Automatic Direction Finding (ADF) Navigation:
On Not Being a Homing Boy
By Rod Machado
It's been said that anyone who keeps an ADF unit in his airplane does so because he can't find anything else to plug up the hole should the ADF be removed. What a shame. I am a very big fan of ADF navigation for one very important reason. If you know how to navigate with ADF, you'll have no problem navigating with any other form of navigation, especially GPS with a moving map display.
Look at the numbers on the compass card of the ADF display in Figure 1. They’re fixed, in that the card doesn’t automatically rotate like the compass card of an RMI. You can manually rotate the card, as we’ll discuss later. For now, however, forget those numbers! Yes, that’s right, forget them! As far as you’re concerned right now, those numbers only identify 30 degree incremental positions to the left and right of the nose. Don’t attach any significance to them. Treat each of these numbers like most people treat the Surgeon General’s warnings on a cigarette package—know that it’s there but don’t look at it.
OK, here is one of the most powerful and simplest rules you’ll ever see for navigation with the ADF. To go to any NDB station, simply tune in that station’s frequency, identify it (usually by Morse code) and turn the airplane so the ADF needle points to the nose of the symbolic airplane on the ADF display, as shown in Figure 2, Airplanes A and B. The symbolic airplane represents the real airplane’s orientation. It’s always pointed toward the white triangle at the top of the ADF indicator, which represents the airplane’s nose. Simply turn toward the tip of the needle to go to the station.
If you keep the needle centered at the top of the display you will eventually end up over the station. As you fly over the station the needle will swing to the tail as shown by Airplane C. This type of tracking is known as homing. It involves no wind correction, but it works every single time. Don’t forget it. Figure 3 shows airplanes A, B and C homing directly to the Flynice NDB. Airplane E is flying directly away from the station by keeping the needle on the tail of the ADF display.
Airplane D’s ADF needle points to the left of the nose. Making a left turn will center the needle at the top of the display. Airplane F’s ADF needle shows the NDB station to the right of the airplane’s nose. Turning right will center the needle at the top of the display.
Tracking a Magnetic Bearing – Homing is the easiest method of ADF navigation (perhaps it’s called homing because pilots can always find their way home using this technique). There is another method, however, that allows you to intercept and track magnetic bearings directly to and from an NDB. This is done in much the same way as the RMI is used. Let’s start from the beginning.
Figure 4 shows an NDB and a VOR station. NDBs are unlike VORs in that their signal is nondirectional. This means the NDB’s signal radiates outward in all directions while VORs have 360 specific radials on which we can navigate. We can, however, imagine 360 bearings running through an NDB in much the same way courses run through a VOR. First, we need to define what a bearing is.
Examine the diagram in Figure 5. If you were instructed to fly from position A directly to position B, you could do so with great precision by using the VOR. You would track inbound on the 270 degree radial and outbound on the 090 degree radial. You would know your exact position with respect to these radials by looking at the VOR needle. Suppose you were asked to fly from position X directly to position Y using your ADF. How would you know your airplane is tracking directly along the specified course?
You need to know two things before answering this question. First, the airplane must be heading in a direction of 90 degrees (we’ll assume no wind for these examples). Heading 90 degrees will certainly point you in the direction of location Y, as shown in Figure 6A. However, heading 90 degrees will also allow you to fly a parallel course offset from the originally desired path, as shown in Figure 6B. We need one additional bit of information. If the ADF needle points to the nose of the display airplane while it is heading 090 degrees, then it is precisely on the course from X to Y, as shown in Figure 6C.
After crossing the NDB station in Figure 6C, the airplane will be flying the targeted course to Y if its heading is 090 degrees and the ADF needle points directly to the tail. Airplanes flying a parallel route between X and Y in Figure 6D don’t meet both requirements to be on the specific course.
In Figure 6D, both airplanes flying from X to Y are heading 090 degrees but the ADF needle points somewhere other than to the nose or the tail. A very important principle has just been uncovered. Even though NDBs are non directional in their signals, we can still fly specific directional routes using the ADF and our heading indicator. These routes are called magnetic bearings and they may be flown to or from an NDB.
