11

The NDB and the ADF

At one time it was commonplace for an instrument student to learn how to fly an NDB approach, but with the growing use of GPS, many pilots no longer use the NDB for instrument approaches or navigation. New RNAV approaches are also rapidly being constructed into airports that were previously only served by NDB (nondirectional beacon).

The long-term plan includes the gradual phase-out of NDB facilities, and eventually the NDB approach will become nonexistent. Until that time, the NDB makes many smaller, remotely located airports available for instrument pilots.

You will not be tested on the NDB or ADF during the Instrument Rating Practical Exam if your aircraft is not equipped with this navigational system.

Instrument rating applicants must accomplish at least two nonprecision approaches for the checkride; NAVAIDs that may be used for these include the NDB, VOR, LOC, GPS and RNAV. The examiner will select nonprecision approaches that are representative of the type the applicant is likely to use. If your aircraft is equipped with an ADF, you are required to understand how to use it for navigation.

The nondirectional beacon (NDB) is the simplest NAVAID used by aircraft. It is a ground-based transmitter which transmits radio energy in all directions, hence its name — the nondirectional beacon.

The ADF, or automatic direction finder, installed in an airplane has a needle that indicates the direction from which the signals of the selected NDB ground station are being received. This is extremely useful information for pilots flying in instrument conditions and/or at night.

Flying to an NDB in an airplane is similar to following a compass needle to the north pole — fly the airplane toward where the needle points, and eventually you will arrive overhead.

Flying away from the north pole, however, with the magnetic compass needle pointing behind, could take the airplane in any one of 360 directions. Similarly, flying away from an NDB using only the ADF needle will not lead the airplane to a particular point (unlike flying to an NDB). The airplane could end up anywhere! More navigation information is required.

ADF and the HI

The extra information required by the pilot, in addition to that supplied by the ADF needle, comes from the magnetic compass or, more commonly, from the heading indicator (which is kept manually aligned with the compass by the pilot, and is easier to use). Accurate navigation can be carried out using these two references:

• an ADF needle that points at an NDB ground station; plus
• a heading indicator that indicates the airplane’s magnetic heading (MH).

  Note. Since a heading indicator will slowly drift out of alignment, it is vital that you periodically realign it with the magnetic compass in straight-and-level flight at a steady speed, say every 10 minutes or so.

A drift out of alignment of 3° in 15 minutes is the maximum acceptable for the HI to be considered serviceable.

NDB/ADF Combination

Before using an ADF’s indications of the bearing to a particular NDB, the airplane must be within the operational range of the NDB and you must have:

• correctly selected the NDB frequency;
• identified its Morse code ident; and
• tested the ADF needle to ensure that it is indeed pointing to the station.

Also, the HI should have been aligned with the magnetic compass (plus or minus the deviation correction, as given on the compass card).

If the NDB bears 40 degrees to the left of the airplane’s magnetic heading, say MH 070, then the situation can be illustrated as in figure 11-5. Since the NDB is 40 degrees left of the nose or, if you prefer, on a relative bearing of 320 (RB 320), it will have a magnetic bearing of 030 (70 MH – 40 = MB 030) from the airplane. This gives you some idea of where the airplane is, and in what direction to travel to reach the NDB.

The ADF/NDB combination, in conjunction with the heading indicator, can be used by the pilot:

• to track to the NDB on any desired course, pass over the NDB, and track outbound on whatever course is desired; or
• to determine the airplane’s position.

The ADF is selected to an NDB relevant to the desired path of the airplane. If tracking en route between two NDBs, the changeover point from one NDB to the next would reasonably be the halfway point, depending of course on their relative power.

If the NDB is ahead, the ADF needle will point up the dial; if the NDB is behind, the ADF needle will point down the dial. As the airplane passes over the NDB, the ADF needle will become quite sensitive and will swing from ahead to behind.

The ADF can also be used for more advanced procedures such as:

• flying an accurate “racetrack” holding pattern based on the NDB;
• using the NDB for guidance when maneuvering in the vicinity of an airport, either as a nonprecision approach aid in its own right, or as a lead-in to a precision approach aid such as an ILS (instrument landing system).

Low-powered NDBs, known as compass locators, are often co-located with one of the marker beacons associated with an ILS. A co-located compass locator and outer marker is designated on approach charts by LOM (for locator outer marker), as illustrated at Columbus Muni, Indiana (figure 11-8).

The compass locator CLIFS (frequency 410 kHz and coded ident dah-dit-dit-dit dit-dah) may be used as:

• a point over which to hold in a racetrack pattern inbound magnetic course (MC) 227;
• the sole tracking aid for the NDB approach for Runway 23;
• an additional tracking guide when using the Runway 23 instrument landing system, and as an outer marker (LOM) or check point on this ILS approach; or
• a point over which to hold following a missed approach.

Some NDBs are not available for holding or for instrument approaches, but only for en route navigation. This information is available in the Chart Supplement U.S. The NDB/ADF combination is the simplest form of navigation in theory, yet it requires a competent instrument pilot to use it accurately. It is a great aid.

The NDB

The nondirectional beacon (NDB) is the ground-based part of the combination. It is referred to as nondirectional because no particular direction is favored in its transmissions; the NDB radiates identical electromagnetic energy in all directions.

Each NDB transmits on a given frequency in the low-frequency or medium-frequency LF/MF band (somewhere between 190 and 1,750 kHz), the transmission mast being either a single mast or a large T-antenna strung between two masts.

To avoid confusion between various NDBs, and to ensure that the pilot is using the correct beacon, each NDB transmits its own particular identification signal (or ident) in the form of a two- or three-letter Morse code signal, which should be checked by the pilot before using the NDB for navigation. The two-letter ident is for an LOM and the three-letter ident is for a stand-alone NDB.

If the NDB is the only NAVAID being used for navigation, for instance during a typical NDB approach, the ident should be continuously monitored in the cockpit by the pilot, since the ADF has no failure flag to indicate a faulty signal or, indeed, no signal at all.

Example 11-1

The Burnet NDB (figure 11-10) operates on frequency 341 kHz and may be identified by the coded ident for BMQ, dah-dit-dit-dit dah-dah dah-dah-dit-dah. A nearby NDB is Marble Falls, which operates on frequency 403 kHz, and may be identified by the coded ident for MFS, dah-dah dit-dit-dah-dit dit-dit-dit.

The normal sources of NDB frequency and identification information are the en route charts, or the approach charts, which you should be carrying in the cockpit. More detailed information on a particular NDB, or any other NAVAID, is available in the Chart Supplement U.S.

NDB Range

For long-range en route navigation where no other NAVAIDs are available, a reasonably powerful NDB with a range of 100 NM or more is usually required. Some NDBs used for long-distance overwater tracking, such as in the Pacific area, may have a range of 400 NM. In the U.S., however, where route segments are relatively short and there are many NAVAIDs, especially VORs, most NDBs have only a short range.

A compass locator positioned at an outer marker of an ILS to assist in holding and local maneuvering may have a range of only 15 NM; an en route NDB might have a range of 75 NM. The range of an NDB depends on:

• the power of transmission (10–2,000 watts);
• the frequency of transmission;
• atmospheric conditions existing at the time — electrical storms, which can generate spurious signals, and periods of sunrise and sunset, which can distort or reflect the signals from an NDB; and
• the nature of the earth’s surface over which the signals travel.

In the U.S., NDBs are classified according to their range, or standard service volume (SSV) radius, as given in table 11-1.

NDB Signal Accuracy

An ideal NDB signal received at an airplane may be accurate to ±2°, however various factors may reduce this accuracy considerably. These factors include:

Thunderstorm Effect

Thunderstorm effect causes the ADF needle to be deflected toward a nearby electrical storm (cumulonimbus cloud and its associated lightning) and away from the selected NDB.

Night Effect

At night, NDB signals can be refracted by the ionosphere and then return to earth as strong skywaves, causing interference with the normal NDB surface waves, and resulting in a fading signal and a wandering ADF needle (most pronounced at dawn and dusk).

Interference

Interference from other NDBs transmitting on similar frequencies can be particularly significant at night.

Mountain Effect

Mountain effect is caused by reflections of the NDB signals from mountains.

Coastal Effect

Coastal effect is caused by the NDB signal bending slightly toward the coastline when crossing it at an angle.

NDB Identification

You must positively identify an NDB before using it for navigation.

Each NDB is identifiable by a three-letter Morse code identification signal that is transmitted along with its normal signal. This is known as its ident.

Each compass locator is identified by a two-lettered coded ident. If it is associated with the outer marker (LOM), the compass locator ident will be the first two letters of the localizer identification group. If it is associated with the middle marker (LMM), the compass locator ident will be the last two letters of the localizer identification group.

Monitor the ident continuously during an NDB approach.

You must identify an NDB or compass locator before using it for any navigational purposes within its operational range and, if using it for some length of time, then it should be periodically re-identified. During an NDB approach, the NDB or compass locator ident should be monitored continuously.

If you hear an incorrect or test ident, the NDB must not be used for navigation.

The lack of an ident may indicate that the NDB is out of service, even though it may still be transmitting (say for maintenance or test purposes), and it must not be used for navigation. When under test, the coded word test may sometimes be transmitted: dah dit dit-dit-dit dah. If an incorrect ident is heard, then those signals must not be used.

