Learn how to use an ADF (Automatic Direction Finder) to navigate using signals from an NDB (Nondirectional Radio Beacon).
You will learn how to read ADF indicators, calculate relative and magnetic bearings, and navigate to or from a station using homing, tracking, and interception techniques.
Learn how ADF and NDB stations work, and understand the differences between fixed-card, rotating-card, and RMI navigation indicators.
Learn to interpret relative bearing and magnetic bearing, and calculate the direction of an NDB station from your airplane.
Navigate toward or away from an NDB, correct for wind drift, and learn how to intercept a selected bearing.
GPS is widely used in modern aviation, and many traditional NDB stations have been decommissioned. However, ADF navigation remains useful for understanding fundamental radio navigation concepts.
ADF indications can be confusing at first. Unlike a VOR course deviation indicator, an ADF needle points toward the transmitting station. Understanding the difference between relative bearing and magnetic bearing is essential for interpreting the instrument correctly.
This lesson uses clear diagrams, practical examples, and interactive exercises to help you understand ADF navigation, calculate bearings, and recognize the effects of wind before practicing these techniques in the airplane.
What Is ADF in Aviation?
ADF stands for Automatic Direction Finder. It is a radio navigation system installed in an aircraft. It allows pilots to determine the direction of a transmitting radio station.
An ADF is commonly used with an NDB (Nondirectional Radio Beacon). The NDB transmits a radio signal from the ground, and the aircraft's ADF receiver receives that signal.
The ADF indicator displays the direction of the NDB. Its ADF needle points toward the transmitting station. The pilot uses this indication together with the aircraft's heading to navigate.
NDB Station and ADF Equipment
NDB Station
The NDB is a ground-based radio transmitter. It sends radio signals in all directions.
Each NDB has an assigned frequency and an identification signal that allows pilots to recognize the station.
ADF Receiver and Indicator
The ADF receiver receives the radio signal transmitted by the NDB and determines its direction.
The ADF indicator, shown above, displays that direction using the ADF needle.
How Does the ADF System Work?
ADF navigation is based on determining the direction of a radio signal. The pilot uses the ADF equipment to select a known radio station and obtain bearing information.
The NDB station transmits a radio signal on its assigned frequency.
The aircraft's ADF receiver receives the signal and determines the direction of the transmitting station.
The ADF indicator shows the direction toward the NDB using the ADF needle.
ADF and NDB Frequencies
NDB stations use low-frequency (LF) and medium-frequency (MF) radio signals. Unlike VHF signals, these signals can travel beyond the visual horizon.
NDB stations in the United States normally operate within this frequency range. Not every frequency in the range is assigned to NDB navigation.
NDB signals may be affected by thunderstorms, atmospheric interference, terrain, and nighttime interference from distant stations.
These disturbances can produce inaccurate ADF indications.
The ADF indicator shows the direction toward a transmitting station, not the distance to it.
The ADF needle alone does not provide a complete aircraft position. Additional navigation information is needed to determine the aircraft's location.
The NDB transmits the radio signal. The ADF receiver receives that signal. The ADF indicator displays the direction to the station through its ADF needle.
Types of ADF Indicators
ADF navigation information can be displayed on three common types of instruments: the fixed-card ADF indicator, the rotatable-card ADF indicator, and the Radio Magnetic Indicator (RMI).
All three can show the direction of an NDB, but their compass cards work differently. Understanding those differences is essential for reading the ADF needle correctly.
Three Common ADF Indicator Types
Fixed-Card ADF Indicator
The fixed-card ADF indicator, also called a Relative Bearing Indicator (RBI), has a compass card that does not rotate.
000° is always at the top and represents the nose of the airplane. The ADF needle shows the relative bearing to the NDB, measured clockwise from the aircraft's nose.
The pilot must also know the aircraft's magnetic heading to determine the magnetic bearing to the station.
Rotatable-Card ADF Indicator
The rotatable-card ADF indicator, also called a movable-card ADF indicator, allows the pilot to turn the compass card manually.
The pilot sets the aircraft's magnetic heading at the top of the card. The head of the ADF needle can then be read directly as the magnetic bearing TO the NDB.
Because the compass card is adjusted manually, the pilot must update it whenever the aircraft's magnetic heading changes.
Radio Magnetic Indicator (RMI)
The RMI has a compass card that automatically rotates to display the aircraft's magnetic heading at the top of the instrument.
When an RMI pointer is connected to the ADF receiver, the head of that pointer shows the magnetic bearing TO the NDB. Its tail shows the reciprocal bearing FROM the station.
Many RMIs have two pointers. Depending on the installation, they may display ADF or VOR navigation information. Always check which source is selected for each pointer.
What Does the ADF Needle Reading Mean?
The meaning of the number under the ADF needle depends on the type of indicator and how its compass card is positioned.
The ADF needle indicates relative bearing measured clockwise from the aircraft's nose.
When the magnetic heading is set correctly at the top, the ADF needle indicates the magnetic bearing to the NDB.
When the compass card is correctly aligned and the pointer is using ADF, its head directly indicates the magnetic bearing to the NDB.
On a fixed-card ADF indicator, the 000° mark represents the nose of the airplane. It does not mean the airplane is heading north.
On a rotatable-card ADF indicator, the pilot places the current magnetic heading at the top. On a RMI, this alignment normally happens automatically.
If the aircraft turns, a manually adjusted rotatable card must be reset to the new heading. The RMI compass card follows heading changes automatically when operating normally.
A fixed-card ADF shows relative bearing. A rotatable-card ADF can show magnetic bearing when manually aligned with the aircraft's heading. An RMI normally keeps its compass card aligned automatically and shows magnetic bearing through the selected ADF pointer.
Relative Bearing and Magnetic Bearing
To use an ADF for navigation, you must understand the difference between relative bearing (RB), magnetic heading (MH), and magnetic bearing (MB).
These angles describe different things. Relative bearing uses the airplane's nose as its reference, while magnetic bearing uses magnetic north.
What Is Relative Bearing (RB)?
Relative bearing (RB) is the angle measured clockwise from the airplane's nose to the direction of the NDB.
On a fixed-card ADF indicator, the ADF needle directly shows this angle.
For example, a relative bearing of 045° means the NDB is 45° to the right of the airplane's nose.
A relative bearing of 330° means the station is 30° to the left of the nose.
The relative bearing changes when the airplane turns, even if the position of the NDB stays the same.