Figure 7 shows several airplanes, all of which are on specific magnetic bearings to and from a station. Airplane A is heading 270 degrees with the ADF needle pointed to its nose. It’s on the 270 degree magnetic bearing to the station. Airplane B is on the 045 degree magnetic bearing from the station and Airplane C is on the 135 degree magnetic bearing to the station. Of course, we don’t have TO/FROM/OFF flags on an ADF. It should be obvious whether you’re going toward or away from any station since the needle is pointed either to the nose or the tail of the ADF’s compass card.
Airplane D is flying the 225 degree magnetic bearing from the NDB and Airplane E is on the 180 degree magnetic bearing from the NDB. Make sure you understand this important principle: when the needle is pointed directly to the nose or the tail of the ADF, the direction the airplane is heading is also its magnetic bearing to or from the NDB (we’re still assuming a no-wind condition).
Now you’re ready for two nifty techniques. First, I’m going to show you how to use the ADF’s rotating compass card to identify the bearing you’re on to or from an NDB. You’ll do a backflip when you see how easy this is. Second, I’ll show you an easy formula you can use to solve any written test question concerning ADF navigation (and there will be some, I can promise you that).
The ADF’s Moveable Compass Card – I’ll have to admit that practical jokes run in my family. Many years ago my girlfriend wanted me to take her out for her birthday. She said she didn’t care where we went as long as it was “expensive.” I took her to the airport for a sandwich. I promised we’d do something really exciting after dinner. I got out my blackboard and started to describe how to use the ADF’s moveable compass card. She loved it (but not me). But you’ll probably love it (if not me) when you see how easy the moveable compass card makes ADF navigation.
Most ADFs have numbered compass face cards that rotate. Let’s examine the utility of this manually rotating compass card, as shown in Figure 8.
A rotating compass card allows you to set the airplane’s heading under the heading index (white triangle at the top of the instrument). Unlike the RMI which had a slaved gyro compass, the moveable ADF card must be manually rotated to the airplane’s current heading. Nevertheless, it has a practical use. Rotating the ADF compass card to the airplane’s heading results in the needle pointing to the bearing the airplane is on to or from the NDB. The ADF’s needle is then interpreted in exactly the same way as the RMI’s needle.
If you manually rotate the ADF card to the airplane’s magnetic heading of 090 degrees, the ADF needle always points to a number that’s the magnetic bearing to the NDB (that’s right, now, and only now, is it OK to pay attention to the numbers on the face of the ADF card). Airplane A in Figure 9 is on the 060 magnetic bearing to the NDB. Airplane B is on the 360 degree (0 degree) bearing to the NDB and Airplane C is on the 300 degree bearing to the NDB.
Pretty easy to use, isn’t it? If you were instructed to identify crossing the 360 degree bearing to the NDB you would wait until the ADF needle pointed to 360 degrees or 0 degrees. If you were in Airplane A, you would know you hadn’t crossed the 360 degree magnetic bearing to the station since the needle doesn’t point to 360 degrees. You know you will eventually cross the bearing if your heading is held constant. Why? Because the ADF needle always moves toward the tail if your heading is constant (any NDB station in front of you will always end up behind you if you hold a constant heading). Airplane C has flown beyond the 360 degree bearing to the NDB because the head (point) of the ADF needle has fallen past the 360 value on the ADF’s card. That needle will not move back up as long as the airplane’s heading is maintained.
These same procedures apply when intercepting a magnetic bearing from the NDB with a moveable ADF card. In Figure 10, Airplanes A, B, and C are on a magnetic heading of 320 degrees. Suppose you are asked to report crossing the 020 degree magnetic bearing from the station. The moveable compass card has been rotated so that your heading of 320 degrees is set to the top of the compass card. How will you know when you’re crossing the designated bearing? You’ll know when the tail of the ADF needle points to 020 degrees, as is shown by Airplane B. In this instance, the tail of the ADF’s needle points to the magnetic bearing from the NDB. Of course, Airplane C has flown beyond the 020 degree bearing. How do you know? If the head of the ADF needle always moves aft on a constant heading, the tail can only rise. There’s no room for argument here: head falls, tail rises.
