NDBs and Visual Navigation
... of Bearings and Beacons

Dereck Haynes discovers some of the tricks of using Non Directional Beacons, explains how they can help with your visual navigation. Slightly less high tech than the GPS, but more of a challenge to use!

The GPS is the answer to everything it seems, in late twentieth century aerial navigation. Mind you, there are those who violently disagree, and take exactly the opposite stance: to them the GPS is the devil in the cockpit and it should be removed immediately from the flight deck, and preferably flung from a great height into the sea. All navigation should be done by map, compass and watch. Nothing else is proper.

But between the two camps of the GPS lovers and the eyeball and chart pilots, there comes a gentler middle ground occupied by those who like to consider other alternatives. Those - like me, for instance - who don't mind using some of the increasingly unloved navigation beacons which are unobtrusively dotted around the British (indeed the whole world's) countryside. We are the NDB users. Okay, we don't use NDBs exclusively or excessively, but they've got me out of some scrapes in the past and although I wouldn't venture to say that NDB navigation is easy, I would suggest that it can improve your airmanship just a little. And knowing about NDBs is invaluable when you come to passing an IMC rating (or whatever it is to become).

What the NDB is

So what are we talking about? Using NDBs isn't currently part of the CAA's basic PPL syllabus, and for that reason many people who only fly VFR seem to assume - even if subconsciously - that they aren't allowed to use them for VFR navigation. Well, you are allowed to use them, and what's more the new JAR-FCL PPL is going to teach PPLs about them. Radio navigation will be part of that syllabus, even if it's only on the basis of teaching a simple appreciation. Nevertheless, future pilots will surely have more confidence in the NDB as a safety net which they can use.

An NDB (Non Directional Beacon) is a ground beacon which emits radio waves in the medium wave band. (200kHz to 1750kHz). Being in the medium band they are not restricted to line of sight, but they are generally only powered to have a range of between 10 and 80 miles, depending on the beacon (more of which later). Often the NDB is a mast-like structure stuck in the ground and held up with wires - they're really not that impressive. However, they don't take up much space and they require comparatively little maintenance, so they are fairly common on airfields - around Britain, which is one of the things which makes them so useful.

The NDB emits a wave which can be picked up by an aerial on an aircraft. This aerial or antenna often looks rather like a fin or a blister module and actually consists of two parts, one to pick up the signal and the other to sense its direction. By combining the two the airborne equipment (known as the ADF or Automatic Direction Finder) is able to work out from which direction relative to the aircraft, the radio wave is coming. This information is displayed on a dial in the cockpit. The dial effectively 'points' at the NDB. It is the art of reading and interpreting this dial which is the most significant part of using an NDB for navigation - but we'll come to that in a minute.

In the meantime, there's another part of the airborne system to mention: the frequency selector box. Each NDB emits its radio wave at a different frequency, and so you need a box on board which allows you to tell the antenna which NDB it should be using. This selector box is, in essence, easy to use - just dial in the correct frequency. There is more to it than that, but all the other procedures are simply back-ups, making sure that everything is working as you want it to.

Some pilots use a little trick to help them read the fixed card type of ADF more easily.

The fixed card type of ADF

All you do is mentally superimpose the needle from the ADF (on the left in picture), in its current angle / attitude, onto the face of the Direction Indicator (on the right).

In the above cockpit, the beacon would be at roughly 315 degrees magnetic. It's not pinpoint accurate, but it'll give you a rough idea of the position.

Using the ADF

Okay, back to the cockpit dial, the ADF. This can come in one of three types, the simplest of which is the 'fixed card'. This type of ADF display has a needle and a compass rose which is fixed (with 360 at the top and 180 at the bottom). The needle points to the NDB's position relative to your direction of travel (usually there's a small representative of the aircraft in the centre of the dial for reference).

