The VHF Omnidirectional Range navigation system, VOR, was probably the most significant aviation invention other than the jet engine. With it, a pilot can simply, accurately, and without ambiguity navigate from Point A to Point B.
The widespread introduction of VORs began in the early 1950s and 50 years later it remains the primary navigation system in the overwhelming majority of aircraft.
If you jumped to this point of the website without proceeding through the earlier sections, I strongly recommend that you return to the Air Navigation section and review the sections on VFR Sectional Charts, IFR enroute low altitude charts, and the basics of plotting a course. Further, you should go to the NDB Approaches/Approach Plates section and read the basics of Instrument Approach Plates, now called Terminal Procedures.
The basic principle of operation of the VOR is very simple: the VOR facility transmits two signals at the same time. One signal is constant in all directions, while the other is rotated about the station. The airborne equipment receives both signals, looks (electronically) at the difference between the two signals, and interprets the result as a radial from the station.
The GPS, Global Positioning System, is making inroads onto the navigation scene and offers a flexibility unavailable with either NDB or VOR systems. However, it is supplementing these systems, not replacing them.
The RMI indicator used in the NDB navigation exercises is as close to a "hands-off" indicator as you will find. In an aircraft the RMI compass card must initially be aligned with the compass before a flight begins and then rechecked every fifteen minutes or so, and that's it.
With VOR, however, course information must be manually entered into the indicator. The VOR indicator below shows an aircraft heading toward, "TO," the Omni station.
NOTE this very important fact, with more info farther down. The radial signals of a VOR always point away from the station. The indicator below shows 345°, but since we are heading toward the VOR, see arrow D, we are actually on the reciprocal radial, or the 165° radial. This aircraft is south of the station. This will become more clear in a moment.
See the text for details on the four components of the VOR Indicator.
The digital indicator is a separate gauge used on the Nav Trainer Panel.
- A Rotating Course Card, calibrated from 0 to 360°, which indicates the VOR bearing chosen as the reference to fly TO or FROM. Here, the 345° radial has been set into the display. This VOR gauge also digitally displays the VOR bearing, which simplifies setting the desired navigation track.
- The Omni Bearing Selector, or OBS knob, used to manually rotate the course card.
- The CDI, or Course Deviation Indicator. This needle swings left or right indicating the direction to turn to return to course. When the needle is to the left, turn left and when the needle is to the right, turn right, When centered, the aircraft is on course. Each dot in the arc under the needle represents a 2° deviation from the desired course. This needle is more-frequently called the left-right needle, with the CDI term quickly forgotten after taking the FAA written exams. Here, the pilot is doing well, and is dead-on course—or maybe lazy and with the autopilot activated in the "NAV" mode.
- The TO-FROM indicator. This arrow will point up, or towards the nose of the aircraft, when flying TO the VOR station. The arrow reverses direction, points downward, when flying away FROM the VOR station. A red flag replaces these TO-FROM arrows when the VOR is beyond reception range, has not been properly tuned in, or the VOR receiver is turned off. Similarly, the flag appears if the VOR station itself is inoperative, or down for maintenance. Here, the aircraft is flying TO the station.
Radials, Radials, RadialsTo grasp the VOR system you must understanding that it is entirely based on radials away from the station.
In the Sandy Point VOR to the left, note first that the arrow on the 0° radial points away from the center of the compass rose. You'll remember that this radial points to the west of true north because of the west magnetic variation. North on a VOR is Magnetic North. So, if you overflew this VOR on the 0° radial, you would be flying away from the VOR.
Similarly, note the arrows by the 30°, 60°, 90° marks and the rest of the way around the compass rose. They all point away from the station. Radials arealways away from the station.
There is only one line on the chart for each numbered radial for a particular VOR station. Whether you are flying it outbound or inbound, or crossing it, a radial is always in the same place.
The only possible complication lies in the reciprocity of the numbers. Whenever you are proceeding outbound, your magnetic course (and heading when there is no wind) will be the same number as the radial. Turn around and fly inbound you must mentally reverse the numbers and physically reverse the OBS setting so that your course is now the reciprocal of the radial. But the radial you are flying on hasn't changed.
Some examples will cement this in your mind.
- The aircraft isn't on the 345° radial because that radial extends from the Omni to the northwest as shown by the arrow.
- The aircraft is actually on the reciprocal radial, the radial pointing towards the plane. That reciprocal radial is 165°, away from the station like all radials.
