Thursday, August 13, 2015

ADF / NDB Navigation (reprint)

Navigating with the ADF

RMIThe LF navigation system was so durable that for twenty years it had no competition. It was widely used long after VOR installations began to sprout up throughout the country. It has the major advantage over a VOR in that reception is not limited to line of sight distances. The LF signals follow the curvature of the earth, and thus the maximum distance that an ADF is usable depends only on the power of the beacon.
Additionally, the ADF indicator provides more information than the VOR indicator. Here it shows that the aircraft is on a 345° heading and that the magnetic bearing to the NDB is 060°.
When I was doing my Instrument Flight training many students dreaded NDB procedures. They hoped that the FAA examiner would "forget" to test those skills. To my knowledge they never "forgot." Currently, it is mandatory to successfully demonstrate an NDB approach on the instrument flight test.
On reflection, one wonders why emotions ran—and run!—so negatively towards ADF procedures. In some aspects it is simpler to track to or from an NDB than to or from a VOR. The VOR, of course, offers the advantage that one can set in the desired radial with the OBS, leaving little doubt that the track to or from the station is correct.
Six ADF navigation procedures will be reviewed here.
  • ADF Time- and Distance-to-Station checks.
  • Intercepting a bearing.
  • Homing.
  • Tracking inbound to an NDB.
  • Tracking outbound from an NDB.
  • NDB Approaches.
On first reading the descriptions, ADF procedures could appear difficult. With modest practice on your Flight Simulator, though, they become routine. They even make sense! The key to successful ADF navigation is to know where you are at all times. Visualization is a must to make the right moves.
Four types of ADF Indicators are in use. In every case, the needle points to the navigation beacon.
  • Fixed Compass Card.
  • Rotatable Compass Card.
  • Single-needle Radio Magnetic Indicator.
  • Dual-needle Radio Magnetic Indicator.

Fixed Compass Card

fixed-card indicator
The Fixed Indicator's compass card is exactly as the name says. It is fixed to the face of the instrument and cannot rotate; 0° is always straight up—the nose of the aircraft. Visualizing the situation with this type of indicator can be daunting.
Consider the present situation. Remember, all numbers are relative to the aircraft's magnetic heading. From before, the aircraft is heading 345°. The ADF indicator shows the beacon 75° relative to the aircraft's heading. Since the beacon is to the right, add numbers: 345 plus 75 equal 420. Subtract 360 and the magnetic bearing to the beacon is 060°. It's enough of a mental calculation sitting at a desk; in a pitching aircraft glopping about in the soup, the math can seem impossible. Some consider using this type of ADF Indicator as cruel and unusual punishment: great as an emergency backup, very unsatisfactory for every-day use.
The fixed-indicator can still be found on many aircraft panels, but seldom in planes where the pilot actually uses the ADF.

Rotatable Compass Card

rotatable-card indicator
The Rotatable Compass Card Indicator was a big step over the fixed-card indicator. The pilot can now rotate the compass card with the heading knob to display the aircraft’s magnetic heading "straight up." Then the ADF needle will directly indicate the magnetic bearing to the NDB. For example, again the aircraft is on a 345° heading. The pilot manually rotates the ADF card to show a 345° heading as shown here. The magnetic bearing to the beacon now correctly shows as 060°. Now, to track to the beacon simply turn the aircraft to a heading of 060°—ignoring any wind correction requirements for the moment ... just remember to rotate the compass card to the new aircraft heading of 060° after the turn.