This is because relative bearing is always measured from the airplane's current heading, not from magnetic north.
How to Calculate Magnetic Bearing (MB)
Magnetic heading (MH) is the direction the airplane's nose points, measured clockwise from magnetic north.
Magnetic bearing TO the station is the direction from the airplane toward the NDB, also measured clockwise from magnetic north.
When using a fixed-card ADF, you calculate the magnetic bearing by adding the airplane's magnetic heading to the relative bearing.
If the result is 360° or greater, subtract 360° to express the answer as a bearing from 000° to 359°.
Your airplane has a magnetic heading of 040°. The fixed-card ADF needle indicates a relative bearing of 050°.
What is the magnetic bearing TO the NDB?
040° + 050° = 090°The NDB is on a magnetic bearing of 090° from your present position.
With no wind, heading 090° would point the airplane directly toward the NDB.
Your airplane has a magnetic heading of 300°. The fixed-card ADF needle indicates a relative bearing of 110°.
What is the magnetic bearing TO the NDB?
300° + 110° = 410°Because 410° is greater than 360°, subtract 360°:
410° − 360° = 050°The NDB is on a magnetic bearing of 050°.
Magnetic Bearing TO and FROM the NDB
So far, we have calculated the magnetic bearing TO the NDB. This tells us the direction from the airplane toward the station.
The magnetic bearing FROM the NDB is the opposite direction: from the station toward the airplane. It is also called the reciprocal bearing.
To find the reciprocal bearing, add or subtract 180° from the magnetic bearing TO.
Choose the result that falls between 000° and 359°.
In our first example, the magnetic bearing TO the NDB was 090°.
The reciprocal bearing is:
Magnetic bearing FROM the NDB: 270°.
This means the airplane is located on the 270° magnetic bearing FROM the NDB.
If you already know the magnetic bearing TO the NDB and the airplane's magnetic heading, you can calculate the relative bearing by subtraction.
RB = MB TO − MH
For example, the airplane's magnetic heading is 090° and the magnetic bearing TO the NDB is 060°.
060° − 090° = −030°. Add 360° to obtain 330° relative bearing.
The fixed-card ADF needle would therefore point 30° to the left of the airplane's nose.
The magnetic bearing TO an NDB identifies the direction of the station from the airplane. It does not automatically tell you which heading will maintain a straight ground track in a crosswind.
Wind correction is necessary when tracking a selected bearing. We will practice that technique later in the lesson.
Relative bearing (RB) is measured clockwise from the airplane's nose. Magnetic bearing (MB) is measured clockwise from magnetic north.
To find the magnetic bearing TO an NDB, use MH + RB = MB. To find the magnetic bearing FROM the station, calculate the reciprocal by adding or subtracting 180°.
Tuning, Identifying and Checking an NDB
Before using an NDB for navigation, the pilot must tune the correct frequency, positively identify the station, and check the ADF equipment.
Proper station identification is especially important because an ADF receiver may display an incorrect bearing without providing a warning.
Tune and Identify the NDB
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1
Find the NDB Frequency
Locate the NDB on a current FAA aeronautical chart or in an appropriate FAA navigation publication. Confirm its frequency and published identifier.
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2
Tune the ADF Receiver
Select the published NDB frequency on the ADF receiver. Make sure the correct frequency is active.
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3
Identify the Station
Select the appropriate audio mode, adjust the volume, and listen to the NDB's Morse code identifier.
Compare the received code with the identifier published for that NDB. Do not use the station until the identification has been confirmed.
Most U.S. NDBs transmit a continuous three-letter Morse code identifier. Compass locators are an exception.
Check the ADF Equipment
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1
Select ADF Mode
Set the function switch to ADF. Allow the ADF needle to stabilize.
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2
Perform the Applicable Test
Follow the operating instructions for the installed ADF equipment.
Some receivers have a TEST control or use ANT mode as part of a needle response check. The expected indication depends on the model.
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3
Verify the ADF Indication
Return to ADF mode after the applicable test. Confirm that the ADF needle responds normally and displays a stable bearing when reception conditions are satisfactory.
ADF receivers do not all use the same controls or test procedures. Follow the instructions for the specific avionics installation.
Where to Find NDB Information in the USA
For flights in the United States, use current FAA aeronautical publications to locate available NDB stations, obtain their frequencies and identifiers, and verify their operational status.
The FAA Aeronautical Chart Users' Guide explains the symbols used for navigation facilities, including NDBs.
The Aeronautical Information Manual (AIM), Section 1-1-2, explains NDB operation and identification.
Always consult current aeronautical information and applicable NOTAMs before relying on a station.
Understanding ADF Receiver Controls
The operating controls depend on the model of ADF receiver installed in the aircraft. The following functions are common on traditional ADF equipment.
Selects normal direction-finding operation. The ADF needle indicates the direction toward the received station.
On receivers equipped with these controls, an antenna or reception mode may be used to listen to the station identifier. Normal bearing indications may not be available in this mode.
Where installed, a test function helps check the equipment's operation. The correct response must be determined from the applicable avionics instructions.
Unlike some other navigation instruments, an ADF receiver normally has no warning flag to indicate an erroneous bearing.
A steady ADF needle does not guarantee that the signal is reliable. Interference can produce a false bearing, sometimes without obvious needle movement.
The FAA advises pilots to continuously monitor the NDB identification. Noise, incorrect identification, or loss of identification may indicate that the bearing information cannot be trusted.
Before navigating with an NDB, tune the published frequency, identify the station by its Morse code, and check the ADF equipment.
During flight, continue monitoring the station identification. Never assume that a steady ADF needle guarantees an accurate bearing.
Homing and Tracking TO an NDB
Both homing and tracking allow a pilot to navigate toward an NDB.
The difference is the flight path over the ground. Homing keeps the airplane pointed toward the station. Tracking keeps the airplane on a selected route toward the station.
You can perform these maneuvers using a fixed-card ADF indicator or a Radio Magnetic Indicator (RMI). The maneuver is the same, but the needle readings are different.
Homing: You simply want to fly toward the NDB, without maintaining a specific published or assigned track.
Tracking: You need to follow a particular magnetic track toward the NDB, even when wind pushes the airplane sideways.
Why Does Wind Make a Difference?
Imagine that your airplane is south of an NDB. The desired inbound route runs straight north on a 360° magnetic track.