- If the 165° radial were set into the VOR, the FROM flag would properly show, because the aircraft is away from the Omni on that radial.
- Here is the important point. If the OBS is rotated until the needle centers and the FROM flag shows, it will always show the correct radial from the Omni that the aircraft is on regardless of the aircraft heading.
- To eliminate the confusion of location relative to an Omni, the magnetic course of the aircraft and the radial setting on the VOR should be the same.
- Presumably the aircraft is flying in the desired course direction, so its heading will be approximately the same as the VOR setting, i.e., the magnetic course. The heading may differ slightly from the VOR because of the correction needed to correct for wind drift.
- Thus, with the OBS set to 345° the left-right needle shows the aircraft on course and the TO flag is showing, pointing up, toward the station ahead.
Next, rotate the OBS to the reciprocal of the course. The needle will again center, but the FROM flag will appear.
A one-line recap: to know whether you are flying TO or FROM an Omni, the OBS setting must be approximately the same as the aircraft heading.
Where am I?
Same example as before. The aircraft is south of the Omni, on the 165° radial. It is flying northwest. Observe the DG. The aircraft is heading 345° as desired. But the OBS was improperly set to 165° and the VOR is falsely informing the pilot, with a nicely centered needle, that he/she is flying away FROM the Omni. The aircraft, of course, is flying TO the Omni.
Hate to beat a dead horse, but again, the TO-FROM confusion disappears if the aircraft heading and the OBS setting are approximately the same which they weren't here. Pay attention to this and you will stay out of trouble.
This sort of error usually happens when the pilot rotates the OBS, watching only for a centered needle, not also paying attention that the setting should approximate the magnetic course, or aircraft heading.
Wandering off course?
This aircraft is 4° off course. Each dot of the arc under the needle is a 2° deviation from the desired course. Don't confuse heading, the direction of the aircraft's nose, with course, the desired track along the ground. Only with no wind will heading and course be the same.
"The needle is centered, my flying is perfect"Nice thought, but not necessarily. The VOR system operates in the VHF frequency band, from 108.0 to 117.95 MHz. Reception of VHF signals is a line-of-sight situation. Nominally, you must be 1000 ft AGL to pick up an Omni within its maximum low-altitude service range.
The VOR indicator is smart enough to know when a usable signal has not been received and displays an "OFF" flag, a red and white barber-pole striped flag in the gauge in the illustration to the left. So when you are flying to or from an Omni station and you're quite content at how stable the CDI needle has been, it's worth taking another glance at the gauge to see if the OFF flag is staring back at you.
The OFF flag also displays if the Nav receiver is tuned to the wrong frequency or, blush, if it's properly tuned but you neglected to turn on the power switch. If you're taking your check ride with an FAA examiner for a real license, that oversight is likely to get you a quick return to terra firma. And, there's also the possibility of a popped circuit breaker interrupting power to the Nav receiver, a connector jiggled loose, etc.
VOR RangeAh, the oft asked and seldom answered question: how far away can I pick up a reliable signal from the Omni and what altitude need I be at? The FAA neatly skirts the answer by classifying Omnis by an altitude code, with the ranges vs. altitudes as shown in the table below.
|Terminal (T)||25||1000 – 12,000|
|Low Altitude (L)||40||1000 – 18,000|
|High Altitude (H)||40|
|1000 – 14,500|
14,500 – 60,000,
18,000 – 45,000
Consider the Bangor VOR, BGR, at Bangor (Maine) Int'l. Airport. Here are the comments in the Airport/Facility Directory:
"VOR unusable 342°—063° below 2500 ft."Pretty significant terrain impact, wouldn't you say? So think of the FAA data in the table as a starting point that may be modified by terrain.
Checking VOR accuracyThe VOR is the most common navigation instrument presently on aircraft panels. We rely on it to accurately track VOR radials, whether flying between Omni stations, or locating intersections, or arriving and departing from airports. We accept at face value that what it displays is accurate. Well, on FS98 and FS2000 it is always accurate. But in the real world, not only can the gauge be wrong, but the FAA requires that a pilot check the VOR for accuracy within 30 days of an IFR flight. Even if a pilot never flys IFR, it is prudent to regularly check the VOR for accuracy.
One acceptable way to formally check VOR accuracy is with a VOR Test Facility, more commonly called a VOT. A VOT is a low-power Omni station located on many of the mid-to-large size airports. A VOT differs from a standard Omni in that it transmits only a single radial, the 360° radial.
