Tracking to an NDB

Tracking to an NDB

A useful ADF application in visual navigation is to locate a particular NDB and then track – or home – directly to it. The ADF receiver is tuned to the NDB frequency, and the audio volume turned up, so that the NDB can be identified as soon as the aircraft comes within range. The ADF needle indicates the bearing to the NDB and the wind correction angle necessary to maintain that track is then ascertained by bracketing, a technique which bears some similarity to the double track error method. The term 'bracketing' is derived from the artillery technique for ranging the target by deliberately placing initial rounds behind and in front of it.

Note: this sequence is best performed if the heading being flown is positioned on the ADF card at TDC, the diagrams in the left column below indicate the readings with those settings.

procedure

Needle position	Compass heading	Event sequence
	060°	Position A. When receiving the NDB signal turn the aircraft so that the head of the ADF needle is pointing to TDC, then check the heading from the compass. That heading is the track required to home directly to the NDB, for our example 060° magnetic. Rotate the ADF compass card to set 060° at TDC and the needle head will also indicate 060°. Remember that all heading changes should be logged.
	060°	Position B. As the flight progresses, holding the 060° heading, the crosswind causes the aircraft to drift to the south of the required track and the ADF needle has moved left about 5° to 055°.
	030°	Position C. We now have to make a first rough cut at the track error – it is best to initially overestimate so let's choose 15° and, applying the double track error technique, we turn left 30° on to an intercept heading of 030° magnetic. Positioning the 030° heading at TDC, the head of the needle will still initially indicate 055° but will move towards 060° as we close with the required track.
	045°	Position D. When the needle reaches 060° the 060° track to the NDB has been regained. Now halve the intercept angle (i.e. subtract the track error) and turn right onto an initial wind correction heading of 045° magnetic, i.e. the estimated track error was 15°, we turned left 30° onto the intercept heading of 030° and now, having regained the required track, we turn right 15° onto a wind correction heading of 045°. Now rotate the card to the 045° heading and the needle remains at the 060° bearing.
	045°	Position E. If the 15° WCA is correct then the ADF needle will remain at the 015° position whilst the 045° heading is maintained. However it is most likely that we have overcorrected, the aircraft will drift north of track, shown by the needle moving clockwise a few degrees from the 015° position so we now have to refine the wind correction angle.
	055°	Position F. We might guess that we have overestimated the WCA by about 5° so, applying the double track error technique, we turn right 10° on to an intercept heading of 055° magnetic. Positioning the 055° heading at TDC, the head of the needle will still initially indicate something greater than 060°, say 063°, but will move towards 060° as we close with the required track.
	050°	Position G. When the needle reaches 060° the 060° track to the NDB has been regained. Now halve the intercept angle and turn 5° left onto a wind correction heading of 050° magnetic, rotate the card to the 050° heading and, if we've estimated correctly, the needle will remain at the 060° bearing, maintaining a 10° WCA, while we continue along the required 060° track to the NDB.

Tracking from an NDB

Another useful application for the ADF in visual navigation is in determining track error when departing from an airfield equipped with an NDB or when overflying an NDB.

For example: the flight plan calls for a departure – from overhead an NDB – on a track of 240° magnetic with any necessary wind correction to be assessed after departure – using the ADF – with the track recovery and heading correction to be made by a slightly modified double track error method. (The modification is that rather than timing the intercept leg to estimate track recovery we will use the ADF needle to indicate when we are back over the required track.)

In this ADF application the ADF card may be used with the 0° position set at TDC or your personal preference may be to set the 240° heading at TDC. The diagrams and the text below indicate the procedure and the readings with 0° positioned at TDC but the additional text in italics is the procedure when rotating the card to the new heading for every change. Hopefully you will be able to see that the latter method is easier to handle.

Note that when tracking away from an NDB we use the tail of the ADF needle, rather than the head, as the indicator.

Needle position	Compass heading	Event sequence
	240°	Departing from overhead the NDB to track 240° magnetic. The magnetic compass heading is 240° ( i.e. no wind correction provision) and the tail of the ADF needle swings to the 0° position. With the 240° magnetic heading set at TDC the position of the needle relative to TDC is exactly the same as in the diagram but, on the background card, the needle tail indicates the 240° heading.
	240°	Position B. As the flight progresses holding the 240° heading the crosswind causes the aircraft to drift to the south of the required track. The tail of the ADF needle has moved about 15° to 345° and is in the left half of the card. Thus the opening angle, or track error, is 15° and the tail of the needle represents the track made good, which is 15° to the left of the required track. With the 240° magnetic heading set at TDC the tail of the needle will indicate the track made good, 225° or an opening angle, or track error, of 15°.
	270°	Position C. Use the double track error method to intercept the required track. The aircraft is turned 30° [2 × 15] onto a heading of 270° magnetic. The ADF needle tail initially moves 30° to 315° then commences to reverse direction as the 270° heading is maintained and the aircraft is closing the 240° track out. The aircraft is turned 30° [2 × 15] onto a heading of 270° magnetic and 270° magnetic is now set at TDC, the tail of the needle will then initially still indicate 225° but will move towards 240° as you close with the required track.
	270°	Position D. When the needle has moved through a 15° arc and is back to the 30° left position [330°], on a heading of 270°, the 240° track out from the NDB has been regained. With the 270° magnetic heading set at TDC the 240° track out from the NDB has been regained when the tail of the needle reaches 240°.
	255°	Position E. Subtract the track error [15°] and turn left onto the new heading of 255° which will then maintain the necessary 15° wind correction angle. The ADF needle moves 15° clockwise and the aircraft should hold the required track – if the heading is maintained and the needle kept at the 345° position. Subtract the track error [15°] and turn left onto the new heading of 255° which will then maintain the necessary 15° wind correction angle. Set the 255° magnetic heading at TDC, the tail of the needle now indicates 255°. After flying this heading for a while you may find that you still have some drift – indicated by movement of the needle. In this case a small heading correction is usually enough compensation.