To identify most NDBs and compass locators, simply select AUDIO on the ADF, listen to the Morse code signal, and confirm that it is indeed the correct one. All NDBs in the U.S. can be identified with the ADF mode selector in the ADF position.

Many NDBs carry voice transmissions, such as the automatic terminal information service (ATIS) at some airports. It is also possible, in a situation where the communications radio (VHF COM) has failed, for ATC to transmit voice messages on the NDB frequency, and to receive them on the ADF if AUDIO is selected. Those that do not have a voice capability will have “W,” which stands for without voice, included in their class designator in the Chart Supplement U.S., for instance as “HW.”

Note. Broadcast stations may also be received by an ADF, since they transmit in the LF/MF bands. It is not good airmanship, however, to use broadcast stations as NAVAIDs, since they are difficult to identify precisely. Even if an announcer says “This is the Memphis Country Hour,” it is possible that the transmission is coming, not from the main transmitter, but from an alternative or emergency transmitter located elsewhere, or even a relay station many miles away from the main transmitter.

To use information from a broadcast station, you must be certain of its geographical position — something which is difficult to determine. Listening to broadcast stations in flight is also distracting from your main operational tasks. Broadcast stations are not used for IFR navigation in the United States — they are not required to have standby generators and do not have to advise the FAA if they are not transmitting for some reason.

The ADF

The airborne partner of the ground-based NDB is the automatic direction finder, usually referred to as the ADF. It operates on the radio compass principle whereby the ADF needle indicates the direction from which the signals are coming. Under ideal conditions, the ADF needle will point directly at the NDB antenna; under less-than-ideal conditions, the signals from the NDB antenna may not follow a straight path, and so the direction indicated by the ADF needle will be somewhat in error.

The automatic direction finder has three main components:

• the ADF receiver;
• the antenna system; and
• the ADF cockpit display.

The ADF receiver is installed in the cockpit radio panel, which the pilot tunes to the frequency of the desired NDB and verifies with the ident.

The antenna system comprises a loop antenna and a sense antenna (or their modern equivalent, a single combined unit) which together determine the signal direction.

The ADF cockpit display is either a fixed-card or a rotatable-compass-card, with a pointer or needle indicating the signal direction. The ADF cockpit instrument is usually installed to the right of the attitude flight instruments, with the top of the dial representing the nose of the airplane and the bottom of the dial representing its tail. Ideally, the ADF needle will point continually and automatically toward the NDB ground station.

The ADF Control Panel

There are various types of ADF that may be fitted to an airplane and, prior to flight, you must be familiar with the set that you will use.

You must be able to select and positively identify the NDB that you wish to use, and then verify that the ADF needle is indeed responding to the signals from that NDB. The correct procedure, any time a new NDB is to be used, is to confirm (verbally if so desired):

• selected;
• identified; and
• ADFing (active needle, giving a sensible bearing to the NDB).

The Mode Selector or Function Switch

The mode selector switches between ADF modes of operation.

OFF. Use OFF to switch the ADF off.

ADF. ADF is the normal position when the pilot wants bearing information to be displayed automatically by the needle. Most NDBs can be identified with the mode selector in this position (and the volume knob adjusted suitably).

ANT (or REC). These are the abbreviations for antenna or receiver. In this position, only the signal from the sense antenna is used, with no satisfactory directional information being available to the ADF needle. The reason for this function position is that it gives the best audio reception to allow easier identification, and better understanding of any voice messages. Never leave the mode selector in this position if you are navigating using the ADF — the ADF needle will remain stationary with no obvious indication that it is not responding! It is possible, however, to identify most NDBs with the mode selector in the ADF position (which is a safer position), and for the ANT position to be avoided.

BFO (or CW). These are the abbreviations for beat frequency oscillator or continuous wave. This position, rarely required in the United States, is selected when identifying the few NDBs that use A0/A1 or A1 transmissions, which are unmodulated carrier waves whose transmission is interrupted to provide the NDB’s Morse code identification. Since no audio message is carried on an unmodulated carrier wave, the BFO (as part of the airborne equipment) imposes a tone onto the carrier wave signal to make it audible to the pilot so that the NDB signal can be identified. Again, do not leave the mode selector switch in this position when navigating using the ADF.

TEST. Placing the mode selector into the TEST position will deflect the ADF needle from its current position. Placing the mode selector back to ADF should cause the needle to swing back and indicate the direction of the NDB. This function should be tested every time as part of the selected, identified, ADFing tuning procedure. Some ADF sets have a separate TEST button which only needs to be pressed to deflect the needle, and then released to check the return of the needle. You only have to deflect the needle approximately 30°, and watch the return, for the test to be satisfactory.

Note. On some ADF equipment, the TEST function is achieved using the ANT/REC position, which drives the needle to the 090 position. Returning the mode selector to ADF should see the needle start ADFing again.

Whenever you select an ADF mode, remember to return the mode selector to ADF when you have completed the task!

VOL. The volume knob will probably be separate to the mode selector. With audio selected to the pilot’s headset or to the cockpit speakers, the volume should be adjusted so that the ident or any voice messages on the NDB or compass locator may be heard. If signal reception is poor in ADF, then try ANT/REC; if there is no signal reception, try BFO/CW. Remember to return the mode selector to ADF!

Frequency Knobs

NDBs transmit on frequencies in the range 190 –1,750 kilohertz, the most common band being 200 – 400 kHz. To allow easier and accurate selection of any particular frequency, most modern ADFs have knobs that allow digital selection, in 100, 10 and 1 kHz steps. Some ADFs may have a band selector (200 – 400; 400 – 800; 800 –1,600 kHz), with either a tuning knob or digital selection for precise tuning.

ADF Cockpit Displays

The ADF needle always points directly toward the selected NDB ground station.

The basic purpose of an automatic direction finder (ADF) in an airplane is for its needle to point directly toward the selected NDB ground station. The ADF cockpit display is a card or dial placed vertically in the instrument panel so that:

• if the ADF needle points up, the NDB is ahead;
• if the ADF needle points down, then the NDB is behind;
• if the ADF needle points to one side, then the NDB is located somewhere to that side of the fore–aft axis of the airplane.

To convey this information to the pilot, various presentations are used, three of which we will consider:

• the fixed-card ADF, also known as the relative bearing indicator (RBI);
• the rotatable-card ADF (the “poor man’s” RMI); and
• the radio magnetic indicator (RMI).

Fixed-Card ADF or Relative Bearing Indicator (RBI)

With each change of magnetic heading, the ADF needle will indicate a different relative bearing.

On the fixed-card ADF, the needle indicates the relative bearing (RB) of the NDB from the airplane.

A fixed-card display has an ADF needle that can rotate against the background of a fixed-azimuth card of 360°, with 000 (360) at the top, 180 at the bottom, etc. The fixed-card ADF is also known as the relative bearing indicator (RBI), and is common in many general aviation aircraft. On the fixed-card ADF, the needle indicates the relative bearing (RB) of the NDB from the airplane. The relative bearing of the NDB from the aircraft is the angle between the aircraft’s heading (or, the nose of the aircraft) and the direction of the NDB. Relative bearings are usually described clockwise from 000 to 360; however, it is sometimes convenient to describe the bearing of the NDB relative to the nose or tail of the airplane.

Each time the airplane changes its magnetic heading, it will carry the fixed card with it. Therefore, with each change of magnetic heading, the ADF needle will indicate a different relative bearing (RB).

It is not the needle that moves, but rather the fixed-card — the needle continues to point at the station. The principle is easily understood if you stand, point at an object, and then turn and face another direction while continuing to point at the object. Your arm indicates the same direction to the object, but it makes a different angle with your body because you have changed your heading. The relative bearing of the object has changed because your heading has changed.

Orientation Using the RBI or Fixed-Card ADF

The airplane can be oriented with respect to the NDB knowing:

• the magnetic heading (MH) of the airplane (from the magnetic compass or heading indicator); plus
• the relative bearing (RB) of the NDB from the airplane.

In practice, magnetic heading is flown using the heading indicator, which should be realigned with the magnetic compass in steady flight every 10 minutes or so. The illustrations will therefore display the heading indicator instead of the magnetic compass. In figure 11-18, the pilot is steering MH 280, and the ADF indicates RB 030 to the NDB.

Visualizing Magnetic Bearing To the NDB (MB)

A quick pictorial means of determining MB to an NDB, using a relative bearing indicator and a heading indicator, is to translate the ADF needle onto the HI by paralleling a pencil or by using your imagination. MH + RB = MB to the NDB ground station.

Visualizing Magnetic Bearing From the NDB

The magnetic bearing of the aircraft from the NDB is the reciprocal of the magnetic bearing to the NDB. In figure 11-19, this is MB 130 from the NDB. MB from can be visualized as the tail of the pencil (or needle) when it is transferred from the RBI on the ADF onto the HI.

Note. An easier method of finding reciprocals than adding or subtracting 180°, is to either add 200 and subtract 20 or subtract 200 and add 20.