A crosswind from the west pushes the airplane toward the east.
If you continually point the nose at the NDB, you may follow a curved path. If you correct for wind, you can maintain the straight route.
Keep Pointing at the NDB
With homing, you keep the airplane's nose pointed toward the station.
When the wind pushes the airplane sideways, the direction of the NDB changes relative to the nose.
You turn toward the station again and continue making corrections to keep it ahead.
You can reach the NDB, but you may drift away from the original straight route.
Stay on the Desired Track
With tracking, you maintain a selected magnetic track toward the NDB.
If a crosswind pushes the airplane sideways, you first correct to regain the track.
You then maintain a heading slightly into the wind so the airplane remains on the route.
With the correct wind correction, the airplane remains on the intended route toward the NDB.
Homing with a Fixed-Card ADF or RMI
During homing, both instruments help you point the nose toward the NDB. However, they show this direction differently.
Keep the Needle at 000°
The fixed-card ADF shows relative bearing.
If the needle points 20° to the right, turn right to point toward the station.
Once the station is straight ahead, the needle indicates 000°.
Continue adjusting the aircraft heading as necessary to keep it there.
Align the Pointer with the Nose
On a correctly aligned RMI, the head of the ADF pointer shows the magnetic bearing TO the NDB.
To home toward the station, turn until the pointer head aligns with the top index of the RMI.
The magnetic bearing displayed at the top will then be the same as the bearing indicated by the pointer head.
Important: homing with an RMI does not mean keeping the pointer on 000°. You keep it aligned with the airplane's nose.
Suppose your magnetic heading is 360°, but after drifting east, the NDB is now on a magnetic bearing of 350°.
The fixed-card ADF shows a relative bearing of 350°: the station is 10° left of the nose.
The RMI pointer head indicates 350° on its compass card.
For homing, turn left to heading 350°. The fixed-card needle will point at 000°, while the RMI pointer aligns with the top index over the 350° heading.
You are again pointing directly toward the NDB, but you have not necessarily regained the original 360° ground track.
How to Track TO an NDB
Tracking requires more than pointing toward the station. You must monitor the selected magnetic bearing and correct for wind drift.
Determine the magnetic bearing TO the NDB you need to follow.
In this example, the selected inbound track is 360°.
Make sure the airplane is on the intended inbound line.
If you are already off the selected track, first intercept it. Interception is covered later in the lesson.
Hold a heading and observe whether the magnetic bearing TO the NDB changes.
If the airplane drifts east of the track, the NDB begins appearing farther to the left.
Regain the selected inbound bearing, then establish a heading into the wind.
Maintain the heading that keeps the magnetic bearing TO the NDB equal to the desired inbound bearing.
Worked Example: Tracking 360° TO an NDB
You are established on a 360° magnetic track toward the NDB.
Wind from the west requires a 10° correction to the left. You therefore maintain a magnetic heading of 350°.
Because the airplane's nose points 10° left of the station, the station appears 10° to the right of the nose.
Both indicators show the same navigation situation, but their readings are different.
The needle indicates a relative bearing of 010°.
The NDB is 10° to the right of the aircraft's nose.
Magnetic bearing TO:
The RMI compass card shows 350° at the top index, because that is the aircraft's magnetic heading.
The selected ADF pointer head indicates 360° magnetic bearing TO the NDB.
The pointer is 10° right of the top index.
The fixed-card ADF displays an angle measured from the airplane's nose. That is why it reads 010°.
The RMI pointer head indicates the magnetic bearing TO the NDB. That is why it reads 360°.
These are not conflicting readings. They describe the same direction using different references.
The RMI must have a correctly aligned heading reference, and the pointer being read must be connected to the ADF receiver.
How to Monitor Tracking with Each Instrument
Once the airplane is established on the 360° inbound track, you must check that it remains there.
| Situation | Fixed-Card ADF | RMI |
|---|---|---|
| No wind and heading 360° | Needle: 000° | Pointer head: 360° |
| Correct tracking with heading 350° | Needle: 010° | Pointer head: 360° |
| Drifted east of the desired line, heading still 350° | Needle moves left of 010° | Pointer head moves below 360°, toward northwest |
With a fixed-card ADF, you combine relative bearing and magnetic heading to check the magnetic bearing TO the NDB.
With an RMI, you read the magnetic bearing directly from the head of the ADF pointer.
In either case, the desired bearing must remain at 360° if you want to stay on the selected inbound line.
Example: Recognizing Wind Drift with an RMI
You are tracking 360° TO the NDB with a magnetic heading of 350°.
The RMI pointer head initially reads 360°. This indicates that you are on the desired inbound bearing.
After some time, the pointer head moves to 355° while your heading remains 350°.
The magnetic bearing TO the NDB has changed. This indicates that the airplane is now east of the intended northbound line.
With the fixed-card ADF, the same situation would give:
In both cases, you must correct back toward the desired 360° line.
Once the airplane has regained the line, establish an appropriate wind-corrected heading to maintain it.
Do not simply turn to point the airplane at the NDB. That would return to homing rather than proper tracking.
Wind Correction in Practice
The diagram below shows how a pilot uses ADF information to detect drift, correct back toward the desired line and maintain a wind-corrected heading.
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A — Start on the Track
The airplane begins on the selected inbound line.
The pilot monitors the ADF bearing while maintaining a heading.
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B — Drift Develops
A crosswind pushes the airplane away from the desired line.
The indicated magnetic bearing TO the NDB begins to change.
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C — Correct Toward the Line
The pilot turns into the wind to regain the selected inbound track.
The correction may be larger than the heading needed later to maintain that track.
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D — Return to the Track
As the airplane regains the desired magnetic bearing, the pilot reduces the correction.
The aim is to regain the route, not merely to point toward the NDB.
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E — Maintain Wind Correction
The pilot now maintains a heading into the wind.
On the fixed-card ADF, the needle may remain away from 000°.
On the RMI, the ADF pointer head remains on the desired magnetic bearing TO.
When Should You Use Homing or Tracking?
Simple Navigation Toward the Beacon
Homing is useful for basic ADF training and for simple steering toward an NDB when no specific inbound track is required.
With a fixed-card ADF, keep the needle at 000°.
With an RMI, keep the ADF pointer head aligned with the top aircraft heading index.
In a crosswind, neither technique maintains a specified ground track by itself.
Maintain a Required Inbound Route
Tracking is used when the airplane must remain on a selected magnetic bearing TO an NDB.