Running fix / distance from NDB

Whenever your track will pass abeam an NDB it is quite easy to obtain a running fix, using the 1-in-60 rule and a little mental arithmetic, providing you have a reasonable idea of your groundspeed. The technique is illustrated in the diagram:

Procedure

1. Tune and identify the NDB, set your heading at TDC and watch the ADF needle, the NDB is directly abeam when it moves to 90° either side of TDC, position A in the diagram.

2. Note the time and continue flying your heading, for example 040° magnetic.

3. When the needle has moved a sufficient amount to get a good reading (position B on the diagram), note the time and the bearing from the NDB, indicated by the tail of the needle. Let's say the needle has moved 10°, the elapsed time is 8 minutes, the bearing from the NDB is 110° magnetic and you reckon your groundspeed at 70 knots.

4. Now we calculate the distance we are along that bearing using the 1-in-60 rule:
i.e the distance [nm] from the NDB = elapsed time [minutes] × ground speed [knots] / degrees traversed = 8 × 70/10 = 56 nm. The aircraft's position at time B is then 110°/56 nm from the NDB. The real difficulty now is to measure and plot that position on the navigation chart whilst flying the aircraft (not forgetting to convert the bearing to °true) – so best to get the passenger to do it.

If you are wondering what happened to the '60' in the 1-in-60 application the answer is it is negated by the usage of minutes in one factor and nautical miles per hour in another. In the diagram the dashed red line outlines the right angle triangle on which the calculation is based – the distance from the NDB to position B forms the hypotenuse.

ADF applications

There are several applications for the ADF in light aircraft cross country VMC navigation – remembering the Visual Flight Rules require that the pilot must be able to navigate by reference to the ground and position fixes must be taken at least every 30 minutes.

Position fixes. If two (or better - three) transmitters are in range then the bearing from each can be ascertained, the lines of position roughly plotted on the chart (after converting to true bearings) and the aircraft position will be close to the intersection point. In most of Australia to have two NDBs in range at the same time is not so common and three would be most unlikely, so the most likely position fixing use is to combine a surface line feature with an NDB bearing.

Running fix / distance from NDB. The 1-in-60 rule can be applied when the aircraft is within range of a transmitter by turning the aircraft so that the station is abeam and then measuring the degrees traversed against time. This is a form of running fix in that two bearings are taken, at an interval, from one source and the aircraft's position is the distance along the second LOP from the NDB. For example:

Distance [nm] to NDB = elapsed time [minutes] × ground speed [knots] / degrees traversed

Homing & tracking to or from an NDB. If there is no crosswind component then tracking toward an NDB is quite simple, just keep the head of the ADF needle at TDC and you will arrive overhead; the track over the ground will be straight and the magnetic heading consistent. However if there is a crosswind component and you just endeavour to keep the head of the ADF needle at TDC, you will eventually arrive but, due to the drift, the track followed will be curved and the magnetic heading will need to be consistently changing. This is called homing, and you will arrive at the NDB on an into-wind heading. Thus tracking, or flying directly towards, or from, an NDB is exactly the same as tracking from A to B – you have to calculate a wind correction angle. Passage overhead an NDB is signified by a "cone of silence" (if the ident volume has been turned up beforehand) and the needle then swinging to the reciprocal bearing.

Using the ADF probably appears to be fairly simple, which it is, but there will be difficulties, for the uninitiated in perceiving, from the position of the needle, the headings to fly when attempting to intercept and then track along a particular magnetic bearing to or from the ground station.

As in all navigation you should always maintain an awareness of the aircraft's position in terms of being north, south, east or west of the NDB and, when initiating a turn, think in the same terms e.g. a left turn will take you further east.

NDB/ADF errors

Electrical interference. Radio waves are emitted by the aircraft alternator in the frequency band of the ADF. An alternator suppressor is fitted to contain those emissions but this component does not have a long life and it is wise to test the ADF for correct operation during pre-flight checks. The test is made by selecting a transmitter – which must be a reasonable distance away, say 30 nm – then watch the ADF needle during the engine run up. If the needle moves as rpm increase there is electrical interference and probably the alternator suppressor should be replaced. Magnetos may also interfere with the ADF.

Thunderstorms emit electrical energy in the NDB band and will deflect the ADF needle towards the storm.

Twilight/night effect. Radio waves arriving at a receiver come both directly from the transmitter – the ground wave – and indirectly as a wave reflected from the ionosphere – the sky wave. The sky wave is affected by the daily changes in the ionosphere, read the ionisation layers section in the Aviation Meteorology Guide. Twilight effect is minimal on transmissions at frequencies below 350 kHz.

Terrain and coastal effects. In mountainous areas NDB signals may be reflected by the terrain which can cause the bearing indications to fluctuate. Some NDBs located in conditions where mountain effect is troublesome transmit at the higher frequency of 1655 kHz. Ground waves are refracted when passing across coast lines at low angles and this will affect the indicated bearing for an aircraft tracking to seaward and following the shore line.

Attitude effects. The indicated bearing will not be accurate whilst the aircraft is banked.