How do you calculate the magnetic bearing to an NDB using ADF?

Add the aircraft's magnetic heading to the relative bearing indicated by the ADF; if the sum exceeds 360°, subtract 360°.

What does ADF stand for and what is its role?

ADF stands for Automatic Direction Finder and is the airborne equipment used to determine the direction to an NDB.

How accurate must you be when flying an ADF approach according to ICAO standards?

You must maintain the required bearing within plus or minus 5 degrees to be considered established.

Can weather affect ADF indications?

Yes, static energy from cumulonimbus clouds can interfere with ADF signals and cause bearing errors.

Relative Bearing and ADF

Q: What is relative bearing in ADF navigation?

Relative bearing is the angle measured clockwise from the aircraft's nose to the direction of the NDB station.

Medium4 min readRadio Navigation

Moderately Examined

Why this matters

Understanding relative bearing and ADF operation is essential for safe and accurate navigation, especially when flying approaches or tracking to NDBs in low-visibility or non-radar environments. Errors in interpretation can lead to significant navigational mistakes, impacting flight safety.

Relative bearing and ADF (Automatic Direction Finder) are key concepts in radio navigation, enabling pilots to determine their position relative to a Non-Directional Beacon (NDB). The ADF instrument shows the angle between the aircraft's nose and the direction to the beacon, known as the relative bearing. By combining this with the aircraft's heading, pilots can accurately calculate the bearing to or from the beacon, which is essential for navigation and approach procedures.

Quick Check

An aircraft is on a magnetic heading of 120°. The ADF shows a relative bearing of 045°. What is the magnetic bearing (QDM) to the NDB?

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In depth

Explanation

What is Relative Bearing and ADF?

The Automatic Direction Finder (ADF) is an aircraft navigation instrument that continuously points to a selected NDB station. The relative bearing is the angle measured clockwise from the aircraft's nose to the direction of the beacon, as shown by the ADF needle on a fixed-card indicator (Relative Bearing Indicator, RBI).

ADF Bearing Calculation

To find the magnetic bearing (QDM) to the beacon:

Add the aircraft's magnetic heading (MH) to the relative bearing (RB): QDM = MH + RB
If the sum exceeds 360°, subtract 360° to keep within 0–359°.
For true bearing (QUJ), use the true heading instead of magnetic heading.

Using ADF for Navigation

In calm conditions, set your heading so the ADF needle points to your desired track; the aircraft will naturally converge on the beacon.
In wind, apply a drift correction to maintain the desired track. The relative bearing will match the drift angle needed to stay on course.
As you approach the NDB, expect erratic needle movement in the 'cone of confusion' directly overhead—this is normal and not suitable for navigation inputs.

Display Types and Interpretation

Fixed-card ADFs show only relative bearing. Pilots must calculate the absolute bearing.
RMIs (Radio Magnetic Indicators) can display magnetic bearing directly, reducing calculation errors.

Operational Accuracy and Limitations

During approach, ICAO standards require the aircraft to be established within ±5° of the required bearing.
Static from thunderstorms (e.g., cumulonimbus clouds) can interfere with ADF signals, causing bearing errors.

Practical Tips

Always verify the NDB ident before using the ADF for navigation.
Remember to apply wind correction and monitor for signal interference.

The essentials

Key Points

ADF stands for Automatic Direction Finder and is the aircraft's navigation receiver for NDBs.

Relative bearing is the angle clockwise from the aircraft's nose to the beacon, shown by the ADF needle.

Magnetic bearing to the beacon (QDM) = Magnetic Heading + Relative Bearing.

True bearing uses true heading instead of magnetic heading in the calculation.

During approach, you must be within ±5° of the required bearing to be considered established.

ADF is susceptible to interference from static, especially near thunderstorms.

Fixed-card ADFs require manual calculation; RMIs can display magnetic bearing directly.

Watch out

Exam Traps & Typical Mistakes

Confusing relative bearing (needle indication) with magnetic or true bearing (absolute direction).

Forgetting to subtract 360° if the sum of heading and relative bearing exceeds 360°.

Assuming the ADF needle always shows magnetic bearing—it shows relative bearing on fixed-card indicators.

Not accounting for wind drift, which affects the heading needed to maintain a desired track.

Misinterpreting the erratic needle movement over the NDB as an equipment fault rather than the cone of confusion.

Test yourself

Example Exam Questions

Question 2Easy

On a fixed-card ADF indicator, what does the needle display?

Question 3Medium

If the aircraft's heading is 270° (magnetic) and the ADF relative bearing is 100°, what is the QDM?

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