Single-needle Radio Magnetic Indicator

rotatable-card indicator
The Single-Needle Radio Magnetic Indicator is the best of all worlds. The Radio Magnetic Indicator, as its name implies, is an instrument that combines radio and magnetic information to provide continuous heading, bearing, and radial information. It is an extremely simple indicator.
The Indicator’s compass card is a directional gyro and so it rotates automatically as the aircraft turns. Thus it always accurately indicates the magnetic heading of the aircraft as well as the magnetic bearing to the beacon. Although it's a "hands-off" instrument, its compass card should be checked against the magnetic compass every fifteen minutes or so and adjusted with the heading knob to correct for any precession errors common to directional gyros. The face of the single-needle RMI is similar to that of the rotatable card ADF just shown.
The ADF gauge on the Cessna Nav Trainer panel is a single-needle Radio Magnetic Indicator. The illustration here includes the digital display added to the RMI which indicates the bearing to the station. The digital display is not a standard feature on an RMI. It was included here because precisely reading the RMI is such an integral part of the practice flights.

Dual-needle Radio Magnetic Indicator

dual-needle RMIThe dual-needle RMI is similar to the single-needle RMI except that it has a second needle. The instrument here presents the same information as the two previous instances: aircraft heading is 345°, and the magnetic bearing to the NDB is 060°—the yellow needle. The second needle, green here, typically points to a VOR station tuned in on the Navigation Receiver—in this case, fly 303° to get to the VOR. The dual-needle indicator is particularly useful in fixing the location of an aircraft. The two lines of position appear right on the face of the gauge.
Sophisticated RMIs show magnetic bearing to any two nav stations chosen by the pilot: ADF and VOR#1, ADF and VOR#2, or VOR#1 and VOR#2. Military pilots routinely navigate with an RMI and wonder why the rest of us fool around with a standard VOR indicator.

Characteristics of ADF Navigation

Please note, that from here on, all ADF discussions and mathematics is limited to navigation using a single-needle RMI.
The most important concept in ADF navigation is that the needle always points to the station. There are three implications to this.
  • First, when tuned to an ADF station for homing, tracking, intersections, holding or an approach, you never have to touch the system. Unlike the VOR, there’s no OBS to think about, no need to "twist" anything in station passage or at any other time. This makes the ADF much simpler to use.
  • Second, with an ADF you always know where you are in relation to the station. There is no TO-FROM to interpret, no confusion about radials and bearings, no way to set the wrong OBS numbers.
  • Third, ADF orientation is much simpler than VOR orientation. With an RMI the head of the ADF needle always indicates the bearing to the station. Simply turn to the heading under the arrow head to fly towards the beacon.
Low frequency beacons can sometimes be received at great distances. It is very important to verify the Morse-code identifier of the station to be certain you are navigating from the proper beacon—even in your Flight Simulator.

ADF Time and Distance Checks

Good exercises to develop NDB awareness are the ADF Time and Distance Checks. Tune in an NDB station, verify its Morse identifier, (Click the ID label in the center of the radio face) and then position the aircraft so that the needle points directly to the left or right, indicating that the station is directly off the aircraft’s wing.
Note the bearing to the station and also the time, or set the timer. Then fly a constant heading until the bearing changes 10°. Note the number of seconds it takes for the bearing to change this 10°. Then simply divide that elapsed time by ten to determine the time to station in minutes.
Time in seconds
Minutes to station =  
(Degrees of bearing change)
You can then estimate the distance to the station:
TAS [kts] × Minutes flown
Nautical Miles to station =  
(Degrees of bearing change)

A Time and Distance Example

enroute headingstart of timingend of timing
  1. You’re en route from New Bedford, Mass. to Manchester, N.H., tracking the 005° bearing to the Haget beacon. Company-President Mr. Benjamin Counter is your sole passenger. Part-way into the flight Mr. Counter decides that a face-to-face meeting would be appropriate with the principal-stockholder, Mr. Well Thee. Mr. Thee is vacationing in Provincetown, on Cape Cod, with his administrative assistant. This working arrangement was mentioned in the Route-selection section.
  2. For the Time and Distance to Provincetown, tune in and verify the Provincetown beacon. Then—in this case—turn to a 025° heading to put Provincetown’s beacon directly off your right wing. The ADF indicator shows a 115° bearing to the beacon. Start the timer.
  3. Maintain a 025° heading. When the ADF needle has shifted ten degrees, to a 125° bearing to the station, note the time—three minutes and forty seconds, or 220 seconds.
Plugging the numbers into the formulas above:
The time to station is:
220 seconds

  = 22 minutes to station.
10 degrees
You can then estimate the distance to the station:
With Mr. Counter on board—"Bean" Counter to all who are out of his hearing range, the aircraft is cruising at an economical 110 kts. The distance to Provincetown's beacon is then:
110 kts × 3-2/3 (3 min 40 sec., 220 sec.)

