¶ Introduction
The relative bearing from the radio navigation aid can be shown on two different instruments:
- Radio Bearing Indicator (RBI)
- Radio Magnetic Indicator (RMI)
Basic instruments consist of a compass rose with one needle that may indicate Relative Bearing (RB) or Magnetic Bearing (MB), depending on the instrument. The head of the needle indicates bearing TO the station and the tail of the needle indicates bearing FROM the station.
¶ Radio bearing indicator with fixed card
It only has one needle and a fixed (not movable) compass rose. It always indicates heading North at the top of the instrument.
The moving needle indicates the Relative Bearing (RB) to the station, relative to the longitudinal axis of the aircraft (fore and aft axis). That Relative Bearing (RB) is read clockwise from 0 until the value the needle is pointing.
This is the simplest instrument but its usage is not easy since the Magnetic Bearing (MB) is not shown at first glance. Relating to the current heading, the pilot will need to check the relative bearing and calculate the Magnetic Bearing (MB) each time.
Magnetic Bearing (MB) = Relative Bearing (RB) + aircraft heading.
¶ Radio bearing indicator with movable card
Similar to the radio bearing indicator with fixed card, this instrument has the advantage to have a compass rose which can be rotated manually by the pilot.
So the aircraft's current heading can be set on the top. This allows the instrument to show directly the Magnetic Bearing (MB), which eases the pilot's work.
In the figure, the movable card was put at a heading 345°. In consequence the NDB magnetic heading is 060°.
¶ Radio magnetic indicator (automatic)
This is an advanced instrument as it automatically rotates the compass rose to represent the current aircraft heading at the top. The Magnetic Bearing (MB) can be obtained easily.
The RMI has one or two needles which can be used to indicate navigation information from either the ADF or the VOR receivers. Both needles are different in appearance, one of them operating with NAV 1 or ADF 1 radio and the other one operating with NAV 2 or ADF 2 radio.
There are two switches that allow the pilot to change each needle source from VOR to ADF or from ADF to VOR.
¶ Electronic horizontal situation indicator EHSI
In more complex instruments mounted on bigger or commercial airplanes, the ADF might be integrated into glass cockpits EHSI where NDB bearings may be shown in the Navigation Display.
¶ Relative position between aircraft and radio navigation aid
The head of the needle indicates bearing TO the station and the tail of the needle indicates bearing FROM the station. The needle is not in function of aircraft heading.
¶ Inbound radio navigation aid
The needle on the RMI is pointing north. Then the radio navigation aid is in front of the aircraft.
¶ Outbound radio navigation aid
The needle on the RMI is pointing south. Then the radio navigation aid is behind the aircraft.
¶ Lateral radio navigation aid
The needle on the RMI is pointing 90°. Then the radio navigation aid is on the right of the aircraft.
¶ 45° track
The needle on the RMI is pointing 45°. Then the radio navigation aid is on the right of the aircraft.
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