Example 11-2

The Rotatable-Card ADF

The rotatable-card ADF is an advance on the fixed-card ADF, because it allows the pilot to rotate the card so that the ADF needle indicates, not relative bearing, but magnetic bearing to the NDB. The pilot does this by aligning the ADF card with the HI compass card each time the airplane’s magnetic heading is changed.

To align a manually rotated ADF card:

• note magnetic heading on the heading indicator; then
• rotate the ADF card, setting magnetic heading under the index.

When the ADF card is aligned with the HI, the ADF needle will indicate the magnetic bearing to the NDB. This eliminates the need for mental arithmetic — an obvious advantage.

Note also that the tail of the needle, 180° removed from its head, indicates the magnetic bearing of the airplane from the NDB.

Any time the aircraft changes magnetic heading, the pilot should manually align the ADF card with the HI (after ensuring, of course, that the HI is aligned with the magnetic compass).

If desired, the rotatable-card can still be used as a fixed-card simply by aligning 000 with the nose of the airplane and not changing it.

The next step up from a rotatable-card ADF is an instrument with a card that remains aligned automatically. This instrument is known as the radio magnetic indicator, or RMI.

The Radio Magnetic Indicator (RMI)

The RMI display has the ADF needle superimposed on a card that is continuously and automatically being aligned with magnetic north. It is, if you like, an automatic version of the rotatable-card ADF — an automatic combination of the heading indicator and RBI.

The RMI is the best ADF presentation, and the easiest to use:

• the RMI needle will always indicate the magnetic bearing to the NDB; and
• the tail of the RMI needle will indicate the magnetic bearing from the NDB.

As an airplane turns and its magnetic heading alters, the RMI card (which automatically remains aligned with magnetic north) will appear to turn along with the ADF needle. In reality, of course, it is the compass card and the RMI needle that remain stationary while the airplane turns about them. Before, during and after the turn, the RMI’s needle will continue to indicate the current MB to the NDB.

Gyro-Stabilized Compass Equipment

In most airplanes fitted with an RMI, the initial magnetic north reference for the RMI card is provided by a fluxgate or fluxvalve, a detector that is sensitive to magnetic north, and situated in a fairly nonmagnetic part of the airplane such as in a wingtip. A heading indicator is electrically slaved to this magnetic reference so that the gyroscope is continually being aligned with magnetic north, and it is this heading indicator that drives the RMI compass card in a process known as slaving.

Most gyro-stabilized compasses have an annunciator near the compass card. This contains a small needle, often triangular in shape, that oscillates when automatic slaving is in process (which should be all the time). When the annunciator needle is hard over to one side, it indicates that the compass card is a long way out of alignment. This can usually be remedied using a manual knob to quickly realign the compass card with the magnetic heading of the airplane, after which the slower, automatic slaving should be sufficient to maintain alignment.

If slaving is not occurring because of some fault in the system (indicated by the annunciator being stationary and not oscillating) then the pilot can revert to using the RMI as a rotatable-card ADF (“poor man’s RMI”) or as a fixed-card ADF (relative bearing indicator).

Indicators with Two Pointers

Some airplanes are fitted with two ADF receivers, and have two needles superimposed on the one indicator (which may be a fixed-card twin-ADF indicator, or a dual-pointer RMI).

Most RMIs have function switches that allow you to select either an NDB or a VOR ground station for the RMI needle to point at. This gives you more flexibility in using NAVAIDs, since you can select the RMI to any suitable NDB or VOR within range or, with two needles, select one to ADF and the other to VOR.

Operational Use of the RBI

Orientation

Using the RBI to Obtain a Position Line

A position line (PL) is a line along which the airplane is known to be at a particular moment. A position line may be obtained by a pilot either visually, or by means of a NAVAID.

Two position lines that “cut” at a reasonable angle, ideally close to 90°, are needed for a fix. For the airplane to be on both position lines simultaneously, it must be at their point of intersection.

Figure 11-27 shows a NAVAID fix obtained using two NDBs in an airplane fitted with two ADFs. It is possible to fix a position using a combination of NAVAIDs, including NDBs, VORs, and DMEs.

A position line can be considered from two perspectives:

• to the NDB from the airplane, that is, the position line to the NDB that a pilot would see from the airplane, as either a relative bearing (RB 030 to NDB 1), or as a magnetic bearing to the NDB (MB 360 to NDB 1); or
• from the NDB to the airplane, as a magnetic bearing from the station (such as MB 180 from NDB 1). The magnetic bearing from the NDB may be converted to a true bearing by applying magnetic variation, if for instance you wanted to plot the airplane’s position on a chart.

Example 11-3

An airplane is heading 015° magnetic. The ADF needle points toward an NDB 75° to the right of the nose on an RBI. Magnetic variation is 5°W. Calculate:

a. the relative bearing (RB) to the NDB from the airplane;

b. the magnetic bearing to the NDB from the airplane;

c. the magnetic bearing from the NDB to the airplane; and

d. the true bearing from the NDB to the airplane.

While it is possible to calculate all of this mentally, at this early stage it is a good idea to sketch a clear diagram to help visualize the situation.

Step 1

Sketch the airplane on heading 015° magnetic.

Step 2

Indicate RB 075.

Step 3

Draw in the position line to the NDB.

Answers:

a. RB 075;

b. MB 090 to NDB;

c. MB 270 from NDB; and

d. TB 275 from NDB.

Easy Method of Visualizing MB

A relative bearing, as well as being specified using the 360° method clockwise from the nose of the airplane, can be specified as either left or right of the nose (or the tail). For instance, a relative bearing of 290 may be thought of as - 70, since the corresponding MB will be 70° less than the current magnetic heading. Similarly, RB 030 may be thought of as +30, since the corresponding MB to the NDB will be 30° greater than the magnetic heading.

Relative bearings off the tail of the airplane may be treated in a similar shorthand fashion. For instance, RB 160 may be thought of as - 20 off the tail; and RB 210 as +30 off the tail. This quadrantal approach to relative bearing and MB problems can simplify your in-flight visualization.

Example 11-4

An airplane is steering MH 340. The ADF needle shows RB 010. Determine the MB to the NDB.

Example 11-5

Refer to figure 11-31. An airplane is steering MH 358. The ADF needle shows RB 352. Determine the MB to the NDB.

Notice that, by coincidence, this airplane has the same magnetic bearing to the NDB as the airplane in Example 11-4, MB 350 to the NDB. In fact, it may even be the same airplane, which has simply altered heading by turning right from MH 340 to MH 358.

Example 11-6

Refer to figure 11-32. An airplane is steering MH 340. The ADF needle shows RB 190. Determine the MB from the NDB to the airplane.

Example 11-7

Refer to figure 11-33. An airplane is steering MH 010. The ADF needle shows RB 160. Determine the MB from the NDB.

Notice that, by coincidence, this airplane has the same magnetic bearing from the NDB as the previous one, MB 350 from the NDB. In fact, it may even be the same airplane, which has simply altered heading by turning right from MH 340 to MH 010.

Visualizing Position on the Heading Indicator

Mentally transferring the RBI needle onto the HI allows quick visualization of MB to NDB on the head of the needle, and MB from NDB on its tail.

If you now imagine a model airplane attached to the tail of the needle, with the model airplane oriented with the actual heading, you have a very good picture of the whole situation.

Example 11-8

Visualize the situation of MH 070 and RB 260 (refer to figure 11-34).

Intercepting a Course

Having oriented yourself with respect to an NDB, you know the answer to the question “Where am I?” Now ask “Where do I want to go?” and “How do I get there?”

Step 1

Orient the airplane relative to the NDB, and to the desired course.

Step 2

Turn to take up a suitable intercept heading, after considering where you want to join the desired course.

Step 3

Maintain the intercept heading and wait:

• for the head of the needle to fall if inbound;
• for the tail of the needle to rise if outbound:

• to ±030 for a 30° intercept;
• to ±045 for a 45° intercept;
• to ±060 for a 60° intercept;
• to ±090 for a 90° intercept, etc.

Step 4

Turn to the desired course, and apply a suitable wind correction angle to maintain it.

Visualizing Where You Are and Where You Want To Go

The heading indicator (HI) helps you visualize the situation. In the previous example, the situation MH 070 and RB 260 was visualized, with MB 330 to the NDB. What if the pilot wishes to intercept a magnetic course (MC) 270 to the NDB?

All the pilot needs to do is visualize the desired course on the heading indicator. With a model airplane on the tail of the needle tracking as desired, it becomes clear what turns are necessary to intercept the desired course. First turn left to a suitable intercept heading, say MH 360 for a 90° intercept of MC 270 to the NDB.

Note. If you become disoriented, a simple procedure is to take up the heading of the desired course. Even though not on course, the airplane will at least be parallel to it, and the ADF needle will indicate which way to turn to intercept it.

Suppose the situation is MH 340, RB 080, and you wish to intercept MC 090 to the NDB. The current magnetic bearing to the NDB is easily found to be MB 060 (MH 340 + RB 080). By continuing to steer MH 340, the airplane will eventually intercept MC 090 to the NDB, but it would be a rather untidy intercept, with the airplane tracking somewhat away from the NDB, and with an intercept turn of 110° being required.

