Get the latest news on coronavirus impacts on general aviation, including what AOPA is doing to protect GA, event cancellations, advice for pilots to protect themselves, and more. Read More
Already a member? Please login below for an enhanced experience. Not a member? Join today

Fear of NeedlesFear of Needles

Why pilots are so squeamish about ADFWhy pilots are so squeamish about ADF

If the day should ever come when the last nondirectional beacon (NDB) is unplugged and hauled off to a museum, I hope that the celebration will pause long enough for someone to write a nice epitaph for the pilots, least favorite nav system. I also hope that the historians responsible for documenting aviation's technological phases will leave room in their volumes for a chronicle of the peculiar spell NDBs and their airborne receiver, the automatic direction finder (ADF), cast on generations of pilots.

To help nonpilots and Generation GPS understand what we went through in the "Age of ADF," said museum's curator might commission an artist to depict NDB's charms with a two-panel cartoon. The first panel would depict a pilot wrestling with a boa constrictor. In the second panel, an FAA-designated examiner would smilingly explain, "I gave him the choice: either that, or an NDB approach."

In 2000, the optimistic among us are counting down the weeks, months, and years until NDB's extinction in the brave new world of satellite-navigation.

But I suspect That's a case of maybe-yes, maybe-no. Remember 1998? That's when the widespread decommissioning of the instrument landing system was to begin, based on projections calibrated in Government Standard Time. Is your ILS gone? Mine sure isn't. Meanwhile, the myth, folklore, and reality of NDB all live on.

You've heard the rap on ADF: Hard to use. Inaccurate to a fault. Great for finding a beacon if you happen to be flying directly above it. Best for listening to baseball games or holding over your local radio station.

Our flying club has been debating adding ADF to its training airplane because the craft would then be usable as an instrument trainer. I support this proposal. it's amusing to listen to the arguments of those who don't. Even pilots who have never witnessed the splendor of an ADF needle swinging smartly to herald station passage have heard the horror stories. In panicky voices they offer warnings such as, "If it's in the airplane, you may have to use it on a checkride." Heaven forfend! It would not have surprised me in the least if one of them had suddenly produced a boa constrictor and began wrestling with it right there on the meeting room floor.

Years ago, I gave an instrument competency check (as it was then known) to an active working pilot who flew complex aircraft in surly weather on a fairly regular basis. We briefed for the flight, and in my innocence, I suggested that we fly the transition to one of my favorite aeronautical destinations, BOOGY intersection. BOOGY is depicted on the approach chart to the Belfast (Maine) Municipal Airport (BST) as the point in space where a radial from a nearby VOR and a bearing from the BST beacon intersect. It is used both as an approach fix and for holding. A pilot can get a lot of work done at places like BOOGY intersection. The idea, I explained cheerfully, was that we'd navigate to BOOGY, hold there for a turn or two, and then do the approach. Great fun! It was at this point that the pilot looked up from his charts with a grim expression and muttered, "Wait a minute. That's an ADF approach!" (So dismayed was he that —I did not aggravate him further by replying, "Actually, it's an NDB approach; ADF is the airborne component." But he probably wouldn't even have heard me.) With dry lips and flushed face, he mumbled something about how nowadays all anyone needed to do was tune in an ILS and fly vectors to the final. Even holding was something you could usually talk your way out of if you tried hard enough, he said. That poor boa constrictor is going to be a very busy beast.

The irony is that textbooks typically introduce the section on ADF with a few words about how simple the ADF system is when compared to omni ranges and the other navigation facilities in the National Airspace System. With ADF, you tune to the appropriate frequency, and the needle points to the beacon. No to/from indications, back courses, or false glideslopes. It just points. What could be bad?

The horror stories come from pilots who discovered that what's simple in concept is not always simple to put into use. First there is this business of "relative bearings." Because the azimuth card on most ADF units is only rotatable by hand, the pilot has a choice of either rotating the bearing indicator everytime the heading flown is changed, or keeping the card set with 360 degrees (actually 0) in the 12 o'clock position, checking the heading indicator, then mentally converting the ADF needle's deflection into a magnetic bearing. An example: You are flying east (090 degrees) with your ADF tuned to a station located 10 degrees right of the nose. To fly toward that station, you would turn 10 degrees right, which means you would turn to a heading of 100 degrees. Looking at it another way—and this is a mental exercise well worth practicing—with the indications given above you are on the 100-degree bearing to that NDB, and you are also on the 280-degree bearing from that NDB. This helps you to visualize your position relative to final approach courses, holding patterns, airway transitions, and the like.