This includes following an applicable published or assigned navigation route with authorized equipment.
With a fixed-card ADF, calculate the magnetic bearing using magnetic heading and relative bearing.
With an RMI, read the selected ADF pointer head directly against the magnetic compass card.
In both cases, use wind correction to maintain the selected line.
An RMI may have more than one pointer. Depending on the equipment, a pointer may receive information from an ADF receiver or from a VOR receiver.
Before using an RMI for NDB navigation, confirm that the correct pointer is selected to ADF and that the station is identified.
Also verify that the RMI compass card is correctly aligned with the aircraft's magnetic heading.
Always follow the operating procedures for the installed avionics and the applicable flight procedure.
Homing keeps the airplane pointed toward the NDB. A crosswind may produce a curved ground track.
Tracking maintains a selected magnetic bearing TO the NDB, using a wind-corrected heading.
On a fixed-card ADF, calculate the magnetic bearing using MH + RB.
On an RMI, read the magnetic bearing TO the NDB directly from the head of the selected ADF pointer.
The navigation technique does not change. Only the way you read the instrument is different.
Tracking Away FROM an NDB
Tracking outbound means following a selected magnetic bearing away from an NDB.
As with inbound tracking, the pilot must compensate for wind to maintain the desired ground track.
The important difference is the location of the station. During outbound flight, the NDB is behind the airplane.
Fixed-card ADF: the needle points toward the NDB, usually near 180° relative bearing. You calculate your magnetic bearing FROM the station.
RMI: the head of the selected ADF pointer still points toward the NDB. Its tail indicates the magnetic bearing FROM the station, which you can read directly on the compass card.
The goal is to maintain the selected magnetic bearing FROM, not to hold a particular needle position relative to the airplane.
Example: Following 090° FROM an NDB
Imagine that your airplane is east of an NDB. You want to continue flying east along the 090° magnetic bearing FROM the station.
A wind from the north pushes the airplane toward the south.
If you maintained a heading of 090° without correcting for this crosswind, the airplane would drift south of the desired route.
Suppose that a 10° wind correction toward the north is required. You must maintain a magnetic heading of 080° to follow the 090° ground track.
Reading the Fixed-Card ADF and RMI
Both instruments below represent the same airplane in the same position.
The aircraft is tracking 090° FROM the NDB with a magnetic heading of 080°.
Read Relative Bearing
The NDB is 10° left of the airplane's tail.
The fixed-card ADF does not directly show the magnetic bearing FROM. You must calculate it.
First, find the magnetic bearing TO:
Then take the reciprocal:
The airplane is on the correct 090° magnetic bearing FROM the NDB.
Read the Pointer Tail
The tail of the selected ADF pointer shows the magnetic bearing FROM the NDB.
The RMI compass card is aligned so that the airplane's magnetic heading, 080°, is at the top index.
The head of the ADF pointer indicates 270° TO the station.
The tail of the ADF pointer indicates 090° FROM the station.
Unlike the fixed-card ADF, you do not need to calculate a reciprocal bearing to identify the 090° outbound line.
You read it directly from the RMI's magnetic compass card.
The RMI pointer head points toward the NDB, which is behind the airplane.
The opposite end, the pointer tail, points in the magnetic direction FROM the NDB. During outbound flight, this is approximately the airplane's direction of travel.
In still air, with the airplane heading 090°, the pointer tail would align with the top index and indicate 090°.
With a 10° left wind correction, the airplane heads 080° while the pointer tail continues to indicate 090° FROM.
The tail is therefore 10° to the right of the aircraft's heading index. This is normal during correct outbound tracking.
How to Track Outbound Step by Step
The navigation procedure is the same with either instrument. What changes is how the pilot reads the magnetic bearing FROM the NDB.
Determine which magnetic bearing FROM the NDB you need to follow.
In our example, it is 090° FROM.
Confirm that you are on the selected magnetic bearing FROM.
If you are not, intercept the bearing before attempting to maintain it.
The next chapter explains interception.
Select a heading that compensates for crosswind.
For a wind from the north, turn the airplane slightly north of the desired eastbound track.
In our example, the wind-corrected heading is 080°.
With a fixed-card ADF, calculate the magnetic bearing FROM using MH + RB and the reciprocal.
With an RMI, read the bearing FROM directly at the tail of the selected ADF pointer.
Adjust the heading if the airplane begins drifting off the selected line.
What Happens If the Wind Pushes You Off the Outbound Track?
Suppose you should be tracking 090° FROM, but you fly a magnetic heading of 090° without correcting for the wind.
The wind from the north pushes your airplane south of the intended eastbound route.
You are no longer on the 090° bearing FROM the station.
Your magnetic heading remains 090°.
The fixed-card ADF needle now indicates 186° relative bearing.
Calculate the magnetic bearing TO:
Take the reciprocal:
You are now on the 096° bearing FROM, which places you south of the desired 090° outbound line.
With an RMI, the same situation would show 096° at the tail of the ADF pointer.
Both instruments indicate the same navigation error. The RMI simply allows the bearing FROM to be read directly.
| Situation | Fixed-Card ADF | RMI |
|---|---|---|
| Still air on 090° FROM, heading 090° | RB 180° | Pointer tail 090° FROM |
| Correct tracking on 090° FROM, heading 080° | RB 190° | Pointer tail 090° FROM |
| Drifted south, now on 096° FROM, heading 090° | RB 186° | Pointer tail 096° FROM |
How Do You Regain the Correct Track?
If the airplane has drifted south of the 090° outbound line, you must first move it back north.
Do not simply change heading until the fixed-card needle returns to 180°.
That would not necessarily put the airplane back onto the desired magnetic bearing FROM.
Select a suitable heading north of east to regain the 090° outbound line.
For illustration, a heading of 060° could provide a 30° interception angle in still-air geometry.
As the airplane moves toward the line, the magnetic bearing FROM the NDB approaches 090°.
Use MH + RB with the fixed-card ADF, or watch the RMI pointer tail approach 090°.
Lead the turn as necessary to avoid crossing beyond the desired outbound line.
Establish the heading required to maintain the 090° track after interception.
In this example, maintain heading 080° while monitoring the selected 090° bearing FROM.
The fixed-card ADF should again indicate approximately 190°, while the RMI pointer tail indicates 090°.
During outbound flight, the head of the RMI's ADF pointer points backward toward the station.