To calibrate a VOR, the pilot tunes in the VOT frequency while on the ground (in rare instances this check is performed in the air). Refer to the back of theAirport/Facility Directory for frequencies and whether it is a ground check (G) or an airborne check (A). See the Connecticut illustration below.
|Facility (Arpt Name)||Freq.||Type VOT||Remarks|
|Bridgeport (Sikorsky Mem)||109.25||G|
|Groton (Groton–New London)||110.25||G|
|Hartford (Hartford–Brainard)||108.2||A||3 nm Radius 1200–5000 ft.|
Make note in the illustration above that the VOT at Bradley Int'l. airport is on 111.4 MHz. That information is important later while performing one of the VOR approach practice flights.
Although I did the keyboarding for this site it contains the fine contributions of many people.
With no prior experience in building a website I was overwhelmed at the generosity of those that I asked for permission to either reproduce their work, or for their technical help, or to use their designs, artwork and descriptions.
Many thanks to those listed below in alphabetical order, who so willingly shared their talent so that I could assemble these webpages:
Bob Austin, of Mission Viejo, California. A scenerio in the VOR Approaches section required an over-kill list of mountaineering equipment for Mr. Benjamin Counter. Counter intends to "scale" southwest New Hampshire's 3165 ft. Mt. Monadnock, the second-most climbed mountain in the world, behind only Mt. Fuji in Japan. Since Florida's highest "mountain" just reaches 345 ft. I had to look elsewhere for an expert. Bob Austin jumped right in, providing an awesome equipment list, which I had to shorten. Any omissions are my work, not his. Bob has the credentials to provide an accurate and complete list, too. In his climbing he's completed six big-walls including two El Capitan routes—The Nose and Triple Direct. Thanks, Bob. Just reading your equipment list I could imagine the wind fiercely blowing off the face of the mountain, daring any climber to move higher.
Wagner Beskow, for his Handy Sheet 3.0. Wagner, a Brazilian in New Zealand taking his Ph.D., assembled this one-page compendium of useful facts for the flight-simmer. "It takes a while for a beginner to find all that information and when he does it's all scattered and difficult to refer to. The idea was to produce something useful that could stay around without cluttering the 'cockpit' too much." Thanks, Wagner. I wish I could say my cockpit was uncluttered now.
Boeing Company, photo of 40-A mailplane. The 40-A, a fabric-covered mail plane, was Boeing's first commercial success. Built in 1927 (the year Charles Lindbergh crossed the Atlantic), it carried mailbags and—load permitting—two passengers. http://www.boeing.com/companyoffices/gallery/boeinghistcom.html
Tom Christine, Electrical Engineer, Federal Aviation Administration. Tom was very helpful on specifics about present-day Non Directional Beacons, including their power levels, uses, their friendly coexistence with GPS, and frequency limits of the LF band. Thanks Tom.
Mario Corral, Sydney, Australia, who sent me the sketch of the homing pigeon. Mario patiently explained to me some similarities and differences between homing pigeons, racing pigeons, and carrier pigeons. Thanks Mario for keeping me on the right track.
Rick Covington, photos of the Piedmont DC-3 in the "On the Beam" section. Thanks Rick ... an excellent photo. You can find more of his photographic expertise at www.airliners.net. He photographed the Piedmont shown here in Durham, N.C.
Mike Genovese for the finest interior shot I've seen of any aircraft. "The shot was taken in Nassau in the early 90's... the plane was just sitting there unattended as we (crew of a major airline) walked by on the ramp... I could not resist the opportunity for the 'Kodak Moment!' ... " Great pic Mike and keep your camera handy as you jet to all those tough destinations. See www.airliners.net for more of Mike's work.
Jesse Kempa for his virtual E6-B calculator program. This program does it all, and best of all, it's freeware. Input True Course, TAS, wind speed and direction and on the click of a button it returns the Wind Correction Angle and Ground Speed. It will calculate distances between points with known lat. and long. coordinates, and on and on. Thanks Jesse for a great utility.internet.oit.edu/~kempaj/e6b.htm
Brian Kostick, the gauge programmer of the very-useful digital Elevator Trim indicator. With it the pilot has repeatability in elevator trim settings. Thanks to Brian, Br5an@aol.com, for an instrument that I rely on and for his quick willingness to let me use it in this panel.
