Automatic Direction Finder (ADF)
An Automatic Direction Finder (ADF) is a navigation receiver that automatically indicates the direction of a received radio signal from a Non-Directional Beacon (NDB) or commercial AM broadcast station, relative to the nose of the aircraft.
RBI vs. RMI
- RBI (Relative Bearing Indicator): Fixed card; needle shows bearing relative to the aircraft nose. 0° = ahead, 090° = right
- RMI (Radio Magnetic Indicator): Rotating card that incorporates heading; needle points to the magnetic bearing TO the station — more intuitive for navigation
ADF Error Sources
- Night effect: Sky wave interference causes erratic needle movement after dark
- Coastal refraction: Signals bend when crossing a coastline at an angle — bearing errors result
- Mountain effect: Terrain reflections cause false bearings
- Thunderstorm interference: ADF needle may point toward nearby lightning rather than the station
- Station passage: Rapid needle oscillation at station passage
NDB Status in Canada
NAV CANADA has been decommissioning NDBs as GPS/GNSS approaches replace NDB approaches. Always verify operational status via NOTAM and CFS before planning to use a specific NDB.
On the TC Written Exam
ADF/NDB navigation remains on written exams. Know relative bearing calculations, tracking to and from an NDB, and the major error sources.