Definition
A radio navigation system that automatically points to a selected ground-based non-directional beacon (NDB) or commercial AM broadcast station. The aircraft's ADF receiver senses the direction of the incoming radio signal and drives a needle on a cockpit indicator to show the bearing from the aircraft to the transmitting station, relative to the aircraft's nose.
Plain English
A simple cockpit instrument with a needle that always points toward a chosen radio station on the ground, so the pilot knows which direction that station is from the aircraft.
Context Anchor
Seen in instrument flying when using older radio navigation equipment, and in precipitation static discussions because electrical static can disturb the ADF indication.
Derivation
The name describes the function literally: the receiver automatically finds the direction of a radio signal, instead of the pilot having to manually rotate a loop antenna and listen for the signal to fade out. Earlier 'direction finding' radios from the 1930s required this manual tuning; the 'automatic' version eliminated that step.
Why Pilots Care
ADF provides a simple backup navigation method but is easily disrupted by precipitation static, requiring pilots to recognize when the needle becomes unreliable.
Intuition Check
ADF does not find the aircraft’s full position by itself. It only shows the direction to the radio signal it is receiving.
Example Sentence 1
With the ADF tuned to the airport's NDB, the needle swung around to point off the right wing, confirming the station was abeam.
Example Sentence 2
Heavy rain static made the ADF needle swing erratically until the pilot switched to another navigation source.