A tidier and more efficient intercept may be achieved by turning to an initial heading of MH 360 for a 90° intercept; or MH 030 for a 60° intercept. (Turning further right to MH 060 would of course point the airplane at the NDB, and MC 090 to the NDB would not be intercepted.)

Using the ADF to Intercept an Inbound Course

Example 11-9

An airplane is steering MH 355, and the RBI indicates RB 005 when tuned to a particular NDB. The pilot is requested to track inbound on MC 340 to the station, intercepting the course at 60°.

Step 1

Orient the airplane. MH 355 + RB 005 = MB 360 to NDB, or MB 180 from NDB. The airplane is south of the NDB and MH 355. The desired course is MC 340 to the NDB (which is on the position line MB 160 from the NDB), to the right of the airplane.

Step 2

To intercept MC 340 from the left at 60°, the airplane should steer (340 + 60 = 400 =) MH 040. As the airplane’s heading alters, the ADF needle will continue to point at the NDB and so the relative bearing will change (in this case, even though it is not an important calculation, from RB 005 to RB 320, or - 40 off the nose, with the 45° right turn).

Step 3

Maintain MH 040 and periodically observe the RBI as the head of the needle falls. Since it is a +60 intercept, wait until the head of the needle falls to - 60 (or RB 300). You are steering course +60, waiting for - 60.

Step 4

At MB 340, and as the needle is falling to RB - 60, turn left to take up the desired course to the NDB, allowing for the estimated crosswind effect on tracking. In this case, a wind correction angle (WCA) of 3° left is used. Maintain the desired course of MC 340 to the station by continually checking that MH + RB = MB 340, for example: MH 337 + RB 003 = MB 340.

Note. An airplane takes some distance to turn, and so you should anticipate the desired course by starting the turn onto course just before MB 340 is reached. You can do this by observing the rate at which the ADF needle falls toward - 060, and commence the turn accordingly.

Example 11-10

ATC gives you radar vector 010 to steer, and instructs you to intercept 055 inbound to an NDB off that heading.

Step 1

Orient the airplane. With radar vector 010 to intercept MC 055 inbound, the airplane must be south of the required course.

Step 2

The intercept has been organized by the radar controller so that the airplane will intercept course at 45° (055 – 010 = 45).

Step 3

Maintain MH 010 and periodically observe the RBI as the head of the needle falls. Since it is a - 45 intercept, wait until the head of the needle falls to +045 (RB 045). You are steering course - 45, waiting for +45.

Step 4

Shortly before MB 055 to the NDB is reached, and as the needle falls to +45, turn right to take up the desired course to the NDB, allowing for the estimated wind drift. In this case, a WCA of 5° right is used, by steering MH 060 with the RBI on - 5 off the nose (RB 355). Maintain the desired MC 055 to the station by continually checking that MH + RB = MB 055, e.g., MH 060 + RB - 005 = MB 055.

Another means of achieving a smooth intercept is to reduce the closing angle as the desired course is approached, for example from 45° to 30° to 15° and finally, to zero as the course is intercepted.

Using the ADF to Intercept an Outbound Course

Example 11-11

The radar controller gives you a radar vector of 340 to intercept an outbound course of 280.

Step 1

Orient the airplane. It must be south of the outbound course.

Step 2

Consider the intercept. A radar vector of 340 to intercept MC 280 outbound means a +60° intercept.

Step 3

Monitor the intercept by steering a steady MH 340 and periodically checking the RBI to see the tail of the needle rising to - 60 (RB 300). You are steering course +60, waiting for - 60.

Step 4

As MC 280 outbound is approached, indicated by the tail of the needle rising to - 60, turn left to pick it up, in this case allowing a WCA of 10° for a wind from the right, to MH 290.

Periodically check that MH ± ADF tail = MB from NDB. In this case, the tail of the ADF needle should be - 10 off the nose (on RB 350), so that MH 290 – 10 = MC 280 from NDB.

Example 11-12

You are steering MH 120, with RB 080. You wish to track outbound from the NDB on MC 090, intercepting as soon as possible.

Step 1

Orient the airplane. MH 120 + RB 080 = MB 200 to NDB, or MB 020 from NDB The airplane is north-northeast of the NDB, steering MH 120. The desired course is MC 090 from the NDB, which is to the right of the airplane.

Step 2

Continuing on MH 120 would give a 30° intercept of MC 090-outbound. To intercept MC 090-outbound as soon as possible, with a 90° intercept, the airplane should be turned to steer (090 + 90 =) MH 180.

Step 3

Maintain MH 180 and periodically observe the RBI as the tail of the needle rises. Since it is a +90 intercept, wait until the tail of the needle rises to - 90 (270). You are steering course +90, waiting for - 90.

Step 4

At MB 090 from the NDB, or shortly before MB 090 from the NDB is reached, and as the tail of the needle approaches - 90, turn left from your current MH 180 to take up the desired course 090 from the NDB, allowing for the estimated crosswind effect on tracking. In this case, with no wind, steer MH 090. The tail of the ADF needle should be on 000. Periodically check that MH ± ADF tail = MB 090 from NDB. For instance, with a northerly wind, you might require WCA 10° left, so that MH 080 + 10 off the tail = MB 090 from NDB.

Note. With such a large intercept angle, 90° in the above case, you will have to watch the rate at which the tail of the ADF needle rises, and anticipate the turn to intercept the desired course. If you do not anticipate the intercept, the airplane will fly through the course, which will then have to be re-intercepted from the other side — not a tidy maneuver.

Tracking with the RBI

Using the RBI to Track Inbound to an NDB

The ADF/NDB combination is often used to provide guidance for an airplane from a distant position to a position overhead the NDB ground station. This is known as tracking. Just how the pilot achieves this depends to a certain extent on the prevailing wind direction and speed, since an airplane initially pointed directly at the NDB will be blown off course by a crosswind.

Tracking Toward an NDB, with no Crosswind Effect

With no crosswind, a direct inbound course can be achieved by simply pointing the airplane directly at the NDB and steering a heading that keeps the ADF needle on the nose (RB 000). If there is no crosswind to blow the airplane off course, then everything will remain constant as in figure 11-45 — the MH 096, the RB 000, and the MB 096 to the NDB will all remain constant. This can only occur in:

• no-wind conditions;
• a direct headwind; or
• a direct tailwind.

Tracking Inbound with a Crosswind

Without Wind Correction Angle. If no wind correction angle is applied, and the airplane is pointed directly at the NDB, so that the ADF needle indicates RB 000, then any crosswind will cause the airplane to be blown off course.

In the case illustrated in figure 11-46, a wind with a northerly component has blown the airplane to the right of course. This is indicated by the ADF needle starting to move down the left of the dial. To return to the desired course, the airplane must be turned to the left, toward the direction in which the head of the needle is moving.

If you turn left to put the NDB on the nose again, so that the relative bearing is RB 000, then after a short while the airplane will again be blown to the right of course, and the ADF needle will again move to the left of the nose. A further turn to the left will be required — and the process will need to be repeated again and again. In this way, the airplane’s ground track to the NDB will be curved and it will finally arrive overhead the NDB heading roughly into the wind. This rather inefficient means of tracking to the NDB is known as homing (keeping the NDB on the nose). It is not a precise procedure and will involve traveling a greater distance than that required to fly a direct course to the NDB from the original position. Professional pilots never use it!

With Correct Wind Correction Angle. With the correct wind correction angle applied, the airplane will track directly toward the ground station in a straight line — a far better procedure than homing. If 5° left is indeed the correct wind correction angle, you can achieve a course of MC 096 direct to the NDB by steering MH 091.

Wind Correction Angles

Different winds require different wind correction angles. An airplane is on course when the relative bearing is equal-and-opposite to the difference between the actual magnetic heading (MH) and the desired magnetic course (MC). This is illustrated in figure 11-48.

In each situation, the airplane is on the desired course of MC 010, but using a different wind correction angle (WCA) to counteract the drift under different wind conditions.

If the precise wind effect is not known, then initially use a best guess WCA estimated from the available information. For the same crosswind, slower airplanes will need to allow a greater WCA than faster airplanes. See how the estimated WCA works, then make an adjustment to heading if required.

It is possible that the wind effect will change as you track toward an NDB, and so regular adjustments to the heading may be required. This is often the case as an airplane descends while using the NDB as the tracking aid, due to variations of wind speed and/or direction, and of airplane TAS, that occur with changes in altitude.

Incorrect Drift Correction

If an incorrect drift correction is made by the pilot, then the airplane will move off the desired course, so the RB indication and the MB to the NDB will change. If a steady heading is being flown, then any divergence from course will become obvious through a gradually changing relative bearing, with the ADF needle moving left or right down the dial.

Suppose you fly a heading with a 5° wind correction angle to the left to counteract the effects of a wind from the left. If the wind effect turns out to be less than expected, then the airplane will gradually move to the left of the desired course to the NDB, and the RB will gradually increase (naturally, the MB to the NDB will also increase). Typical cockpit indications could be:

The head of the ADF needle falling away to the right indicates that a turn right must be made to track to the NDB. Conversely, the head of the ADF needle falling away to the left indicates that a left turn must be made to track to the NDB. Just how great each correcting turn should be depends on the deviation from course.