Paul Lutus ... who wrote the freeware HTML editor "Arachnophilia" which I used in assembling these webpages. Arachnophilia is a full-featured, nearly perfect editor—and now v4.0 is even better with a 120,000 word spell-checker. Paul has an outstanding philosophy on life, too. Thanks, although we've never "met." www.arachnoid.com
Alice Marks ... for the extraordinary sketch of McGirr field. That picture is the essence of the intermediate fields that Air Mail pilots aimed for when facing an emergency. Her artistry instantly backed me onto McGirr field—into the new era of Air Mail, with the wind sweeping across the plains, striking me in the face. But then, it should have because Alice Marks's father was McGirr field's caretaker and Alice grew up there. She has a wonderful first-person account of the activities at the field that beats anything you'll read in stuffy history books. Take five and go to her site and relive the experience. http://www.halcyon.com/cliffsan/airmail/air_mail.html. Thanks Alice, twice ... for sketching McGirr field to keep that kernel of history alive and for letting me share it with others.
Ned Preston, Agency Historian, Federal Aviation Administration. Ned provided the chronology on when VORs were introduced, a whole wealth of information on the four-course radio range and on the light-beacon system used to identify the airways at night before radio navigation facilities were in place. Never met a more helpful person. Thanks Ned.
Bill Rambow for his many e-mails answering so many questions. Bill designed the incomparable dual panel for the Douglas DC-3 / R4D / C-47. A restored, flyable U.S. Navy R4D at the Mid Atlantic Air Museum in Reading, Pa. was the model for his work. A finer piece of work I've never seen. Thanks for the encouragement, Bill.
Tom H., "Rumple," a gentleman who always sets his work aside to answer the other fellow's questions. I reaped the benefits of his very-methodical air-file testing. He also designed the manifold pressure gauge for the training panel in my Virtual Airline. Tom is very visible on newsgroups and forums patiently answering questions that are asked over and over. Thanks for being there, Tom.
Sarah at Airline History Website for the photo of the KLM Douglas DC-5 which opens the VOR Navigation section. I only recently learned that a DC-5 had ever existed and wanted to include a picture of it. Since Sarah's site has 350 Airline Histories, 1700 Airline Pictures, and 150 Airliners from 1920 to 1999, I knew it would be there. Why is Sarah interested in all this stuff? ... because she's a pilot—and more. She writes software for and sells IFR flight Simulator programs for real IFR training. Thanks Sarah for the great work that you put into your site.airlines.freeuk.com/airlines
Chris Sheldon, firstname.lastname@example.org, from the U.K. He photographed G-AMPZ at Manchester Airport, and it appears here in the IFR Charts section. The aircraft belongs to Atlantic Airways (previously called Air Atlantique) who are based at Coventry Airport. The first letter in the registration number, "G," denotes a U.K. aircraft. Chris exhibits his work at www.airliners.net. Thanks Chris.
Russ Strine, Mid Atlantic Air Museum in Reading, Pa. Here's another fountain of information. A very busy man at the museum, he pushed his papers aside and spoke with me for an hour on the telephone about aviation in the old days and VORs and NDBs and answered questions that I just wasn't savvy enough to ask. He gave me some anecdotal stuff, too, that you can't find in books. Thanks for your time, Russ, and all the information. www.maam.org
Barry Thomas, Silversmith, who created the magnificent chalice pictured in the section on tracking ADF's inbound. That section's lead-in story centers around Mr. Benjamin Counter's desire to fly to Meriden, Connecticut, 'The Silver Capital of the World' to buy some silverwork. The chalice pictured was not created in Meriden, nor even in the U.S. Barry Thomas is a silversmith in rural Derbyshire, England. Meriden earned the Silver-Capital title in the late 19th century and it is doubtful that it still retains the rights to that sobriquet. Thanks Barry for the great picture, too. More of his masterpieces can be found athttp://easyweb.easynet.co.uk/bthomas.
Rod Watson, for his stunning photograph of the Gay Head Lighthouse on Martha's Vineyard, Massachusetts. Rod has visited over 170 lighthouses in the US. and the Lighthouse Gallery section of his website, Rod's Photo Gallery, includes photographs of most of those lights, organized by geographic locations. He photographs more than lighthouses, too. Thanks, Rod, for a great picture.Rod's Photo Gallery
Roland Zuiderveld, from Sweden, for his totally awesome picture of a DC-3 descending out of the clouds at dusk to land. Roland also displays his work onwww.airliners.net. Thanks Roland, and if you take any more pictures like this one I want to hear about them.
Many others contributed to my virtual airline webpages and were just as generous. They are identified on that site. Assembling a webpage is so much easier with the assistance of people like these.