NDB / ADF

The NDB (Non-Directional Beacon) is an omnidirectional radio transmitter. The aircraft receives it via the ADF (Automatic Direction Finder), which indicates the direction to the station.

Operating principle

The NDB transmits a simple radio signal on low frequency (LF/MF). The aircraft's ADF antenna automatically determines the direction from which the signal originates.

The result is displayed on the RMI: the ADF needle always points towards the station.

Reading the RMI

Reading the NDB on the RMI is intuitive:

• Needle head → QDM (heading to reach the station)
• Needle tail → QDR (bearing from the station)

Reading example

If the heading is 090° and the ADF needle points to 045° on the compass rose:

• QDM = 045° (the station is 45° to the left of the nose)
• QDR = 045° + 180° = 225°

Homing to an NDB

Direct homing

The simplest method: keep the ADF needle at the 12 o'clock position (top of the compass rose). The aircraft heading then points towards the station.

Drawback: In crosswind conditions, the flight path curves (known as a "dog leg" track).

Homing with drift correction

To follow a straight-line track towards the NDB:

1. Estimate the drift caused by wind
2. Apply a correction: heading = QDM +/- drift
3. Verify: if the QDM remains stable, the correction is correct

Technical characteristics

Limitations

• Less accurate than VOR (+/-5° vs +/-1°)
• Susceptible to atmospheric interference (thunderstorms, night effect)
• Coastal refraction (signal deviation when crossing coastline/sea boundary)
• No direct radial: requires QDM/QDR calculation via the RMI