The tail of airplane C’s ADF needle has risen above 020 and it certainly won’t move back down. Knowing the ADF tail will rise allows you to determine what magnetic bearing you’ll intercept from any NDB station. The only problem with using a moveable compass card is that you must constantly keep twisting it as your heading changes.
Hey, you can’t believe everything you hear. A friend of mine told me that watching fish is relaxing. Well, that surely explains why I doze off when I’m snorkeling. You can, however, believe that ADF navigation is made very simple when you use the rotating compass card to help determine your magnetic bearing to and from the station.
The ADF Fixed Compass Card – At the very beginning of our discussion I said to think about the numbers on the ADF’s fixed card as index marks. I didn’t want you to think they represented anything useful at that stage of the presentation. However, the numbers on the fixed card (the card with “0” set at the top), do have value, especially when solving knowledge exam questions. The following information is for doing just that—answering written test questions only. Don’t attempt to navigate this way in an airplane. It’s just not practical. If you do try it in the air, I guarantee your mind will come to a stop quicker than a lawnmower cruising over a tree stump. We only do practical things in airplanes. It’s like the old sage advice to yell “fire” if you’re being attacked. This surely works a lot better than, “Help, this guy has a gun!”
First let’s define a few terms. The relative bearing is not a round metal ball underneath your Aunt Mabel. It is the actual number the needle points to on the face of the ADF’s fixed compass card. It’s called relative because it’s measured clockwise from 0 degrees to 360 degrees to the right of the nose. In other words, if the needle were pointed 10 degrees left of the top of the compass card, as in Figure 11, we could say its relative bearing is 350 degrees. ADF card B shows a relative bearing of 70 degrees; Card C shows a relative bearing of 270 degrees and Card D shows a relative bearing of 330 degrees. Remember, relative bearings are “relative” or counted to the right of the top (nose) of the ADF compass card.
During a written test, there is a nifty way to find your bearing to the NDB station using the relative bearing and the airplane’s magnetic heading. By adding the relative bearing, shown on the face of the ADF compass card, to the airplane’s magnetic heading, found on the heading indicator, you can find your magnetic bearing to the NDB. In other words:
RB + MH=MBTS
Relative Bearing + Magnetic Heading =
Magnetic Bearing To the Station
Don’t be scared. This is a very simple math formula. I know! I know! Many people have difficulty with math. In fact, a friend who has math phobia once said, “Hey Rod, did you know that 5 out of 4 people have trouble with math?” As you can see, he really has trouble with math.
To understand how to use this formula, examine Figure 12. Airplane A has a relative bearing of 080 degrees showing on the face of its ADF compass card. Its heading is 310 degrees. Use the formula to find its magnetic bearing to the station. First we add, RB + MH=MBTS or (080 degrees) + (310 degrees) = 390 degrees —the magnetic bearing to the station. As we’ve previously learned, any magnetic bearing greater than 360 degrees means a complete circle has already been made and the heading count should begin anew. So, subtract 360 degrees from 390 degrees to find the 30 degree magnetic bearing to the station, as depicted Figure 12.
Suppose we wanted to know our magnetic bearing from the station. Simply add or subtract 180 degrees from the bearing to the station to get its reciprocal. If we are on the 030 degree magnetic bearing to the station, the magnetic bearing from the station is (30 degree + 180 degrees)=210 degrees.
