Definition
A ground-based, low or medium frequency radio transmitter that radiates a signal equally in all directions. An aircraft equipped with an Automatic Direction Finder (ADF) receiver can determine the bearing to the station and use it for navigation, instrument approaches, or as a transition fix.
Plain English
A radio station on the ground that sends a steady signal out in every direction. A receiver in the aircraft points to the station so the pilot knows which way it lies relative to the airplane.
Context Anchor
Pilots may see nondirectional beacons on aeronautical charts, in instrument procedure descriptions, or when using older radio navigation equipment.
Derivation
Nondirectional' simply means the signal is not aimed in any particular direction — it goes out evenly all around the transmitter. This contrasts with a VOR, which sends out direction-specific information. Knowing the signal is omnidirectional explains why the airplane's receiver, not the station, has to work out the bearing.
Why Pilots Care
NDBs are older technology and being phased out, but they still appear on charts, on checkrides, and at some airports as the basis of an instrument approach. A pilot needs to recognize what one is and how an ADF uses it, even if they rarely fly to one.
Intuition Check
Do not assume beacon means a flashing light here. In this FAA context, a nondirectional beacon is a radio navigation station, not a visual airport light.
Example Sentence 1
The approach begins at the nondirectional beacon, where the pilot turns inbound on the final approach course.
Example Sentence 2
After passing the nondirectional beacon the aircraft began its descent on the published approach.