The tail points in the reciprocal magnetic direction, which is the bearing FROM the NDB.
To track outbound, watch the tail reading. The heading required to maintain that bearing may be different because of wind correction.
Confirm that the RMI pointer is connected to the ADF receiver, the NDB is correctly identified, and the compass card is properly aligned with magnetic heading.
Tracking FROM an NDB means maintaining a selected magnetic bearing away from the station.
With a fixed-card ADF, the needle is normally near 180°. Calculate MH + RB and take the reciprocal to determine the bearing FROM.
With a RMI, the selected ADF pointer head shows the magnetic bearing TO the NDB, while its tail shows the bearing FROM.
The objective is to maintain the selected magnetic bearing FROM with an appropriate wind-corrected heading.
Intercepting an NDB Bearing
Intercepting an NDB bearing means flying toward a selected magnetic bearing until the airplane joins it.
You use an interception when you are not already on the magnetic track you need to follow TO or FROM the NDB.
The objective is to reach the selected line, turn onto it, and then maintain the required track.
With a fixed-card ADF, combine magnetic heading and relative bearing to determine the magnetic bearing TO the NDB. Take the reciprocal when you need the bearing FROM.
With an RMI, read the magnetic bearing TO directly at the head of the selected ADF pointer. Read the bearing FROM at its tail.
When the indicated magnetic bearing reaches the selected bearing, the airplane is on that line. Begin the rollout early enough to avoid overshooting it.
Intercepting TO and FROM: Which RMI Pointer End Matters?
Intercept a Bearing TO the NDB
The selected bearing leads toward the station.
Fixed-card ADF: calculate the magnetic bearing TO using MH + RB.
RMI: watch the pointer head approach the selected magnetic bearing TO.
After interception, turn onto the required inbound track and correct for wind.
Intercept a Bearing FROM the NDB
The selected bearing extends away from the station.
Fixed-card ADF: calculate the magnetic bearing TO, then find the reciprocal bearing FROM.
RMI: watch the pointer tail approach the selected magnetic bearing FROM.
After interception, establish the appropriate outbound tracking heading.
Example 1: Intercepting 355° TO an NDB
Imagine that you need to follow the 355° magnetic bearing TO an NDB.
Your airplane is west of that inbound line. Before intercepting, you fly parallel to the desired bearing on a magnetic heading of 355°.
The fixed-card ADF needle indicates 040°. The NDB is 40° to the right of the airplane's nose.
The inbound line is the route the airplane must eventually follow.
The interception path crosses that line at an angle.
When the airplane approaches the selected bearing, the pilot prepares to turn onto the inbound route.
The ADF indication changes continuously as the airplane moves toward the line.
The illustration shows the interception principle. The numerical example below uses the FAA's 355° inbound bearing.
The station is 40° right of the nose.
In this FAA training example, double the initial relative bearing to choose the interception angle.
Since the station is to the right, turn 80° right from the initial 355° heading.
Fly a magnetic heading of 075° toward the selected 355° inbound line.
Inbound Interception: What Do the ADF and RMI Show?
The four stages below describe the same airplane during the interception. The example assumes still air to make the geometry easy to understand.
Fly heading 355°. The NDB is 40° to the right.
On the fixed-card ADF, 355° MH + 040° RB gives 035° TO.
The RMI pointer head shows 035° directly.
Turn right to heading 075°. Immediately after the turn, before significant aircraft movement, the NDB bearing has not changed.
The fixed-card needle now points 40° left of the nose.
The RMI still indicates the station's magnetic bearing of 035°.
Maintain heading 075°. As the airplane moves toward the desired line, the magnetic bearing TO the NDB approaches 355°.
On heading 075°, a 280° relative bearing gives 355° magnetic bearing TO.
Begin the turn with suitable lead to avoid overshooting the line.
Complete the turn onto the desired inbound track.
In still air, the tracking heading is 355°.
With crosswind, use the heading required to maintain the selected 355° inbound track.
The airplane is still flying the interception heading of 075°.
The selected magnetic bearing TO the NDB is 355°.
The NDB is therefore 80° to the left of the airplane's nose.
On the fixed-card ADF, you recognize this by the needle approaching 280° relative bearing.
On the RMI, you recognize it by the pointer head approaching 355° magnetic.
They indicate the same interception point using different references.
Example 2: Intercepting 090° FROM an NDB
Now consider an outbound interception.
You want to follow the 090° magnetic bearing FROM an NDB, but your airplane is south of that line.
You are currently on the 110° bearing FROM the station.
To reach the desired eastbound line, you must fly toward the northeast.
You are south of the desired 090° outbound line.
Select an illustrative 30° interception angle north of the desired eastbound direction.
Fly heading 060° and monitor the magnetic bearing FROM the station as it approaches 090°.
This example assumes still air. An actual intercept heading may need adjustment for wind.
Outbound Interception: Fixed-Card ADF vs. RMI
The table follows the same airplane from its initial position until it is established on the 090° outbound line.
| Stage | Magnetic Heading | Fixed-Card ADF | RMI Pointer Tail |
|---|---|---|---|
| Initial position on 110° FROM | 090° | RB 200° | 110° FROM |
| After turning toward the selected line | 060° | RB 230° | 110° FROM |
| At the selected line, before rollout | 060° | RB 210° | 090° FROM |
| Tracking outbound in still air | 090° | RB 180° | 090° FROM |
Recognize the Outbound Line
While flying heading 060°, watch the ADF relative bearing.
At the selected 090° bearing FROM, the magnetic bearing TO the station must be 270°.
The fixed-card ADF needle therefore indicates 210° when the airplane is on the desired line and still heading 060°.
Begin turning toward the outbound tracking heading with suitable lead.
Watch the Pointer Tail
On a correctly aligned RMI, the tail of the selected ADF pointer initially reads 110° FROM.
As the airplane approaches the desired line, the tail reading decreases toward 090°.
When the tail reaches 090°, the airplane is on the selected outbound bearing.
The pointer head simultaneously indicates 270° TO.
Lead the turn as appropriate, then maintain the 090° track.
The needle readings at the exact interception point are useful reference values.
They are not instructions to wait until the airplane has completely crossed the selected line before beginning the turn.
Anticipate the rate of bearing change, aircraft turn rate, distance from the NDB, and wind conditions.
An ADF bearing changes more rapidly when the airplane is close to the station. Poor turn timing can lead to overshooting the line.