For instance (Fig. 1), if you are flying on a heading of 010 degrees (information which the DI gives you) and the ADF needle is showing that the beacon is 30 degrees to the right of the aircraft's nose (in other words, the needle's pointing at 030), you can work out that the beacon is actually on a bearing of 040 degrees from you by adding together your heading and the relative bearing of 040 shown by the ADF (10 + 30 = 40 degrees).

Fig.1

You can then use that information to, say, draw a line on a chart along the 40 degree radial to the beacon: your position will lie along that line - although the ADF can't tell you how far you are from the beacon. (By the way, the bearings here are Magnetic rather than True because they're based on compass information via the DI.)

As long as don't move in space, the magnetic bearing to the beacon will always be the same, whatever your heading.

In the above scenario, if you were immediately to turn onto a heading of 100 degrees (Fig. 2), the ADF needle will alter its reading (it will point at 300 on the dial): although you have changed direction, the beacon on the ground hasn't moved so the needle still points at it.

Fig.2

The needle is now on 300 and your heading is 100, so a little mathematics will tell you that you're still on the 040 radial to the beacon.

But that's problem with the ADF, at least using this type of fixed card ADF and this technique: 'a little mathematics' can be a very hard thing in a cockpit in flight. In this example we not only have to add the heading and the ADF reading together, you also have to accommodate the fact that there are only 360 degrees in a circle by subtracting 360 from the answer. (An alternative method is to subtract from your heading the number of degrees left of 360 the needle's pointing: here the needle is on 300 degrees, which is 60 left of the 360 mark, so you subtract 60 from 100 and get 40 degrees. Phew!) It's also far from easy to see the 'big picture' - situational awareness doesn't always mix well with maths. However, with practice all this becomes more intuitive and with experience you can begin to guesstimate quite accurately what is happening.

Before moving on we must point out that the ADF works all round the aircraft. If, for instance, you now tune into (and identity - see the panel) an NDB at the airport from which you took off, the ADF pointer will swing to point at that beacon (Fig. 3).

Fig.3

It could be that the pointer is now on the 140 mark on the ADF's compass rose; since you are still flying 100 degrees M you are, therefore, on the NDB's 240 degree radial (again, a little maths is all it takes!). If you've the time and presence of mind to work out your bearing from both stations, you can even do a crosscut and work out your exact position!

Now that's better

So, problem identified: that 'little bit of maths'. One solution adopted in some light aircraft ADF dials: the rotating compass rose. The pilot, using a knob found along side does the rotating the dial. The idea is that you actually set your rose to reflect the direction in which you are flying, so that your heading is at the top of the instrument. This effectively cuts out the need for a lot of the arithmetic: the number which the ADF needle points at on the compass rose represents your bearing to the beacon.

Taking the last example again, let's set it up (Fig. 4) so that the compass rose on the ADF is now showing the direction of travel( we've also skewed the whole thing round so that it appears as it would in the aircraft).

Fig.4

It's now much clearer what you're doing. As you change position in relation to the NDB (as long as you're not heading directly towards or away from it on a constant radial), your bearing relative to the beacon, and therefore the radial you are on, will change. But as long as you always fly the heading shown at the top of the ADF card, your pointer will always be indicating the magnetic bearing / radial to the beacon (fig 5).

Fig.5

Easier as the rotating card method may be, it still has its drawbacks. One is that you must be accurate in your flying - for the radial information to be accurate you must be flying the heading on the compass rose. If you change heading (at the start of a new leg, say) you have to make sure that you remember to adjust the rotating card to match.

