Definition
A cockpit instrument associated with the Automatic Direction Finder (ADF) that displays the bearing to a non-directional beacon (NDB) relative to the nose of the aircraft. The RBI has a fixed compass card with 0° permanently at the top, and a single needle that points toward the tuned station. The angle between the nose (0°) and the needle, measured clockwise, is the relative bearing.
Plain English
An instrument with a needle that points at the radio station you're tuned to. Because the dial doesn't move, the needle shows where the station is in relation to the front of your airplane, not in relation to north.
Context Anchor
Seen in ADF instrument flying, especially when using an ADF needle to find the direction to a selected radio station.
Derivation
Relative because the bearing is measured relative to the aircraft's nose, not relative to magnetic north. Bearing is the direction to something. Indicator simply means a gauge that shows you a value.
Why Pilots Care
It lets the pilot determine the direction to an NDB and intercept a desired course without first knowing their exact magnetic heading.
Grounding Statement
Picture the airplane’s nose as 0 degrees; the RBI needle shows where the station sits around the airplane from that starting point.
Intuition Check
Do not treat an RBI like a normal compass display. It shows direction from the aircraft’s nose; it does not automatically show the magnetic direction to the station.
Example Sentence 1
With the aircraft heading 090° and the RBI needle showing 30°, the magnetic bearing to the station is 120°.
Example Sentence 2
With the RBI showing a relative bearing of 090, the station was directly off our right wing.