Common ADF and RMI Interception Errors
The RMI pointer head indicates the magnetic bearing TO.
The pointer tail indicates the magnetic bearing FROM.
Choosing the wrong end can make the interception calculation incorrect by 180°.
On a fixed-card ADF, the same magnetic bearing to a station can produce different relative bearing readings when the airplane turns.
Always combine magnetic heading and relative bearing.
A fast bearing change or a late rollout can carry the airplane beyond the desired line.
Use appropriate turn lead, especially when close to the NDB.
Confirm that the pointer being read is connected to the identified ADF station.
Verify that the RMI compass card has a valid magnetic heading reference.
The FAA's example of doubling the initial ADF needle deflection demonstrates one way to choose an inbound interception angle.
Actual interception angles depend on aircraft performance, wind, station proximity, and applicable navigation instructions.
Do not assume that every ADF interception should use twice the needle deflection.
Always comply with the applicable ATC clearance, published procedure, and approved avionics instructions.
Intercepting means joining a selected magnetic bearing before tracking along it.
With a fixed-card ADF, use magnetic heading and relative bearing to determine when the selected bearing has been reached.
With a RMI, use the pointer head for TO and the pointer tail for FROM.
Choose the intercept heading, monitor the bearing, anticipate the rollout, then maintain the selected track with wind correction.
Station Passage and ADF Limitations
When an airplane flies over or very close to an NDB, the direction of the station changes rapidly. The ADF needle may swing through a large angle or become temporarily unstable.
Recognizing NDB station passage is important because it may mark a navigation fix, a required turn, or another point in a published instrument procedure.
Pilots must also understand the limitations of ADF navigation. Not every movement of the needle is caused by the airplane changing position.
What Happens When You Fly Over an NDB?
As you approach the station, the ADF needle points toward the NDB. Near the station, even a small change in aircraft position can produce a large change in the indicated bearing.
If you fly directly over the NDB and continue along the same track, the station changes from being ahead of the airplane to being behind it.
This produces an approximately 180° change in the direction to the station. The actual transition may not be smooth or instantaneous.
Three Stages of NDB Station Passage
The following example continues the northbound tracking scenario from the earlier chapter. The airplane maintains a 360° ground track with a 350° magnetic heading to compensate for wind.
The airplane is south of the station, flying north.
The NDB lies directly along the 360° inbound line, but the airplane's nose is pointed 10° left into the wind.
As the airplane approaches or passes very near the NDB, the direction to the station changes quickly.
The ADF needle may rotate rapidly or oscillate. The RMI's selected ADF pointer may show similar behavior.
The airplane continues north, beyond the station.
The NDB is now to the south. Once clear of the immediate station area, the bearing indication should stabilize if reception is reliable.
Fixed-Card ADF and RMI: Before and After the Station
The instruments below show the same aircraft with a constant magnetic heading of 350°. They assume the airplane passes directly over the station and continues along the same 360° ground track.
Relative Bearing Reverses
Before passage, the needle points 10° right of the nose.
After passage, it points 10° left of the tail.
The indicated relative bearing changes by approximately 180° because the NDB is now behind the airplane.
Magnetic Bearing Reverses
Before passage, the RMI pointer head indicates 360° TO.
After passage, the pointer head indicates 180° TO.
The opposite end of the pointer now indicates 360° FROM, which is the magnetic bearing extending north from the NDB.
These illustrations show an idealized direct overflight of the NDB.
In real flight, the needle may swing rapidly or oscillate near the station. The behavior depends on the airplane's flight path and distance from the beacon.
If you pass beside the NDB rather than directly over it, the indication may change through a wingtip direction instead of showing a perfectly timed 180° reversal.
Do not use a single needle movement alone to determine an exact crossing time. Interpret it in conjunction with the expected flight path and the applicable procedure.
How to Recognize Station Passage During Flight
Know where the NDB should be along the planned route.
Before reaching it, verify the expected magnetic bearing and any published crossing instructions.
Close to the beacon, small lateral changes in position may cause large ADF pointer movements.
Do not chase every rapid swing with an abrupt heading correction.
The NDB transitions from the forward to the rear part of the airplane's relative direction.
On a fixed-card ADF, the indication moves toward the rear half of the dial.
On an RMI, the selected ADF pointer head indicates a magnetic bearing back toward the station.
Once clear of the immediate station area, the bearing normally becomes more stable if reception is reliable.
Follow the appropriate published or assigned procedure after station passage.
Confirm your position using other suitable navigation information when available.
ADF Errors and Limitations
ADF navigation uses low-frequency and medium-frequency radio signals. Their propagation can be affected by the environment, weather and aircraft equipment.
These effects may produce an incorrect bearing even when the ADF needle appears to move normally.
Electrical activity from thunderstorms can interfere with ADF reception.
Lightning and precipitation static may cause the ADF needle to move erratically or display an inaccurate direction.
The pointer may be influenced by electrical activity rather than the intended NDB signal.
At night, low-frequency and medium-frequency signals may be affected by changes in ionospheric propagation.
Distant stations can produce interference that alters the received bearing.
These effects may be noticeable around sunrise and sunset as propagation conditions change.
NDB radio waves can change direction when crossing a coastline, particularly at an oblique angle.
The received signal may therefore appear to arrive from a slightly incorrect direction.
A bearing measured near a coastline may not represent the true direction of the station.
Mountains and terrain can reflect or alter radio signals.
Reflected signals may combine with the direct signal and create inaccurate ADF bearing indications.
This is particularly important when using NDB navigation in mountainous areas.
During turns, bank angle and aircraft installation characteristics may affect ADF indications.
Airframe-related signal distortion can also create systematic bearing errors, sometimes called quadrantal errors.
Interpret ADF indications carefully while maneuvering.
An ordinary ADF indicates bearing, not the distance to the NDB.
The ADF receiver also normally lacks a warning flag to indicate an erroneous bearing.
A steady needle does not guarantee that the signal is accurate.
What Should You Do When an ADF Bearing Looks Unreliable?
Continue listening to the station's Morse code identifier. Noise, an incorrect identification or unexpected audio may indicate interference.
Compare the ADF indication with your expected route, heading and other suitable navigation sources when available.
A rapidly moving ADF needle near station passage does not automatically mean that the airplane needs a large heading correction.
