What is the difference between ADF and NDB navigation?

ADF is the aircraft instrument that receives signals from NDBs, which are ground-based radio beacons; together, they allow pilots to determine bearings to the station.

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

Add the aircraft's true heading to the relative bearing indicated by the ADF to get the true bearing to the NDB.

What is the meaning of QDM and QDR in ADF/NDB navigation?

QDM is the magnetic bearing to the station (direction to fly to reach the NDB), while QDR is the magnetic bearing from the station (direction to fly away from the NDB).

How is position fixing achieved using ADF and NDB?

By taking bearings from two or more NDBs, the intersection of the plotted lines on a chart gives the aircraft's position.

Why must variation and deviation be considered in ADF/NDB navigation?

Variation and deviation correct compass and bearing readings to ensure accurate plotting and navigation relative to true north.

ADF and NDB Navigation

Hard4 min readGeneral Navigation

Occasionally Examined

Why this matters

Mastering ADF and NDB navigation ensures pilots can navigate safely even when modern systems fail or are unavailable, supporting robust situational awareness and redundancy in flight operations.

ADF and NDB navigation is a fundamental radio navigation method where pilots use the Automatic Direction Finder (ADF) to track to or from a Non-Directional Beacon (NDB). The ADF displays the relative bearing to the NDB, which must be converted to a true or magnetic bearing for accurate chart plotting and position fixing.

Quick Check

An aircraft is flying with a true heading of 120°. The ADF shows a relative bearing to the NDB of 045°. What is the true bearing from the aircraft to the NDB?

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In depth

Explanation

Understanding ADF and NDB Navigation

ADF navigation relies on the aircraft's ADF receiver to detect signals from ground-based NDBs. The ADF needle points towards the NDB, showing the relative bearing (the angle between the aircraft's nose and the station). To use this information for navigation, you must convert the relative bearing into a usable bearing for plotting or tracking.

Key Calculations

Relative Bearing (RB): The angle from the aircraft's nose to the NDB, as shown by the ADF.
Magnetic Bearing to the Station (QDM): Add the aircraft's magnetic heading (MH) to the relative bearing (RB). QDM = MH + RB.
True Bearing to the Station: If plotting on a chart, apply magnetic variation to convert the magnetic bearing to a true bearing.
Bearing from the Station (QDR): The reciprocal of the bearing to the station (QDM ± 180°).

Position Fixing and Plotting

To fix your position using an NDB, you can take bearings from two or more NDBs, or combine an NDB bearing with another navigation aid. For plotting, always ensure you apply deviation (if using a magnetic compass), variation (to convert between magnetic and true), and consider chart projection corrections (such as convergency on Lambert charts).

Exam and Operational Focus

Expect exam questions to test your ability to:

Convert ADF readings into true or magnetic bearings.
Apply deviation and variation correctly.
Plot bearings accurately on aeronautical charts.
Understand the difference between a dead reckoning (DR) position and a fix.

While NDBs are less common today, understanding ADF and NDB navigation remains essential for foundational radio navigation knowledge and for handling legacy or backup navigation scenarios.

The essentials

Key Points

ADF shows the relative bearing from the aircraft to the NDB.

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

Apply deviation and variation to convert between compass, magnetic, and true bearings.

Plotting on charts requires true bearings; always correct for variation.

QDR is the reciprocal bearing from the NDB to the aircraft (QDM ± 180°).

Position fixing with NDBs may require intersecting bearings or combining with other aids.

Chart projection effects (like convergency) can affect plotted bearings on certain charts.

Watch out

Exam Traps & Typical Mistakes

Forgetting to apply magnetic variation when converting magnetic to true bearings.

Confusing relative bearing (from aircraft) with bearing from the station (QDR).

Incorrectly adding or subtracting deviation and variation in the wrong order.

Assuming the ADF needle shows a true or magnetic bearing directly—it only shows relative bearing.

Neglecting chart projection corrections (e.g., convergency) when plotting long-range bearings.

Test yourself

Example Exam Questions

Question 2Medium

You are given a magnetic bearing to an NDB of 090° with a local magnetic variation of 10°W. What is the true bearing to the NDB?

Question 3Hard

An aircraft at 50°N 017°W has a true bearing of 087° to an NDB at 52°N 009°W. The convergency between positions is 6°. What is the true bearing to plot from the NDB to the aircraft?

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