= 40 nautical miles to station.
10 degrees
One can only hope that Mr. Counter is not inspired to again change your destination before reaching Provincetown.
As seen, the calculations for time and distance to station are quite simple. What's even easier is to read those numbers from a table. Since you're flying the Cessna Nav Trainer which cruises at 110 kts, a table can be prepared with a spreadsheet showing both time to station and distance to station for any value of time to cross 10°. The table below is for 20-second increments of time to cross 10° but it is just as easy to use your spreadsheet and prepare it for 10 or 15 second increments, interpreting for the in-between values.
If your aircraft differs from the Cessna Nav Trainer, just redo the Distance to Station columns for the appropriate cruise speed. The Time to Station column is independent of speed.

Time and Distance to Station vs. Time to cross 10° for 110 kts
Time to
cross 10°
Time to
to Station
Time to
cross 10°
Time to
to Station

Intercepting a Bearing

RMI-on bearing
Probably the most common ADF navigation procedure is intercepting a specific bearing of a Non Directional Beacon. The technique is similar whether intercepting an inbound bearing or an outbound bearing.
Until this easily-understood maneuver is learned, VFR approaches to an airport will be sloppy, and all but impossible in IFR conditions. This procedure becomes second nature after a few Flight Simulator practice sessions.
The illustration above shows the aircraft nicely tracking inbound—no wind—on the 075° magnetic bearing to the Provincetown—PVC—beacon. Three simple flight maneuvers, as shown in the illustrations below, will accurately put you on any inbound or outbound NDB bearing. Here, the desired bearing is 075° to the PVC beacon to line up with Provincetown's Runway 7.
RMI, 075° hdg, 125° bearingRMI, 175° hdg, 125° bearingRMI, 175° hdg, 075° bearing

  1. Turn to the desired bearing (075°), and note the number of degrees of needle deflection to the left or right—in this case it's 50° (125° bearing minus 075° heading). Then double this amount to determine the interception angle (100°).
  2. Turn toward the needle the number of degrees determined above (100°). Adding that to the desired bearing—right turns, add numbers; left turns, subtract numbers—gives an intercept heading of 175°.
  3. Maintain the new intercept heading (175°) until the needle points to the desired bearing (075°). Then turn to the (075°) bearing. Of course, the turn should be started shortly before the desired bearing is reached so as to roll out on the correct heading without any overshoot.
With no wind you need only continue on the 075° inbound heading to Provincetown. But there is always wind. The next chapter provides you with the techniques to correct for the effects of the wind.
But first, open up your Flight Simulator and randomly select an NDB. Then choose a bearing to that NDB and practice intercepting it from several positions around the NDB. Do this for several other randomly chosen NDBs until you can comfortably and accurately intercept any bearing to an NDB no matter where you are or your direction of flight in relation to the NDB.

And Finally

Nothing is quite so important as recognizing when your RMI cannot receive signals. The situations that create that dilemma are:

  1. The ADF Receiver is turned off.
  2. The ADF Receiver is not tuned to the station.
  3. The NDB is too distant to provide a reliable signal.
  4. The aircraft's altitude is too low (reception is generally reliable at and above 1000 ft AGL).
When "Off," or receiving no usable signal, the RMI needle points directly to the right, 90° away from vertical. In addition to that, the digital bearing indicator on the Nav Trainer's panel also displays three dots as shown in the illustration.


thank-youAlthough 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.
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 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 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
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,, 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."
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. 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
Chris Sheldon,, 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 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.
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 at
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 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.
With thanks,

Charles Wood 

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