If the NDB signal cannot be trusted, do not rely on its bearing to continue a navigation procedure. Follow the applicable clearance and procedures, and use an authorized alternative navigation source when available.
The RMI makes magnetic bearing information easier to read when its heading reference is valid.
However, if the selected ADF receiver is affected by radio interference, the RMI pointer can display the same erroneous station direction.
A heading-reference problem can also make the RMI's magnetic bearing readings incorrect.
Always verify the selected navigation source, the heading reference and the NDB identification.
During NDB station passage, the direction to the station changes from ahead to behind the airplane. The ADF needle may swing rapidly or become temporarily unstable.
With a fixed-card ADF, expect a substantial change in relative bearing. With an RMI, the magnetic bearing indicated by the selected ADF pointer changes accordingly.
Lightning, nighttime interference, terrain, coastal refraction and equipment-related errors can affect ADF accuracy.
Never rely on the ADF needle alone when its indication is uncertain. Monitor station identification and cross-check suitable navigation information.
Estimating Time and Distance with ADF
An ADF indicator tells you the direction of an NDB, but it does not directly show how far away the station is.
However, you can estimate your distance from an NDB by measuring how quickly its bearing changes while flying a constant heading.
This technique is called an ADF time and distance check. It can be performed with a fixed-card ADF or an RMI with its pointer selected to ADF.
When you fly across the direction to a nearby station, its bearing changes more quickly than the bearing of a distant station.
By measuring the time required for a 10° bearing change, you can estimate your distance from the NDB.
Important: for the standard method, first place the station approximately 90° to the left or right of your airplane's nose. Then hold a constant heading during the timed measurement.
Why Must the Station Be at Your Wingtip?
To perform this check, the airplane flies across the direction to the NDB rather than directly toward it.
This creates a measurable change in the direction of the station as the airplane moves sideways relative to it.
If you simply fly directly toward the NDB, the bearing may remain almost unchanged until you get very close to the station. That does not provide the same measurement.
Example: Measuring a 10° Bearing Change
Imagine the airplane is north of an NDB.
The NDB is due south, on a magnetic bearing of 180° TO.
You fly heading 090°, toward the east. The station is now directly off your right wingtip.
ADF Time and Distance Check: Step by Step
Turn the airplane until the selected NDB is approximately 90° to the left or right of the nose.
With a fixed-card ADF, the needle should indicate 090° or 270°.
With an RMI, the selected ADF pointer head should be aligned with a wingtip index, 90° from the top.
Once established on a steady heading with the station at the wingtip, note the bearing and start timing.
Maintain a constant heading during the measurement. Avoid turns that would change the relative bearing simply because the airplane rotates.
Keep monitoring the ADF or RMI as the airplane moves across the station's direction.
In our example, the magnetic bearing TO changes from 180° to 190°.
Stop the timer when the selected bearing has changed by 10°.
Divide the elapsed time in seconds by the number of degrees of bearing change.
The result estimates the time equivalent to the distance from the NDB.
Multiply this estimate by speed in nautical miles per minute to estimate distance.
Reading the Bearing Change: Fixed-Card ADF vs. RMI
The procedure works with either instrument. The difference is where you read the bearing.
Watch the Relative Bearing
With a magnetic heading of 090°, the NDB initially lies directly off the right wing.
The needle moves from 090° to 100° while the airplane maintains heading 090°.
Since the heading remains constant, this 10° relative bearing change corresponds to a 10° change in the magnetic bearing TO the NDB.
At the beginning: 090° MH + 090° RB = 180° TO.
At the end: 090° MH + 100° RB = 190° TO.
Watch the Pointer Head
On a correctly aligned RMI, magnetic heading 090° appears at the top of the compass card.
The selected ADF pointer head moves from 180° to 190° on the magnetic compass card.
The RMI pointer head already indicates the magnetic bearing TO the NDB.
Therefore, you can read the 10° bearing change directly without adding heading and relative bearing.
The pointer must be selected to the identified NDB through the ADF receiver.
Calculate the Estimated Time and Distance
Now use the measured bearing change and elapsed time.
The example assumes a steady speed of 120 knots and negligible wind during the measurement.
Divide the elapsed time in seconds by the number of degrees of bearing change.
In our example, the bearing changes 10° in 40 seconds.
This is an approximate direct-flight time corresponding to the estimated distance from the NDB at the assumed speed.
Convert speed from knots to nautical miles per minute.
Multiply the estimated time by the distance covered in one minute.
The airplane is approximately 8 nautical miles from the NDB. This is an estimate, not a precise distance measurement.
If you measure exactly a 10° bearing change, divide the elapsed seconds by 10.
For example, 40 seconds becomes 4.0 minutes.
You can then estimate distance by multiplying the result by ground speed in nautical miles per minute.
Does This Mean You Will Reach the NDB in Four Minutes?
Not necessarily. This is an important distinction.
During the time and distance check, the airplane flies across the direction to the station, not toward it.
The calculated time represents an approximation of how long it would take to cover the estimated direct distance at the assumed speed.
If you continue flying your present cross-station heading, you will not reach the NDB simply because four minutes pass.
To use the time as a practical estimate for flying directly to the station, you would need an appropriate route, a suitable heading, and a realistic ground speed along that route.
How Accurate Is the ADF Time and Distance Check?
This method is a useful navigation estimate, but several factors can affect its accuracy.
Wind can change the airplane's ground speed and its flight path.
Ground speed is generally preferable when a reliable value is available. True airspeed can be used for an approximation when wind is negligible or its effect is understood.
The calculation depends on observing the bearing change while maintaining a steady heading.
Turns during the timed interval can change the ADF relative bearing even without a comparable change in position.
A small mistake in reading the needle or measuring the elapsed time can significantly affect the result.
The estimate is especially sensitive when the measured bearing change is small.
ADF reception can be affected by interference and propagation errors.
Use this method only when the selected NDB is correctly identified and its bearing indication is reliable.
ADF time and distance checks are approximate navigation techniques. They do not replace approved navigation equipment or the position information required by a published procedure.
If flying under an ATC clearance or on an instrument procedure, do not make a heading change for a time-distance check unless the maneuver is permitted.
An accurate measurement also requires a suitable station signal and a stable flight path.
To estimate distance from an NDB, first place the station at a wingtip and maintain a constant heading.
Measure the time required for a known change in bearing, commonly 10°.
With a fixed-card ADF, observe the change in relative bearing while maintaining a constant heading.
