position relative to the station. For example, by using two different VOR frequencies, a lost pilot can find out exactly where he is. If you have two VOR indicators, tune each one to a different VOR frequency. Then center the CDI needles with a From indication and note the radials. Get out a chart and plot the extended radials. Where the lines intersect, there you are. In addition, pilots can use VORs to fly Victor airways, find airports where VOR beacons are colocated, tell controllers their position relative to VORs, and much more.

All types of VOR navigation are based on just four sets of procedures that every pilot should know. These procedures deal with identifying the radial you are on, intercepting and flying a specific radial away from a station, flying directly to a VOR station, and flying a specific radial to a VOR station.

To identify the radial that the aircraft is on, center the CDI with a From indication. The number on the top of the course index is the radial that you are on.

To intercept and fly a radial away from a station, the pilot should turn the OBS until the desired radial is on top of the VOR indicator. With that done, the To/From indicator should show a From indication and the CDI will deflect in the direction that the pilot must turn to intercept the desired radial. In other words, turn toward the needle.

To fly directly to a VOR station, turn the OBS until the CDI needle is centered with a To indication. The heading to the VOR station is on the course index. All you need to do is turn until the aircraft's heading matches the number on top of the course index. If there is no wind to blow you off course, flying this heading will take you directly to the station. Adjust the CDI to keep it centered since your objective is to fly to the beacon and not to track any specific radial.

To fly a specific VOR radial inbound to a VOR station, use the OBS to place the radial you want to fly inbound on the reciprocal course index on the bottom of the VOR indicator. You should have a To indication, and the CDI needle will deflect in the direction you need to turn to get to the station. (Once again, turn toward the needle.)

Putting it Together--VOR Orientation

Look at the figure on page 34. Each T or F represents the position of an aircraft which has selected the same VOR frequency, the same (90-degree) radial, and its corresponding To/From flag indication. If an aircraft's VOR is tuned to the 90-degree radial it will indicate a To flag when west of the VOR and a From flag when east of the VOR regardless of aircraft heading.

Each R, C, or L represents the position of an aircraft which has selected the same VOR frequency, the same (90-degree) radial, and its corresponding CDI needle indication. If an aircraft has tuned to the 90-degree radial it will have a right CDI indication north of the 90 degrees, a centered CDI indication when on the 90-degree radial (the same west of the VOR), and aircraft south of the radial will have a left CDI indication. These CDI indications tell the pilot which way the selected radial (90 degrees) is relative to the aircraft position regardless of aircraft heading.

If you put these two concepts together, you will have the needle/flag indications shown in the bottom illustration on page 34, regardless of aircraft heading.

Again, each RT, CT, LF, etc., represents the position of an aircraft which has selected the 90-degree radial and its respective CDI and flag indication. With this information the pilot knows where he or she is in relation to the VOR beacon and the selected radial and which way to turn to intercept the desired radial, regardless of aircraft heading.

Does Heading Matter?

Understanding how to interpret the To/From flag is critically important because, if you get it backward, the Left/Right CDI needle will give you opposite (and therefore inaccurate and unsafe) course interception information. In other words, the CDI needle will tell you to turn left when you should turn right and vice versa. This is known as reverse sensing.

To avoid reverse sensing, remember that radials are always radiating away from the VOR station. So, you normally want a From indication. The exception is when you are specifically told to fly directly to a station or track a radial inbound. Then you are going to the VOR, and you want a To indication.

It is always important to remember that your aircraft's heading does not affect the To/From flag and Left, Right, and Centered CDI indications on your VOR instrument. Your aircraft's position relative to the VOR beacon and the radial you have selected does. (Think of your aircraft as a dot in space without regard to heading). However, if your aircraft's heading is on the opposite hemisphere of the heading indicator to your selected VOR radial course index, you will see a To flag and you will be flying away from the VOR beacon (or a From flag and be flying toward the VOR beacon). Similarly, the CDI needle pegged to the left means flying left you are flying away from the selected radial and not toward it. (The opposite is true with a right-pegged CDI needle.) A centered CDI needle indication is not affected and is, therefore, accurate. For example, if you have selected the 90-degree radial (on the course index) but your heading is 270 degrees, the CDI needle is to the left, and the To/From flag is indicating To, then the 90-degree radial is actually right of you and the VOR is actually behind you and not in front of your aircraft. It is obvious that this could get you into trouble on a checkride and put you in danger when navigating.

Your Ultimate Goal--Navigating With the VOR

In a no-wind situation, your ultimate goal is to have the same heading on your heading indicator, magnetic compass, and course index (on the top) of your VOR indicator. The needle on the VOR indicator should be centered whether you are tracking a radial inbound or outbound.

Imagine that you want to track a radial inbound. When you set up your VOR so that the desired radial appears in the reciprocal course index window at the bottom of the VOR and you have a To flag, you see that the needle is deflected to the right. But turning to the right only until your heading matches the course index at the top of your VOR will take you parallel to your desired radial. No matter how long you fly that heading, the needle will always remain to your right because you have not intercepted the radial.

To correct this, you must turn to the right past the heading shown in your course index. Most instructors recommend turning 30 degrees to 45 degrees past your desired course. This is called an intercept angle, and it will allow you to fly a course that will ultimately intersect the desired radial. (Using a 45-degree intercept angle can be the easiest way to go. Since most directional gyros have a hash mark at 45 degrees, you don't need to do the math. Just look at the number under the hash mark and then turn to that heading.)

It may take a few minutes, depending on how far you are from your desired radial and from the VOR station, but eventually, the CDI needle will begin to move back toward the centered position. Once that happens, you must begin turning left, back toward the desired course heading as shown at the top of the VOR's course index. Ideally, you can turn at approximately the same rate as the CDI needle moves so that you can roll wings level on the desired course at the same time the needle centers. Needless to say, this takes some practice. Don't worry if you miss intercepting your radial and the CDI swings past center to the left side of the VOR. Simply follow the same procedure - turning toward the needle and adding an intercept angle - until you are flying on the desired heading with the needle centered.

Once you are flying the radial, the CDI needle will act as a wind direction indicator. If all the numbers are lined up and the CDI starts to move to the left, wind is causing the aircraft to drift to the right. To correct, make a slight turn toward the needle - only about 5 to 10 degrees - to allow you to reintercept the desired radial. Depending on the wind, you may need to fly with a correction angle to stay on the desired radial. (This is just like crabbing the airplane to stay on the extended centerline during final approach with a crosswind.)

Test Your Knowledge

Try your new VOR knowledge on the practice exercise at the bottom of this page. (Remember, each aircraft has the same VOR frequency and has selected the 60-degree radial on the course index). Choose any aircraft. You should be able to tell if it has a To or From flag and a Left, Centered, or Right CDI needle indication. Good luck!


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