Note. Be careful of terminology. Drift is the angle between heading and the achieved ground track, which may not be the desired course. The perfect wind correction angle will counteract any drift exactly, and the actual ground track will follow the desired course; this is the aim of tracking.

Maintaining Course

Tracking is a series of small turns made in an attempt to maintain the desired course perfectly.

In reality, flying level is a series of small and gentle climbs and descents made by the pilot in an attempt to maintain the desired altitude precisely. Similarly, tracking is a series of small turns made in an attempt to maintain the desired course perfectly.

Re-intercepting a course, having deviated from it, involves the same procedure as the initial intercept of a new course, except that the intercept angles will be smaller (provided that the pilot is vigilant and does not allow large deviations to occur). Realizing that the airplane is diverging from the direct course to the NDB, the pilot has several options — either:

• fly direct from the present position (along a new course); or
• regain the original course.

Flying Direct to the NDB

To fly direct to the NDB from your present position (even though the present position is not on the originally desired course), turn slightly right (for example, 3° in this case), and fly direct to the NDB from the present position. Normally, this technique is used only when within one or two miles from the NDB, when there is insufficient distance remaining to regain the original course.

Regaining Original Course

To regain the original course, turn further right initially (for example, 5° to MH 096), and reintercept the original course by allowing the wind to blow the airplane back onto it. Once the desired course is regained, turn left and steer a heading with a different wind correction angle (WCA 3° left instead of 5° left), such as MH 093 instead of MH 091. This is a relatively minor correction, something you would expect to see from an experienced instrument pilot who would have noticed any deviation from course fairly quickly.

If MH + RB = desired MB constantly, then ADF tracking is good.

Attempting to maintain the desired course (remaining on a constant MB to the NDB) is the normal navigational technique when more than just a mile or two from the NDB. If, when steering a steady magnetic heading, the ADF needle indicates a constant relative bearing near the top of the dial, then the airplane is tracking directly to the NDB, and no correction to heading is necessary.

Just how great each correcting turn should be depends on the deviation from course. A simple method is to double the error. If the airplane has deviated 10° left indicated by the RBI moving 10° right, then alter heading by 20° to the right. (If you alter heading by only 10° to the right, the result will probably be a further deviation to the left, a further correction to the right, with this being repeated again and again, resulting in a curved homing to the NDB.)

Having regained course, turn left by only half the correcting turn of 20°, that is, turn left 10° to intercept and maintain course. This leaves you with a WCA different to the original one (remembering that the original WCA caused you to deviate from course).

The new WCA should provide reasonable tracking. If not, make further minor corrections to heading.

“Bracketing” Course

In practice, an absolutely perfect direct course is difficult to achieve. The actual ground track by the pilot will probably consist of a series of short segments either side of the desired course, which corresponds to minor corrections similar to those described above. This technique is known as bracketing the course, and involves making suitable heading corrections, left or right as required, to regain and maintain the desired course.

The aim of bracketing is to find the precise WCA needed to maintain course. If, for instance, a WCA of 10° right is found to be too great and the airplane diverges to the right of course, and a WCA of only 5° right is too little and the wind blows the airplane to the left of course, then try something in between, such as WCA 8° right.

Bracketing is a technique used to get you to a point where you can begin tracking.

Monitor the tracking of the airplane on a regular basis, and make corrections earlier rather than later, the result being a number of small corrections rather than just one big correction. However, if a big correction is required, as may be the case in strong winds, make it.

Wind Effect

If the wind direction and strength is not obvious, then the best technique is to initially steer course as heading (make no allowance for drift). The effect of the wind will become obvious as the ADF needle moves to the left or right. Observe the results, and then make appropriate heading adjustments to bracket course.

Tracking Over an NDB

The ADF needle will become more and more sensitive as the NDB station is approached. Minor displacements left or right of course will cause larger and larger changes in RB and MB. For a very precise course to be achieved, you must be prepared to increase your scan rate as the NDB is approached, and to make smaller corrections more frequently.

Close to the station and just prior to passing over the NDB, however, the ADF needle can become very sensitive and agitated. Relax a little and steer a steady heading until the airplane passes over the NDB, indicated by the ADF needle moving toward the bottom of the dial.

Having passed over the NDB, tracking from the NDB should be checked and suitable adjustments made to heading. If the course outbound is different from that inbound, then a suitable heading change estimated to make good the new desired course could be made as soon as the ADF needle falls past the 090 or 270 position on its way to the bottom of the dial.

The ADF needle becoming extremely active, and then falling rapidly to the bottom of the dial, indicates that the airplane has passed directly over the NDB.

The ADF needle moving gradually to one side, and slowly falling to the bottom of the dial indicates that the airplane is passing to one side of the beacon, the rate at which the needle falls being an indication of the airplane’s proximity to the NDB. If it falls slowly, then the tracking by the pilot could have been better.

Time over (or abeam) the NDB with no WCA can be taken as the needle falls through the approximate 090 or 270 position.

Time over (or abeam) the NDB with a WCA 10° right can be taken as the needle falls through the approximate 080 (090 – WCA 10) or 260 (270 – WCA 10) position.

Tracking Away from an NDB

When tracking away from an NDB, the head of the ADF needle will lie toward the bottom of the dial.

Tracking Away from an NDB with no Crosswind Effect

If the pilot tracks over the NDB and then steers course as heading, the airplane will track directly away from the NDB with the head of the ADF needle steady on 180, and the tail of the ADF needle steady at the top of the dial on 000. The airplane in figure 11-59 has MB 040 from the NDB, and MB 220 to the NDB.

Tracking Away From an NDB with a Crosswind

Suppose that the desired course outbound from an NDB is MC 040, and the pilot estimates that a WCA of 5° to the right is necessary to counteract a wind from the right. To achieve this, the pilot steers MH 045, and hopes to see the tail of the ADF needle stay on - 5 off the nose (RB 355). The magnetic course away from the station is found from: MB from NDB = MH ± deflection of the tail of the needle. In this case, MH 045 – 005 tail = MB 040 from NDB, and the chosen WCA and magnetic heading to steer are correct.

If the estimated WCA is incorrect, then the actual ground track achieved by the airplane will differ from the desired course. If, in the previous case, the wind is stronger than expected, the airplane’s ground track may be 033, to the left of the desired MC 040.

Whereas inaccurate tracking to an NDB is indicated by the ADF needle falling, incorrect tracking away from an NDB can occur with the ADF needle indicating a steady reading. Having passed overhead the NDB, an airplane can track away from it in any of 360 directions. You must always ensure that you are flying away from the NDB along the correct course, and the easiest way to do this is to calculate MB from the NDB using the HI and the RBI.

Regaining Course Away from an NDB in a Crosswind

If an incorrect wind correction angle is flown, then the airplane will be blown off the desired course. The vigilant pilot will observe the incorrect ground track, probably by visualizing MB from the NDB (or MB to the NDB), while steering a constant magnetic heading.

Example 11-13

The pilot is flying course as heading, initially making no allowance for wind effect. If the head of the ADF needle moves right from RB 180 into the negative quadrant, then the airplane must be turned right to regain course.

Note. It is generally easier to work off the top of the dial, since that is where the airplane is going, rather than off the bottom of the dial. The right turn necessary to regain course, turning right toward the head of the needle and therefore away from the tail of the needle, can be thought of as pulling the tail of the ADF needle around or trailing the tail. Some instructors, however, prefer to say that if the head of the needle is moving right, then turn right (and vice versa), even though the head of the needle is at the bottom of the dial. Your instructor will recommend a method.

In Example 11-13 (figure 11-62), the airplane has been blown to the left of course. The off-course MB from the NDB is given by MH 040 – 015 tail = MB 025 from NDB which is left of the desired MB 040 from the NDB.

To regain course, the pilot has turned right by 30° (double the error) from MH 040 to MH 070, which causes a simultaneous change in the relative bearing of the NDB, the ADF needle tail moving from - 015 (345) to - 045 (315). (The head of the needle, indicating relative bearing, will move from RB 165 to RB 135, but this is not a calculation for the pilot to make, only an observation.)

The relative bearing will change as heading changes. Wait until you achieve a steady intercept heading to determine if additional corrections are needed.

The relative bearing will naturally change as the airplane is turned but, once the airplane is flown on its steady intercept heading of MH 070, the tail of the needle will be gradually pulled around.

The pilot will continue to steer the intercept heading until the airplane approaches the desired course, MC 040 from the NDB. This is indicated by the tail of the needle moving up toward - 30 (since MB 040 from NDB = MH 070 - 30 tail). For a 30° intercept of MC 040 outbound, the pilot will steer a MC + 30 heading (MC 040 + 30° intercept = MH 070), waiting for the tail of the needle to rise to - 30 off the nose. Steering course +30, waiting for - 30 on the tail of the needle.

As the desired outbound MC 040 is intercepted from MH 070, the pilot turns left to maintain MC 040 from the NDB. Estimating a WCA of 10° into the wind to be sufficient, the pilot steers MH 050 and checks regularly that the needle tail stays on - 010.

Example 11-14

Conversely, if the head of the ADF needle moves left from RB 180 into the positive quadrant, then the airplane must be turned left to regain course.