With an RMI, observe the change in the magnetic bearing indicated by the selected ADF pointer head.
Estimated time (min) ≈ elapsed seconds ÷ degrees of bearing change.
Estimated distance (NM) ≈ time (min) × speed (kt) ÷ 60.
Remember that the result is approximate and depends on wind, speed, timing and bearing accuracy.
Interactive ADF Simulator
Move the airplane around the NDB and change its magnetic heading. Compare the relative bearing on a fixed-card ADF with the magnetic bearing TO and FROM on an RMI. Practice homing, tracking, and interceptions using the examples below.
Try These ADF Navigation Examples
Inbound tracking: the airplane is south of the NDB on the 360° TO line, with heading 350° as an example of a wind correction. The instruments read 010° and 360° TO. Movement in this tool does not model wind.
- Select Homing, then click Point Nose at NDB. The fixed-card needle moves to 000° and the RMI pointer head aligns with the heading index.
- Select 360° TO. The fixed-card reads 010° while the RMI head reads 360°. Both are correct with heading 350°.
- Select 090° FROM. The fixed-card reads 190°, and the RMI tail reads 090°. The NDB is behind the airplane.
- Change the Selected practice bearing and click Place Aircraft on Selected Bearing. The airplane moves onto your chosen TO or FROM line, and both instruments update while its heading stays the same.
- Select an intercept example. Move the airplane onto the dashed line and watch the magnetic bearing reach the selected value.
Remember: An ADF has no OBS, CDI, or VOR-style TO/FROM flag. The selected line is an on-screen practice guide. This simulator assumes magnetic north equals map north, no wind during movement, a valid heading reference for the RMI, and an ideal NDB signal. It does not model propagation errors, station-passage uncertainty, or aircraft performance. The map distance is simulated; ADF does not measure distance.
ADF Review Questions
Test your understanding of ADF navigation before moving on. These questions review fixed-card ADF indicators, manually rotatable cards, the RMI, magnetic and relative bearings, homing, tracking, interception, station passage, and distance estimation.
Try each question before opening the answer. Use the Interactive ADF Simulator above to check the instrument-reading exercises.
Part 1 — Check Your Knowledge
Show Answer
An ADF is the aircraft receiver and direction-finding system. An NDB is a ground-based non-directional beacon. The ADF indicates the direction toward a received station. Depending on the indicator, this is shown as a relative bearing or a magnetic bearing. A conventional ADF does not measure distance or display a CDI.
Show Answer
Use current aeronautical information for the frequency and identifier. Tune the ADF receiver, select the correct operating mode, and listen for the station's Morse code identification. Continue monitoring the identification during navigation. A moving needle alone does not prove that the signal is reliable.
Show Answer
Set the aircraft's magnetic heading under the top index of the rotatable card. The needle then points to the approximate magnetic bearing TO the NDB. Unlike an RMI, the card does not automatically follow heading changes; reset it whenever the heading changes.
Show Answer
The RMI's compass card automatically follows aircraft heading when its heading reference is valid. The selected ADF pointer head indicates the magnetic direction toward the NDB; its tail indicates the reciprocal magnetic direction away from the NDB. Check the pointer's navigation source before using the reading.
Show Answer
During homing, keep the NDB ahead of the nose: fixed-card ADF approximately 000° or RMI pointer head at the top index. A crosswind can produce a curved ground track. During tracking, use a wind-corrected heading to maintain the selected magnetic bearing TO the station. The needle may be away from 000° even while tracking correctly.
Show Answer
A fixed-card needle at 180° means the NDB is directly behind the nose direction. That is not necessarily the heading needed to maintain an outbound ground track in wind. Maintain the selected bearing FROM using wind correction. On an RMI, monitor that bearing at the pointer tail.
Show Answer
The direction toward the beacon changes from ahead to behind as the airplane passes it. Near the NDB the needle may rotate or oscillate. An off-center passage may not show a neat 180° reversal. Interpret the change using the planned flight path and other suitable navigation information.
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Other examples include coastal refraction, precipitation static, and aircraft-related errors. The RMI does not correct these ADF signal problems. If a bearing is unreliable, verify station identification and cross-check other suitable navigation information rather than chasing the needle.
Part 2 — Read the Instruments
Use each instrument drawing to work out the answer before revealing it. The compass settings and needle directions refer to the same conventions used in the lesson and simulator.
Show Answer
Take the reciprocal to obtain 180° FROM. The station is 10° right of the aircraft's nose because the heading is 350°.
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The RMI pointer head gives 270° TO. The tail gives the reciprocal, 090° FROM. With the same heading, a fixed-card ADF would show 190° relative bearing.
Part 3 — Apply Your Navigation Skills
Show Answer
The magnetic bearing TO the station is 090°. Its reciprocal, the magnetic bearing FROM, is 270°.
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At the ideal intercept point, the NDB is 80° left of the nose. The fixed-card shows 280° relative bearing; the RMI pointer head shows 355° magnetic TO. Lead the turn appropriately before reaching the line so you do not overshoot it.
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Doubling the initial relative bearing is used in a particular FAA training example. For example, starting on heading 355° with the NDB at 040° RB, an 80° right intercept turn gives heading 075°. In real navigation, choose an appropriate interception angle for the geometry, wind, aircraft and procedure; do not mechanically apply one rule in every situation.
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This is an approximation based on the wingtip-bearing method, not an ADF distance readout. You are flying across the station direction while measuring, not directly toward the NDB. Wind and measurement errors can change the estimate.
Show Answer
A real ADF has no OBS that sets its needle direction. Changing the selected practice bearing changes the training line, not the station's actual location. Clicking Place Aircraft on Selected Bearing moves the aircraft onto that line without changing its heading. The RMI then shows 120° FROM at the tail and 300° TO at the head. The fixed-card relative bearing depends on the aircraft heading.
Show Answer
Do not chase an untrustworthy signal. Check station identification, equipment and other suitable navigation sources. If an NDB is required for the procedure and its signal cannot be trusted, follow the applicable ATC instructions and approved procedures rather than assuming the ADF remains accurate. An RMI displaying the same ADF source is affected by the same reception problem.
If a question was difficult, return to the relevant chapter or use the Interactive ADF Simulator. Move the aircraft, change its magnetic heading, select TO or FROM, and compare the fixed-card and RMI readings. Remember: the ADF indicates bearing, not distance or course deviation.