If that sounds too much like work, go ahead and add the effects of wind-correction angles on tracking and bearing indications, and you can see why people complain. But I think one of the things that compounded pilots' fear of the ADF system was the way it was taught. Although the idea of converting a zero-referenced needle deflection to a magnetic indication is simple to grasp, some of the illustrations published to show the idea in action—with all the bearing angles, relative and magnetic, crammed onto the same diagrams—were positively terrifying. That problem gave rise to a rash of "ADF made easy" articles promoting various 12-step systems, or other nonintuitive gimmicks for at least blundering your way through a flight test where using ADF might be required. Most of these methods made a mountain out of a molehill.

It wasn't until I became an instrument flight instructor and had to help other people overcome nondirectionalbeaconophobia that I realized that this simple system really is simple. This is so for only two (not 12) reasons: Not only does the ADF needle always point at the station, but if you become disoriented, you can easily make that iconoclastic needle point the way to the course you need to fly. Having done that, any reasonably competent pilot can use the same intercept-and-tracking techniques we use with any nav system, to make the ADF needle behave.

Even better, it doesn't matter at all whether you are flying to or from the station—with the following method, the needle always points toward the course. Not sure which way the course lies? Fly a heading parallel to the course. The ADF needle will point left or right of the nose or tail, depending on whether you are flying inbound to or outbound from the NDB. If it points left, make your correction to the left. If it points right—you got it, turn right! To the station, from the station—doesn't matter. Just turn left or right as indicated.

As for how much to turn to make the intercept, That's a function of the wind, your speed, and distance from the station, just as it is with any other intercept. ADF makes no unusual demands on the pilot in that department. The only thing certain is that you must turn more degrees than the bearing error displayed on your ADF, or you will never reintercept the course. Then when the needle moves across the nose the same number of degrees as the angle you have chosen, you are back on the course. Roll out again on the course heading; the ADF needle should be pointing straight ahead.

Remember that if drift caused the original problem, a wind-correction angle must be established. One of the most common errors I see pilots commit is trying to track a course before it has been intercepted. The exact opposite error occurs when a pilot continues to fly the intercept angle after it is time to turn and start tracking again. Navigating is always a two-step rhythm: intercept, track; intercept, track.

In truth, I used to hate ADF as much as you do. It was only after spending numerous hours practicing, and helping others practice using it, that I began to appreciate how wonderfully simple the language of a fixed-card ADF really is. The fun truly began when I came to see how this simple language lets you not only interpret but also anticipate the success of the NDB procedure you are flying. Say you are holding at an NDB, and you have just completed an outbound leg in an unknown wind. As you begin the standard-rate turn to the inbound course, you know that there is a possibility that the wind has caused you to drift. Will you roll out on the course? Overshoot? Come up short? This is the challenge of any holding procedure, and it may take a few turns before you have the proper correction angles pinned down.

But ADF gives you a head start. As you are flying the final 90 degrees of your 180-degree turn to the inbound course, glance over at the heading indicator now and then and see how many degrees of turn remain. Then look at your ADF needle. Suppose you see that you have 40 degrees of turn remaining. If the ADF needle is also deflected 40 degrees, you will roll out exactly on course. If the needle is deflected less than 40 degrees at that point, your rollout will leave you still on the holding side of the course; you will have to reintercept with a correction in the opposite direction from the original turn. (That is, if you were in a right turn in the hold, the turn to reintercept will be to the left.) If the ADF needle is deflected more than 40 degrees when you have 40 degrees left to turn, you will fly through the course; you will have to reintercept by continuing your turn past the course heading, and recapturing it from the other side.

Drawing this out on an approach chart or just a piece of paper shows the idea nicely. It is when pilots try to figure out what the needles are saying after rolling out on the wrong heading that they waste precious time and get behind the airplane. What we are really talking about, of course, is developing the ability to visualize the airplane's relationship to fixes and courses, a skill that always makes for more efficient piloting.

A comparison for the ages: Just as many pilots who trained in tailwheel airplanes fly with better coordination, and do better in crosswinds, than their tricycle-reared kin, those who can navigate the hard way are more skilled in the use of easier, "smarter" boxes. Hundreds of NDBs still dot the landscape. In Canada, they remain a mainstay of the system, occasionally used in combination with distance measuring equipment in the approach environment. So while it is true, as the critics proclaim, that NDB put the non in nonprecision approaches, a pilot who has tamed ADF will be able to wring more precision out of more-modern systems. He or she will also be better able to manage the work load in phases of flight when the ability to divide attention between multiple tasks becomes not merely necessary, but critical.

Dan Namowitz is a writer and flight instructor living in Maine.

Dan Namowitz

Dan Namowitz

Associate Editor Web
Associate Editor Web Dan Namowitz has been writing for AOPA in a variety of capacities since 1991. He has been a flight instructor since 1990 and is a 30-year AOPA member.

Related Articles