Looking at the top of the ADF dial and the tail of the needle, turn left and trail the tail. In figure 11-63, the airplane has been blown to the right of course. The off-course MB from the NDB is given by: MH 040 + 015 tail = MB 055 from NDB; right of desired MC 040-FROM.

To regain course, the pilot has turned left by 30° from MH 040 to MH 010, which causes a simultaneous change in the relative bearing of the NDB, the tail of the ADF needle moving from 015 to 045. The relative bearing will naturally change as the airplane is turned but, once the airplane is flown on its steady intercept heading of MH 010, the tail of the needle will be gradually pulled around.

For a 30° intercept of MC 040 outbound, the pilot is steering a heading of 010°M (MC - 30), waiting for the tail of the needle to reduce to +30 off the nose. Steering course - 30, waiting for +30 on the tail of the needle.

The pilot will continue with the intercept heading until the airplane approaches the desired MC 040 outbound from the NDB. This is indicated by the tail of the needle moving up toward RB 030 (since MB 040 from NDB = MH 010 + 30 tail).

As the desired course outbound is approached, the pilot turns right to maintain MC 040 from the NDB. Estimating a WCA of 10° into the wind to be sufficient, the pilot steers MH 030 and checks regularly that the tail of the needle stays on +10 (RB 010).

The NDB Approach

The NDB/ADF combination can be used for an instrument approach procedure called the NDB approach — typical examples follow. The top section of each instrument approach chart shows a plan view for tracking purposes, and the bottom section shows a profile view for vertical navigation (descent).

The Valparaiso County Municipal NDB Rwy 27 approach uses an NDB situated away from the airport, while other NDB approaches may use an NDB situated on the airport. As you will see later, the same situation can apply with VOR approaches — some use a VOR on, or very near to, the airport, others use a VOR sited some distance away. NDB and VOR approaches are similar in design, the only significant difference being the type of navigational aid used for approach course tracking.

The Valparaiso Rwy 27 NDB Approach

This approach is based on the compass locator VP situated at the outer marker (LOM) 5.3 NM from the Runway 27 threshold. From overhead the LOM, track outbound on MC 092 and descend to not below 2,600 feet MSL. You must remain within 10 NM of the LOM. Course reversal is achieved with a procedure turn to the right, to MH 137 for 1 minute, followed by a left turn to MH 317 to intercept the inbound course of MC 272.

Track into LOM not below 2,600 feet MSL, continuously monitoring the VP ident. The ADF needle will become increasingly sensitive as you approach the compass locator, and then fall toward the bottom of the dial as you pass over it. The LOM is the final approach fix (FAF), and you should start timing as you pass it. This allows you to determine arrival at the MAP, which will be in 3 minutes 32 seconds if your groundspeed is 90 knots (as per the timing table at bottom right of approach plate).

From the FAF, you will be back-tracking on the compass locator, remaining on course by trailing the tail of the needle. Descent is now approved to the MDA 1,280. To land straight-in, you require a visibility of ¾ SM; to circle to another runway, you require a visibility of 1 SM.

If you do not become visual at or above the minimum, then you must begin a missed approach, climbing away and turning right at the MAP (determined by stopwatch) direct to the VP LOM at 2,500 feet MSL, where you should enter the holding pattern and consider your options.

The RMI and Rotatable-Card ADF

The radio magnetic indicator combines the relative bearing indicator and the heading indicator into the one instrument, where the ADF card is aligned automatically with magnetic north. This considerably reduces your workload by reducing the amount of visualization and mental arithmetic required. Even the manually rotatable-card (the “poor man’s RMI” which allows you to align the ADF card manually with magnetic north) lightens the workload, since it also reduces the amount of visualization and mental arithmetic required. The following discussion applies to both the RMI and the manually rotatable-card ADF, except that:

• the RMI is continuously and automatically aligned with magnetic north; while
• the manually rotatable-card must be realigned with the heading indicator by hand following every heading change (and of course the HI must be realigned with the magnetic compass by hand every 10 minutes or so).

Orientation

An RMI gives a graphic picture of where the airplane is:

• the head of the RMI needle displays magnetic bearing to the ground station; and
• the tail of the RMI needle displays magnetic bearing from the ground station.

One significant advantage of the RMI over the RBI is that you can select it to either an NDB or a VOR ground station. The method of use is the same in each case. If the head of the RMI needle indicates 030, then we write this as RMI 030. It tells us that the magnetic bearing to the ground station from the airplane is 030 degrees magnetic. The bearing from the ground station to the airplane is, of course, the reciprocal 210 degrees magnetic.

Example 11-15

The RMI is selected to an NDB. Orient the airplane with MH 320 and RMI 050. Determine the magnetic bearing to the ground station, and the magnetic bearing from the ground station.

Note. RMI 050 means MB 050 to the NDB (whereas RB 030 means a relative bearing of 030 to the NDB, relative to the airplane’s nose and its heading).

The Initial Interception of Course

Intercepting an Inbound Course

A common use for the RMI, after you have used it to orient yourself with respect to the NDB, is to track to the NDB. The RMI makes it easy to visualize:

• where you are;
• where you want to go; and
• how to get there.

Example 11-16

An airplane has MH 340 and RMI 030. You are requested to intercept a course of 090 to the NDB.

Step 1

Orienting the airplane is made easy by the RMI. The magnetic bearing to the NDB from your present position is 030. If you now imagine a model airplane attached to the tail of the needle, with the airplane on the actual heading (which in this case is MH 340), then you have a clear picture of the situation.

The desired course of 090 to the NDB is ahead of the present position of the airplane. If you visualize the desired course on the RMI, with the model airplane on the tail of the needle tracking as desired, it becomes quite clear what turns are required to intercept the desired course.

Step 2

To intercept MC 090 to the NDB, the airplane should be turned to a suitable intercept heading, as in figure 11-70.

Step 3

Maintain the chosen intercept heading and periodically observe the RMI needle as it falls toward the desired inbound course of 090.

Step 4

As MC 090 to the NDB is approached (indicated by the RMI needle approaching 090) turn right to take up the desired course to the NDB, allowing for any estimated wind drift. In this case, a WCA of 10° right has been used. With MH 100, and the RMI steady on 090, the airplane now tracks MC 090 to the NDB.

Example 11-17

An airplane is given a radar vector by ATC to steer MH 010, and then to intercept MC 055 inbound to an NDB.

Visualizing the situation confirms that radar vector 010 will intercept the inbound MC 055, and it will in fact be a 45° intercept (055 – 010 = 45). The RMI needle falls toward 055 approaching the desired course inbound, and you should commence a turn shortly before reaching it to avoid overshooting the course. This is known as leading in (which is really anticipating), and the amount of lead-in can be judged by the rate at which the needle is falling, and the distance required for the airplane to turn to a suitable heading to fly inbound.

Another means of achieving a smooth intercept is to progressively reduce the closing angle as the course is approached, for example from 45° to 30° to 15° and finally, to zero as the course is intercepted.

In this case, the pilot has chosen to fly inbound with a WCA of 15° left to counteract drift caused by a strong northerly wind. Correct tracking to the NDB will be confirmed by the RMI needle staying on 055.

Intercepting an Outbound Course

Example 11-18

You are given a radar vector of 340 to intercept 280 outbound from an NDB.

Step 1

Orient the airplane.

Step 2

Consider the intercept, 60° in this case (340 – 280 = 60). Visualize the situation. Again, the model airplane imagined on the tail of the needle helps.

Step 3

Monitor the intercept by steering a steady MH 340 and periodically checking the tail of the RMI needle rising to 280.

Step 4

As the desired course 280 outbound is approached, and as the tail of the needle approaches 280, turn left to pick up the MC 280, in this case allowing no wind correction angle, since you expect no crosswind effect.

Maintaining Course

Tracking Toward an NDB, with no Crosswind Effect

With no crosswind, a direct course inbound can be achieved by pointing the airplane directly at the NDB. The magnetic heading will, in this case, be the same as the desired course, and the RMI needle will be on the nose indicating the course.

If there is no crosswind to blow the airplane off course, then everything will remain constant as in figure 11-73; MH 250 and RMI 250 will remain constant. This can only occur in:

• no-wind conditions;
• a direct headwind; or
• a direct tailwind.

Tracking Inbound with a Crosswind

Without Correction for Drift. If no correction for drift is made by the pilot, and the airplane is pointed straight at the NDB with the RMI needle initially on the nose, any crosswind will cause the airplane to be blown off course.

In figure 11-74, the wind, with a northerly component, has blown the airplane to the left of course. This is indicated by the head of the RMI needle starting to move down the right of the dial. To return to course, the airplane must be turned toward the right, toward the direction in which the head of the needle is moving.

If you turn right to put the NDB on the nose again, MH = RMI 255, then after a short period the airplane will again be blown to the left of course, and the RMI needle will move to the right of the nose. Another turn to the right will be required.

In this way the path to the NDB will be curved, with the airplane finally approaching the NDB heading roughly into the wind, and a longer distance will be traveled compared with the direct course from the original position. This is known as homing (keeping the NDB on the nose).

