Definition
A cockpit instrument, typically associated with the Automatic Direction Finder (ADF), that displays the angle between the aircraft's nose and the bearing to a tuned non-directional beacon (NDB). The needle points directly at the station, and the angle is read clockwise from the aircraft's nose (000 degrees at the top of the dial) around to 360 degrees. The card on a basic RBI is fixed with 0 always at the top, so the indication shows where the station is relative to the aircraft, not relative to north.
Plain English
An instrument with a needle that points to a ground radio station. The angle from straight ahead of the aircraft to that needle tells the pilot where the station is in relation to the nose of the airplane.
Context Anchor
Seen in instrument navigation when using older bearing displays, especially with ADF-style navigation equipment.
Derivation
Relative' here means 'measured in relation to something else' -- specifically the aircraft's nose, not magnetic north. 'Bearing' comes from the old nautical sense of the direction in which something 'bears' or lies from the observer. So the name literally describes what the instrument shows: the direction of a station as it lies relative to the aircraft.
Why Pilots Care
Enables pilots to determine the direction to a station and home toward it without first converting to a magnetic heading.
Analogy
It is like saying a sound is at your 3 o’clock instead of saying it is east. The direction is based on where you are facing.
Intuition Check
Relative does not mean approximate here. It means measured from the aircraft’s nose instead of measured from north.
Example Sentence 1
With the aircraft on a heading of 360 and the RBI showing a relative bearing of 045, the NDB is off the nose to the right at a magnetic bearing of 045.
Example Sentence 2
We turned to place the RBI reading at zero and flew straight toward the station.