ADF Principles and Usage

What is the ADF and How Does It Work?

The ADF (Automatic Direction Finder) is fitted in the aircraft and receives signals from NDB (Non-Directional Beacon) stations, which are ground-based transmitters operating in the LF (Low Frequency) and MF (Medium Frequency) bands. The ADF uses a combination of loop and sense aerials to determine the direction of the incoming signal, displaying the bearing to the NDB relative to the aircraft's nose.

ADF Display Interpretation

ADF information is typically shown on an RMI (Radio Magnetic Indicator) or a fixed-card indicator. The needle points towards the NDB, showing the relative bearing. Pilots must interpret this information to determine their QDM (magnetic heading to the station) or QDR (magnetic bearing from the station).

ADF Navigation Procedures

• Homing: Flying the aircraft so the ADF needle stays on the nose. This leads directly to the NDB but does not correct for wind, resulting in a curved track if wind is present.
• Tracking: Adjusting heading to compensate for wind drift and maintain a constant bearing to the station, ensuring a straight track over the ground.
• Intercepting QDM/QDR: Pilots can intercept inbound (QDM) or outbound (QDR) bearings by flying specific headings to join and maintain the desired track.
• Changing Bearings: To switch from one QDM/QDR to another, pilots adjust heading to intercept the new bearing, monitoring the ADF needle for correct alignment.
• Station Passage and Abeam: Station passage is indicated when the ADF needle swings rapidly through 90 degrees. The abeam point occurs when the needle is at 90 degrees to the aircraft's longitudinal axis.

Types of NDB Stations

• Locator Beacons: Short-range, low-power, used for approaches (10–25 NM coverage).
• Homing/Holding NDBs: Medium power, used for en-route and terminal navigation (up to 50 NM).
• En-route/Long-Range NDBs: High power, used for long-distance navigation, especially over oceans.

ADF Usage and Limitations

ADF can receive signals from any station in the LF/MF band, including commercial radio broadcasts, but only aviation-approved NDBs (with published locations and Morse identifiers) are valid for IFR navigation. Pilots must always identify the NDB audibly before using it for navigation.

Ground Direction Finding (DF)

Ground DF stations can determine the bearing of an aircraft's transmission, but this is not part of the airborne ADF system.