With Correct Drift Correction. If a correct drift correction is made by the pilot — a far better procedure than homing — then the airplane tracks direct to an NDB by crabbing into the wind to counteract drift. If 15° right is indeed the correct WCA, then the airplane will fly MC 250 direct to the NDB by steering MH 265.

Wind Correction Angles

Different winds require different wind correction angles. An airplane is on the desired course when the RMI indicates that same course. This is illustrated in figure 11-76. In each situation, the airplane is on the desired MC 355, but using a different wind correction angle to counteract the drift under different wind conditions.

Figure 11-76 Crabbing into the wind to achieve the desired course.

If the precise wind effect is not known, then initially use a best-guess WCA, estimated from the available information. For the same crosswind, slower airplanes will need to allow a greater WCA than faster airplanes.

It is possible that the wind effect will change as you fly toward an NDB, and so regular alterations of heading may be required. This is often the case as an airplane descends using the NDB as the approach tracking aid.

With Incorrect Drift Correction. If an incorrect drift correction is made by the pilot, then the airplane will move off the desired course and the magnetic bearing to the NDB will change. This will become obvious through a gradually changing RMI reading (since the RMI indicates MB to the ground station).

Suppose, for instance, the pilot steers a heading with a 5° wind correction angle to the right to counteract the effects of a wind from the right.

If the wind effect turns out to be greater than expected, the airplane will gradually deviate to the left of the desired course to the NDB, and the RMI reading will gradually increase. Typical cockpit indications could be:

The head of the RMI needle falling away to the right indicates that a turn right must be made to track to the NDB. Conversely, the head of the RMI needle falling away to the left indicates that a left turn must be made to track to the NDB. Just how great each correcting turn should be depends on the deviation from course.

Maintaining Course

Re-intercepting a course, once having deviated from it, requires basically the same procedure as the initial intercept of a new course, except that the angles will be smaller, provided the pilot is vigilant and does not allow large deviations to occur.

Realizing that the airplane is diverging from the direct course to the NDB, you have several options. You may either:

• track direct to the NDB from the present position even though it is slightly off the original course; or
• regain the original course.

To Track Direct to the NDB. Turn slightly right (say 5° in this case), and track direct to the NDB from the present position, even though it is not on the originally desired course. Normally, this technique is used only when very close to the station (say 1 or 2 NM).

To Regain the Original Course. Turn further right initially (say 10° to MH 030), and reintercept the original course, indicated by the RMI needle moving down to read 015 again.

Once course is regained, turn left (say by half the correcting turn of 10°) to MH 025. This is a moderate correction, something you would expect to see from an experienced instrument pilot.

Attempting to maintain the desired course (maintaining a constant MB to the ground station) is the normal navigational technique when at some distance from the NDB. If the RMI remains on a steady reading, then the airplane is tracking directly to the NDB.

Just how great each correcting turn should be depends on the displacement from course and the distance from the station. A simple method is to initially alter heading by double the error.

If the airplane has deviated 10° left (indicated by the RMI moving 10° right), then alter heading by 20° to the right. (If you alter heading by only 10° to the right, the result will probably be a further deviation to the left, a further correction to the right, ultimately resulting in a curved homing to the NDB.)

Having regained course, turn left by only half the correcting turn of 20°; i.e., turn left 10° to intercept and maintain course. This leaves you with a WCA different from the original one (that caused you to deviate from course), and one that should provide reasonable tracking. If not, make further corrections to heading!

Bracketing Course

In practice, absolutely perfect tracking along the desired course is difficult to achieve. The actual ground track achieved will probably consist of a series of minor corrections to heading, a technique known as bracketing the course, making regular corrections, left or right as required, to maintain or regain the desired course.

The aim of bracketing is to find the precise WCA needed to maintain course. If, for instance, a WCA of 10° right is found to be too great and the airplane diverges to the right of course, and a WCA of only 5° right is too little and the wind blows the airplane to the left of course, then try something in between, say WCA 8° right.

A precise instrument pilot will monitor the tracking of the airplane on a regular basis and make corrections earlier rather than later, the result being a number of small corrections rather than just one big correction.

Wind Effect

If the wind direction and strength is not obvious, then a useful technique is to initially steer the course to the station as heading (make no allowance for drift), observe the results, and then make heading adjustments to bracket course. This is illustrated in figure 11-80.

Tracking Over an NDB

The RMI needle will become more and more sensitive as the NDB station is approached, minor movements left or right of course causing larger and larger changes in the RMI reading. For a precise course to be achieved, you must be prepared to increase your scan rate and respond more frequently.

Close to the station and just prior to passing over the NDB, however, the RMI needle can become agitated, and the pilot should relax a little and steer a steady heading until the RMI needle moves toward the bottom of the dial and settles down, at which time tracking from the NDB should be checked and suitable adjustments made to heading.

If the course outbound is different from that inbound, then a suitable heading change estimate could be made as soon as the RMI needle falls past the mid-position on its way to the bottom of the dial.

The RMI needle becoming extremely active and then falling rapidly past the abeam position to the bottom of the dial indicates that the airplane has passed directly over the NDB.

The RMI needle moving gradually to one side and slowly falling to the bottom of the dial indicates that the airplane is passing to one side of the beacon — the rate at which the needle falls being an indication of the airplane’s proximity to the NDB. If it falls very slowly, then possibly the tracking by the pilot could have been better. Time over (or abeam) the NDB can be taken as the needle falls through the approximate mid-position.

Tracking Away from an NDB

When tracking away from an NDB, the head of the RMI needle will lie toward the bottom of the dial, and the tail of the RMI needle will be toward the top of the dial.

Tracking Away from an NDB with no Crosswind Effect

If the pilot tracks over the NDB and steers course as heading, the airplane will track directly away from the NDB with the head of the RMI needle steady at the bottom of the dial, and the tail of the RMI needle steady at the top of the dial under the MH lubber line.

The airplane in figure 11-84 has a magnetic bearing from the station indicated by the tail of the RMI needle, and a magnetic bearing to the station indicated by the head of the RMI needle. Since it is course outbound that is being considered here, the position of the tail of the needle is of more use to the pilot.

Tracking Away from an NDB with a Crosswind

Suppose that the desired course outbound from an NDB is MC 340, and the pilot estimates that a WCA of 12° to the right is necessary to counteract a wind from the right. To achieve this, the pilot steers MH 352, and hopes to see the tail of the RMI needle stay on 340, the desired outbound course. In figure 11-85, the chosen WCA and MH are correct, and the desired MC 340 outbound is maintained.

If the estimated WCA is incorrect, then the actual ground track achieved by the airplane will differ from that desired. If, in figure 11-85, the wind is stronger than expected, the airplane’s ground track may be 335°M, and to the left of the desired course of MC 340.

Whereas inaccurate tracking to an NDB is indicated by the RMI needle falling, incorrect tracking away from an NDB can occur with the RMI needle indicating a steady reading — but the airplane may be on the wrong MC. Having passed over the NDB, an airplane can track away from it in any of 360 directions.

You must always ensure that you are flying away from the NDB along the correct course, and the easiest way to do this is to observe the MB from the ground station on the tail of the RMI needle. A magnetic course outbound from a VOR is known as a radial so, if you have the RMI selected to a VOR or VORTAC, the tail of the RMI needle will tell you what radial you are on.

Regaining Course Away from an NDB in a Crosswind

If an incorrect wind correction angle is flown, then the airplane will be blown off course. The vigilant pilot will observe the incorrect ground track, probably by noting that the tail of the RMI needle is indicating something other than the desired outbound course.

Example 11-19

The pilot is flying course as heading, initially making no allowance for drift. If the tail of the RMI needle moves right, then the airplane must be turned left to regain course.

In figure 11-87, the airplane has been blown to the right of the desired MC 035, to MB 043 from the NDB, as indicated by the tail of the RMI needle.

To regain course, the pilot has turned left by 16° (double the error) from MH 035 to MH 019. As the airplane is flown on its steady intercept heading of MH 019, the tail of the needle will be gradually pulled around.

The pilot will continue with the intercept heading until the airplane approaches the desired course, MC 035. This is indicated to the pilot by the tail of the RMI needle moving up the dial toward 035.

As the desired course outbound is approached, the pilot turns right to maintain the RMI tail on 035, which is MB 035 from the NDB. Estimating a WCA of 8° into the wind to be sufficient, the pilot steers MH 027 and checks regularly that the tail of the RMI needle stays on 035.

Similarly, if the tail of the RMI needle is left of where it should be, then the desired course is out to the right, and a right turn should be made to trail the tail.

Note. If using the head of the needle is the preferred technique, then the need for a left turn is indicated by the head of the needle moving to the left of the datum MB 215 to the NDB.

It is generally easier to work off the top of the dial, since that is where the airplane is going, rather than off the bottom of the dial. The left turn necessary to regain course (turning left toward the head of the needle and therefore away from the tail of the needle) can be thought of as pulling the tail of the RMI needle around or “trailing the tail.” Some instructors, however, prefer to use the head of the needle — in this case, the head of the needle (now at the bottom of the dial) moves left, indicating that a correcting turn to the left is required. Your instructor will recommend a method.