What is the magnetic bearing to the station of Airplane B in Figure 12? Adding the relative bearing of 300 degrees to a magnetic heading of 270 degrees gives us a magnetic bearing to the station of 210 degrees (570-360; your answer always has to be 360 or less). What is airplane
Advanced ADF Navigation
Now that we understand determination of when an airplane is on a specific NDB magnetic bearing, let’s examine what happens when the airplane’s heading changes. Airplanes A, B, C and D in Figure 13 are all situated on the 360 degree magnetic bearing to the NDB. Conversely, we can say that all four airplanes are also on the 180 degree magnetic bearing from the station. For this discussion, let’s talk about magnetic bearings to the station. Of course, because these airplanes are not flying 360 degrees, they will eventually move away from this magnetic bearing. Let’s assume they remain stationary, as shown in Figure 13.
Airplane A’s ADF needle points to the left wing or 90 degrees to the left of the nose. How many degrees would Airplane A have to turn to the left to get its ADF needle on the nose? Yes, the answer is 90 degrees. As Airplane A turns left the ADF needle will continue pointing to the NDB. If Airplane A’s heading is east or 90 degrees, at the completion of a 90 degree left turn its new heading will be 360 degrees and the needle will be on the ADF’s nose.
Airplane B is heading 270 degrees and the ADF needle is pointing directly off the right wing. If Airplane B turns 90 degrees to the right, the ADF needle will end up pointing to the nose and the airplane’s heading will be 360 degrees. Sound familiar? This means that both Airplane A and B, if they immediately turned to a heading of 360 degrees, would be flying to the NDB station on the 360 degree magnetic bearing.
These same principles apply on a magnetic bearing from the station. The ADF needle on Airplane E is 30 degrees to the left of the tail. If Airplane E is turned 30 degrees to the right, where would the ADF needle be? Yes, right on the tail. Remember, the needle will remain pointing directly to the NDB and the airplane (our ADF compass card) appears to rotate around the needle. To clearly understand this point, look at all the airplanes in Figure 13. The ADF needle points to the NDB despite all the variations in airplane headings. Which way and how many degrees would Airplane F need to turn to establish itself outbound on the 360 degree magnetic bearing? The answer is 30 degrees to the left.
Now you understand how the airplane can be established on a magnetic bearing while the ADF needle points to the right or left of the nose or tail. This becomes important when making corrections for wind while remaining on the specified magnetic bearing.
Another Way of Determining Your Magnetic Bearing to or from an NDB – Figure 14 shows ADF indications on several different airplanes. Each airplane is obviously on one of the marked magnetic bearings to or from the station. Yet each airplane is headed in a direction different from that magnetic bearing. Here’s the question. Is it possible to determine your magnetic bearing to or from the station by examining the airplane’s heading and the angle the needle is deflected to the right or left of the nose or tail? Yes, it is. To find your magnetic bearing to or from any NDB station, simply ask yourself the following two questions:
- How many degrees must you turn the airplane to the right or left to center the ADF needle on the nose (if you want a magnetic bearing to the NDB) or the tail (if you want a magnetic bearing from the NDB)?
- What is the airplane’s new heading after turning by this number of degrees?
The answer to question two is your magnetic bearing to or from that specific NDB. Let’s see how this works. Using the method above, what magnetic bearing is Airplane H on from the NDB? Since we’re dealing with a magnetic bearing from, we’ll concern ourselves with the angle between the needle and the tail. The ADF needle in Airplane H is 75 degrees to the left of the tail. A right turn of 75 degrees puts the needle on the tail (this answers question 1). Since Airplane H is heading 330 degrees, a right turn of 75 degrees puts the airplane on a new heading of 405 degrees. Remembering that directional answers must always be 360 degrees or less, you subtract 360 from 405. This give you a value of 45 degrees, which answers question 2.
What is Airplane D’s magnetic bearing to the NDB? Airplane D must turn 45 degrees to the left to center the ADF needle on the nose (this answers question 1). Starting on a heading of 090 degrees, a 45 degree turn to the left puts the airplane on a new heading of 045 degrees with the needle centered on the nose (this answers question 2). Airplane D’s magnetic bearing to the NDB is 045 degrees.