Using the ADF There's a certain amount of ritual attached to using the ADF, all of which is there to ensure that things go smoothly. Before taxiing, if there's an NDB on the field or within range, select the frequency for that beacon. Now, it's likely that the ADF will have been left tuned to this local beacon from the previous flight, so just dial the tuner away from the frequency before reselecting it, to make sure that the box is not stuck. When you do this, you should find that the needle points at the beacon relative to the aircraft (if you're not sure where precisely the beacon is physically situated, airfield charts will show you). You can now identify the beacon. Next choose the 'Test' or 'TST' setting on the unit (some units may have a different procedure). The pointer will swing away from the correct setting and, when you return the setting to 'ADF', it should move back to show the correct bearing to the beacon. If you wish - it's not always possible on the ground - you can then tune into another NDB, to check thoroughly that the equipment is working. Then go back to the more local beacon, and keep an eye on the needle as you taxi - it should track the beacon closely as you move round the airfield. If you can't do any of this on the ground because there is no convenient beacon to tune to, you will have to do it in the air. Make sure that you remember to fly the aircraft and navigate correctly while part of your attention is taken up with the ADF! This may all seem rather over the top for a piece of equipment that you're effectively only using as a back up for visual navigation, but it could save embarrassment and worse if something's not working properly.

Better Still

Wouldn't it be nice if that rotating card would actually rotate automatically - so that whenever you changed heading the rotating card would adjust accordingly, keeping the new heading at the top. Some top flight ADF's actually do that for you: the instrument will continually be adjusted magnetic north. Now you don't have to remember to adjust the card when you turn onto a new heading, because the card will automatically follow you; equally it doesn't matter that you wander a degree or two off course - the card will still indicate the correct magnetic bearing to the currently tuned beacon.

Clearly, this makes it a lot easier to find which radial you are on from the NDB, thereby increasing the usefulness of the ADF. Many such ADFs actually sport two needles, with the second one being able to indicate NDB or VOR information, at your whim.

Best of all are the complex instruments and displays which show NDB information on a navigation or even primary instrument (artificial horizon with knobs on) or screen - but you don't get many of them in your average light aircraft, so we'll leave them to one side and keep using the steam driven types.

You can do more!

So we know what radial we are on from the NDB. Whoopee! But what does that mean in practice? For a start, it means that you can draw a line from the NDB along the radial you have identified, and know that your aircraft is somewhere along that line. In many visual flying situations that's enough: it will just confirm what you already know (or it will help you pinpoint your position if you were slightly uncertain).

However, a more useful application might be to use the ADE / NDB combination to actually find somewhere. As we've said, many airports have NDBs either on them or in their near vicinity, so it's not out of the question that you can use the NDB to find an airport (and how many tines have you heard pilots complain about the difficulty of doing just that?).

Tracking towards an NDB is fairly simple in theory, but more tricky in practice. In an ideal world you'd stick the beacon on the nose of the aircraft and just aim at it full speed ahead. But your ideal world doesn't have any wind, does it?

If you are aiming to track (say) 360 degrees towards a beacon which is directly to the north of you, but you've got a healthy wind coming from the west, you can steer 360 as steady as you like but you'll be blown way off track to the east.

As you do so, the needle on the ADF will slowly slip away to the left, and you'll pass abeam the NDB to the east of it. On the other hand, if you make decision to forget the 360 degree heading and just keep the ADF needle pointing vertically on the dial, you'll certainly arrive overhead the beacon: it's just that you'll end up doing it by steering into the wind, as the needle will be falling to the left of the dial, and you'll be constantly turning left to keep it vertical.

The way to get there

The real secret of tracking towards an NDB is to allow for the wind drift in your calculations, and to monitor every so often how you're getting on.

For instance, you decide to track towards a beacon (let's call it 'END'). You tune the ADF and ident the beacon successfully and discover that the NDB is 360 degrees from you - so you have to fly the 360 degree radial to reach the beacon. However, you know that the wind is from the west - your left since you'll be flying north - because you've done your preflight weather checking and have put a big arrow on your chart.

Now, just as in visual flight planning, you must counteract a little of the expected drift by 'crabbing' into wind (of course, if this is a planned leg you'll already have worked out how much drift is likely, but even then there is no certainly that your calculation will be dead accurate, so follows is still true). So, your aircraft's nose will be pointing left by about 10 degrees (so your DI says 350 degrees); however, the ADF needle will now be showing 10 degrees to the right of upright, or 360 degrees if you have a (correctly aligned) rotating card.