The NDB Approach

The NDB approach discussed at the end of the previous chapter is made simpler for pilots with an RMI to use for NDB tracking, rather than an RBI.

The actual NDB approach procedure for the pilot to follow in terms of flight path is identical for all ADF presentations — the fixed-card ADF (or RBI), the manually rotatable-card ADF, and the RMI.

Using the RMI to Fly a DME Arc

The RMI simplifies flying a DME arc, and this is considered in Chapter 28.

Review 11

The NDB and the ADF

1. What is NDB the abbreviation for?

2. What is an NDB?

3. Which frequency bands do NDBs transmit on?

4. What is ADF an abbreviation for?

5. What is an ADF?

6. How can a particular NDB be identified?

7. What are the three basic steps you should follow before using a particular NDB?

8. What is RBI an abbreviation for?

9. If an airplane steering MH 250 has a reading of 030 on its relative bearing indicator (RB030), what is:

a. the magnetic bearing to the NDB from the airplane?

b. the magnetic bearing of the airplane from the NDB?

10. If an airplane steering MH 250 has a reading of RB 350 on its RBI, calculate:

a. the magnetic bearing to the NDB from the airplane.

b. the magnetic bearing of the airplane from the NDB.

11. What is an NDB used to locate the airplane on an instrument approach called?

12. What letters on the instrument approach chart designate an NDB positioned so that it provides a fix for an airplane during an instrument approach, and is co-located with the outer marker?

13. What would you expect the ident of the LOM to be if a localizer has the following coded idents:

a. I-UKI?

b. I-RDD?

14. If a localizer has the coded ident I-DAN, what would you expect the ident of the LMM to be?

15. What effect can atmospheric conditions, such as electrical storms or the periods of sunrise and sunset, have on NDB signals?

16. What effect can mountains have on NDB signals?

The Relative Bearing Indicator

17. What is the MB to the NDB in the following situations:

a. on MH 020 with RB 010?

b. on MH 020 with RB 005?

c. on MH 020 with RB 000?

d. on MH 020 with RB 355?

18. When steering MH 180, MB 240 to the NDB is indicated by what RB?

19. When steering MH 315, MB 060 to the NDB is indicated by what RB?

20. What is the MB to and from the NDB in the following situations:

a. when steering MH 340 with RB 010?

b. on MH 150 with RB 350?

c. on MH 340 with RB 180?

d. on MH 340 with RB 190?

21. What RB indicates MB 120 from the NDB when steering MH 270?

22. What RB indicates MB 255 from the NDB when steering MH 225?

23. What RB indicates MB 090 from the NDB when steering MH 315?

24. An airplane is steering MH 035. Its RBI indicates 040. Magnetic variation in the area is 4°W. Calculate:

a. MB to the NDB.

b. MB from the NDB.

c. True bearing from the NDB.

25. An airplane is steering MH 335. Its RBI indicates 355. Magnetic variation in the area is 4°W. Calculate:

a. MB to the NDB.

b. MB from the NDB.

c. True bearing from the NDB.

26. With MH 080 and RBI 000, what heading would you steer to make a 90° intercept of a course of MC 040 to the NDB? What would the RBI indicate at the point of intercept?

27. With MH 080 and RBI 000, what heading would you steer to make a 60° intercept of a course of MC 040 to the NDB? What would the RBI indicate at the point of intercept?

28. With MH 070 and RBI 010, which way would you turn to intercept MC 075 to the NDB?

29. With MH 155 and RBI 180, which way would you turn to intercept a course of MC 140 away from the NDB?

30. With MH 155 and RBI 180, which way would you turn to intercept a course of MC 180 away from the NDB?

31. When tracking toward an NDB, the ADF readings are:

• time 1 — MH 055, RBI 005; and
• time 2 — MH 055, RBI 005.

What course is the airplane maintaining to the NDB?

32. When tracking toward an NDB, the ADF readings are:

• time 1 — MH 055, RBI 005 and on course; and
• time 2 — MH 055, RBI 002.

What is the desired MC? Is the airplane left or right of this?

33. To track toward an NDB on MC 340, with an expected crosswind from the right causing 5° of drift, what magnetic heading would you steer? What would you expect the RBI to indicate?

34. To track away from an NDB on MC 120, with an expected crosswind from the right causing 8° of drift, what magnetic heading would you steer? What would you expect the RBI to indicate?

35. You wish to track MC 360 in no-wind conditions. What magnetic heading would you steer? What would the RBI indicate as you pass abeam an NDB 10 NM to the right of course (when the NDB is on magnetic bearing MB 090 to the course)?

36. You wish to track MC 360 and expect 10° of drift caused by a wind from the east. What magnetic heading would you steer? What would the RBI indicate as you pass abeam an NDB 10 NM to the right of course?

37. You wish to track MC 030 in no-wind conditions. What magnetic heading would you steer? What would the RBI indicate as you pass abeam an NDB 10 NM to the right of course?

38. You wish to track MC 030 in no-wind conditions. What magnetic heading would you steer? What would the RBI indicate as you pass abeam an NDB 10 NM to the left of course?

39. You wish to track MC 030 and expect 7° left drift. What magnetic heading would you steer? What would the RBI indicate as you pass abeam an NDB 10 NM to the left of course?

40. You are tracking MC 278 with 6° of left drift. You can determine your position abeam an NDB to the right of track by waiting for what RBI indication?

41. You are tracking MC 278 with 6° of left drift. You can determine your position abeam an NDB to the left of course by waiting for what RBI indication?

42. You are tracking MC 278 with 5° of right drift. You can determine your position abeam an NDB to the left of course by waiting for what RBI indication?

The RMI and Rotatable-Card ADF

43. If the head of the RMI needle reads RMI 070, what is the magnetic bearing to the ground station from the airplane?

44. If the head of the RMI needle reads RMI 070, what is the magnetic bearing from the ground station to the airplane?

45. An airplane is flying MH 035. Its RMI indicates RMI 075. Calculate:

• MB to the NDB.
• MB from the NDB.

46. An airplane steers MH 335. Its RMI indicates RMI 330. Calculate:

• MB to the NDB.
• MB from the NDB.

47. The desired course is MC 040 inbound to an NDB or VOR. The WCA is 10° left. What will the head of the RMI needle indicate when the airplane is achieving this course?

48. The desired course is MC 230 inbound to an NDB or VOR. The WCA is 7° right. What will the head of the RMI needle indicate when the airplane is achieving this course?

49. The desired course is MC 120 outbound from an NDB or VOR. The WCA is 3° left. What will the head of the RMI needle indicate when the airplane is achieving this course?

50. With MH 080 and RMI 080, what heading would you steer to make a 90° intercept of a course of MC 040 to the NDB? What would the RMI indicate at the point of intercept?

51. With MH 080 and RMI 080, what heading would you steer to make a 60° intercept of MC 040 to the NDB? What would the RMI indicate at the point of intercept?

52. With MH 070 and RMI 080, which way would you turn to intercept MC 075 to the NDB? What would the RMI indicate at the point of intercept?

53. With MH 155 and RMI 330, which way would you turn to intercept MC 140 away from the NDB? What would the RMI indicate at the point of intercept? What would the tail of the RMI pointer indicate?

54. With MH 155 and RMI 130, which way would you turn to intercept MC 090 away from the NDB? What would the RMI indicate at the point of intercept? What would the RMI tail indicate?

55. When tracking toward an NDB, the RMI readings are:

• time 1 — MH 055, RMI 060; and
• time 2 — MH 055, RMI 060.

What course is the airplane maintaining to the NDB?

56. When flying toward an NDB, the RMI readings are:

• time 1 — MH 055, RMI 060 and on course; and
• time 2 — MH 055, RMI 057.

Is the airplane off course to the left or right?

57. To track toward an NDB on MC 340, with an expected crosswind from the right causing 5° of drift, what magnetic heading would you steer? What would you expect the RMI to indicate?

58. To fly away from an NDB on MC 120, with an expected crosswind from the right causing 8° of drift, what magnetic heading would you steer? What would you expect the RMI to indicate?

59. You fly MC 360 in no-wind conditions. What magnetic heading do you steer? What does the RMI indicate as you pass abeam an NDB 10 NM to the right of course?

60. You fly MC 360 and expect 10° of drift caused by a wind from the east. What magnetic heading do you steer? What does the RMI indicate as you pass abeam an NDB 10 NM to the right of course?

61. You track MC 030 in no-wind conditions. What magnetic heading do you steer? What does the RMI indicate as you pass abeam an NDB 10 NM to the right of course?

62. You fly MC 030 in no-wind conditions. What magnetic heading do you steer? What does the RMI indicate as you pass abeam an NDB 10 NM to the left of course?

63. You fly MC 030 and expect 7° left drift. What magnetic heading do you steer? What does the RMI indicate as you pass abeam an NDB 10 NM to the left of course?

64. You are flying on MC 239 with 7° of left drift. At a position directly abeam an NDB to the left of course, what will the RMI read?

65. Which part of an RMI selected to a VOR tells you what radial you are on?

66. If the head of the RMI needle selected to a VOR reads RMI 010, what radial are you on?

67. If the tail of the RMI needle selected to a VOR reads 089, what radial are on?