Use the preceding two questions to determine the other airplanes’ magnetic bearing to and from the NDB. Remember, if you obtain a value greater than 360 degrees, subtract 360 from the number. There’s no such thing as a heading of 400 degrees. If a controller ever asks you to turn left or right to 400 degrees, be very suspicious. If an instructor asks you to do this, tell him or her to get their gyro stabilized and their bearings dry lubed. Coming from an instructor, this is roughly the equivalent of being sent out to find a left-handed monkey wrench. Don’t go.
Correcting for Wind – Is it possible to do everything in aviation and never make a mistake? No way. It’s not possible in aviation or in life. Perhaps this explains why many pilots are not sure they are going to heaven, but they sure hope that God grades on a curve. It’s important to think of your first few times navigating with ADF as being graded on a curve, because you’re going to make a few mistakes. This is especially true if you’re attempting to track NDB bearings in strong winds. Fortunately we can apply some of the same philosophy that worked for making VOR wind corrections to ADF navigation.
Figure 15 shows several airplanes tracking to and from the NDB on the 360 degree magnetic bearing. The wind conditions vary for all four airplanes. Notice that Airplane A has a wind from the left (west) and is angled 15 degrees into the wind. Even though the airplane is flying a heading of 345 degrees, the airplane remains on the 360 degree magnetic bearing to the station. Since the NDB station is to the right of Airplane A’s nose, the ADF needle remains deflected 15 degrees to the right. A general rule about ADF wind correction is, If your wind correction angle is sufficient, the angle between the ADF needle and the nose or tail remains equal to that wind correction angle. In other words, if your wind correction angle is 15 degrees, the ADF needle deflects 15 degrees on the appropriate side of the nose or tail.
Airplane B has a wind from the right (east). Crabbing into the wind on a heading of 015 degrees, Airplane B tracks directly to the NDB. Consequently, the ADF needle deflects 15 degrees to the side the station is on. In this instance, the station is to the left of Airplane B’s nose and the needle deflects 15 degrees to the left of the ADF’s nose.
Airplane C is experiencing a wind from the left (west) as the pilot attempts to track outbound on the 360 degree magnetic bearing from the NDB. The pilot applies a 15 degree wind correction angle to the left by turning to a heading of 345 degrees. The ADF needle deflects 15 degrees left of the tail—the side of the tail where the NDB is located. If this wind correction angle is sufficient, Airplane C will remain on the 360 degree magnetic bearing while on a heading of 345 degrees.
Airplane D experiences a wind from the right (east) as the pilot attempts to track outbound on the same magnetic bearing from the NDB. A 15 degree wind correction angle is applied to the right. Even though the airplane is heading 015 degrees, it remains on the 360 degree magnetic bearing from the station. The ADF needle shows this wind correction angle by deflecting 15 degrees to the right of the tail—the side of the tail where the NDB is located.
Figure 16 shows a smaller and more common wind correction angle, experienced under light wind conditions. Airplanes A and B need to angle only 5 degrees into the wind to remain on track to and from the NDB.
Here’s the most important thing I want you to understand about wind correction with the ADF. When a wind exists, the airplane can track a specific magnetic bearing to and from an NDB while pointing in a direction different from that of the bearing. This is the basic premise for wind correction regardless of whether you’re using the VOR, ADF or any other means of navigation.
I’m often asked if private pilots really need to be masters of ADF navigation. Perhaps this is similar to the question, “Do hunters really need assault rifles?” I suppose they do if they are really bad hunters. If you are really bad at VOR navigation, don’t have VORs where you come from, and the airplane you use isn’t equipped with GPS, then you’d better have good ADF skills. Fortunately, most of the 48 contiguous states in the U.S. are well equipped with VOR stations.
While modern navigational methods are rapidly pushing ADF into the background, at least a working knowledge of this stalwart system could prove to literally be a beacon in the night for you when other methods fail. Being a good pilot means knowing how to use everything at your disposal to navigate. ADF remains one of those tools.
Besides, it’s the only radionavigational aid that receives baseball games.