You can now fly like this for a few seconds or so, just to see how things are settling down. Perhaps, after all that concentration on the ADF and navigation, it's time for a FREDA check?

When you come back to the ADF what do you see? If the ADF needle is still on 360 degrees you've done well - or have been lucky! You are tracking that 360 radial towards 'END'. However, it's more likely, this being an unequal world, that your ADF needle is no longer indicating the 360 degree radial. Over a short space of time it's not likely to have moved that far (perhaps a degree or two), but it's a trend that you want to stop sooner rather later.

The first thing to do in this situation is to find out in which direction you've drifted - are you left or right of the desired track? This is where most pilots training on the ADF find that their brains go blank. However, it's not that difficult: if the needle is to the right of the track which you want to fly, then you've got to turn right; if it's to the left, you need to turn left. Even on a fixed card ADF it's quite simple to see what's happening: since the movement away from track is a trend, the needle will be consistently moving one way or another, and that's the direction in which you must turn. (All of this relies, of course, on the fact that you are holding a steady heading!). Some instructors will tell you to 'turn in the direction that the needle falls' on the dial - that's not a bad way of looking at it.

So, how large a turn should you make? This requires a certain amount of assessment on your part. How far off track have you wandered? How long has it taken? Moving 2 degrees off track over the course of ten minutes clearly isn't going to require as decisive a manoeuvre as veering 10 degrees off in the space of 15 seconds! A good rule of thumb is to correct your track by twice the amount of the error. So, if the needle has drifted five degrees to the right of the desired figure (360 degrees in this case), a good plan would be to alter your heading 10 degrees to the right until you intercept the originally intended track, at which point you can adopt a revised heading which takes into account the drift that you have experienced. So if you drifted to the left (the needle was dropping to the right) you will need to add a degree or two (or more) to your original heading, again depending upon how dramatic your initial deviation from the desired track had been.

But be warned (and also take heart) - you've never likely to keep precisely on the track of an NDB for long. Generally, tracking a beacon is a matter of weaving from one side of the desired track to another. The secret is to always think clearly and react slowly but decisively.

6. So, now you can adopt a new heading, with less wind correction, in order to maintain the correct radial. But keep an eye on that needle ...   5. When the needle swings back to indicate 360 degrees you're back on the correct radial to the beacon   4. Doubling the deviation angle of 5 degrees gives 10 degrees, to turn 10 degrees to the right. Although this is parallel to the correct radial, you can rely on the wind to blow you back on course   3. After a short while the needle indicates 005 degrees. It's 'fallen' to the right, so you need to turn right   2. Your calculations suggest a 10 degree wind correction, so steer 350 degrees. The needle still indicates 360 degrees   1. Work out the magnetic bearing to the beacon. It's 360 degrees in this case

Near the beacon

The only stage of the tracking procedure for which the above correction sequence doesn't hold true is when you are nearing the beacon. As you get closer to the NDB the radials begin to get closer together and so the ADF needle begins to wander from its desired position much more quickly and further. As this starts to happen do not try to chase it - simply keep on the heading that you are currently flying.

Since you are flying visually you will probably not reach this situation. You should know by this stage where you are in relation to the NDB and, if the beacon is on an airfield, you should be able to see the airport in question. If you can't then it's probably underneath you and you've busted their airspace, which is one reason it's important to look out of the window when you're tracking NDBs, and not stay fixated on the dial.

However, if you plan is to fly over the beacon - and if you've been accurate - the needle will become very uncertain as you pass overhead the beacon, and will then swing right round as you leave the NDB behind. This is known as station passage, and seeing it happen crisply and neatly (with the needle swinging about a quarter of a turn in one direction and then all the way back round the other way to point to the rear of the aircraft) is one of the big kicks in flying!

Other uses of an NDB

That's two uses of the NDB, then: how to find the radial you're on, and how to track to the beacon from your current position. There are many more things that you can do - for instance, you can plan to intercept and then track a radial away from the beacon, or fly an approach and so on. However, these both take a little more brainpower than the simpler examples shown above so we're not going to deal with them here. However, the ADF basics that we've discussed here will stand you in good stead when you move onto more complicated procedures.

The next step is to practice your new skills. PC flight simulators are excellent for this, but there's no substitute for real, in flight, experience. It would be best to take another pilot - or an instructor - with you for the early sorties while you are getting to grips with the APF, and remember: you're flying visually, so don't forget to keep a lookout and do the aviating first!

Also, see article on tracking away from NDBs

Identifying your beacon One of the nightmares of flying in IMC is tracking the wrong beacon. Even worse, you could find yourself believing your ADF when it's tuned to a beacon that doesn't exist, or is out of range or is turned off. When you're flying visually this could prove embarrassing, when you're flying totally on instruments it could prove fatal. One means which has evolved to stop this sort of thing happening is the 'ident'. As well as broadcasting its position, an NDB (or any other sort of radio navigation beacon) transmits a sequence of morse code letters - either two, three or four - on the same frequency. So once you've dialled up the frequency on your ADF console you should listen to the ADF to ensure that it matches what you're expecting to hear. If you can't hear the ident, or if you get the wrong one, then don't use the information from that beacon. 'Listening' to the ADF will probably require a couple of button pushes - it's best to get an instructor to initiate you into the secrets of this on the aircraft you'll be flying. Some beacons, especially on the continent, may require you to select the BFO - beat frequency oscillation - setting on your selector box in order that you can hear the ident A beacon is often referred to, among pilots, by its identifier. "I tracked the BRI" means "I tracked the Bristol Airport beacon", whose morse identifier is BRI Does this mean that you have to learn morse code? Well, yes it does. Either that or have the morse code table handy when you're planning on using NDB Charts and flight guides carry full details of NDBs, their location, their frequency and their ident code. Make sure that you look up any you are planning on using It is possible for NDBs to carry voice information (in fact, many medium wave radio broadcasting stations can be picked up on the ADF - although it's not good practice to listen to Radio Five Live when you're flying), so it could be that you'll get an airport information service when you ident the beacon.

NDB / ADF Problems

Although the ADF is not as complicated a system as some - and is, therefore, on the whole, more reliable as there's less to go wrong - it's still not immune to errors.

In some cases the beacon will have been turned off, either for maintenance or for some other reason, so always check NOTAMs to make sure that any beacon you're intending to use is still supposed to be operational! You should, of course, be able to check whether a beacon is working or not by listening to its ident code when you've tuned it in. It's then good airmanship to re-ident it regularly.

NDBs have limited range. Range depends on power, and as doubling the range requires a quadrupling of power, it's perhaps not surprising that many NDBs have a fairly limited range. Flight guides and official documents will list the range of all beacons - they vary between 10 miles and 70 miles or more.

Other things which can cause errors on ADFs include thunderstorms (which attract the pointer to themselves - it's clearly very dangerous to try to track into a thunderstorm); mountains, which can reflect the signal; the waves can be affected (refracted) as they cross the coast, so if you're flying in the vicinity of the coast make sure that you're aware of this; at night strong sky waves can distract the ADF, luring it into giving false readings; and there's always good old interference from nearby stations to keep you on your toes

Some Jargon Explained

• ADF - Automatic Direction Finder. Indicates the position of a beacon relative to the aircraft as an aid to navigation.
• Beacon - A ground station emitting a radio wave which can be picked up by equipment in the aircraft. A Non Directional Beacon NDB doesn't transmit information about your position relative it, but aircraft systems can sense the direction of the beacon relative to the aircraft.
• BFO - Beat Frequency Oscillation, a means of making audible the ident tone of some NDBs. Also called Carrier Wave.
• Ident - the morse sequence which identifies the beacon
• Radials - imaginary (straight) lines emanating from a beacon
• Station Passage - when you pass overhead the beacon, the needle goes potty and then points behind you ... that's station passage!

Also, see article on tracking away from NDBs