The Radio Magnetic Indicator (RMI) is an instrument that consolidates fluxgate, heading indicator, and relative bearing indicator functions into one display. By automatically rotating the compass card through the aircraft's compass system, it presents the aircraft's heading while simultaneously showing relative bearing to any selected navigation aid. It offers all ADF-indicator capabilities and depicts the aircraft's heading and position with respect to a chosen VOR/TACAN course, enabling straightforward VOR navigation.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is an RMI in aviation?
A radio magnetic indicator (RMI) provides a magnetic bearing to the station, and it is one solution to the ADF's shortcomings. RMIs provide information for the ADF and VOR. All RMIs are dual-needle, so you cannot select a different radio for each needle.
RMI is a radio magnetic indicator, a steam-gauge instrument that is part of an instrument panel or a glass-panel display. It combines a magnetic compass card with one or two pointers to provide a unified view of bearing to navigation aids. The RMI is slaved to a remotely mounted compass or magnetometer so the compass card rotates automatically to show the aircraft's current magnetic heading beneath the top index, while the pointers are driven by VOR or ADF receivers.
In a single-needle installation the needle simply points to the selected station, indicating magnetic bearing to the NDB or VOR and, by reading the tail of the needle, the bearing from the station. With two needles, one needle is used for VOR and the other for ADF, permitting simultaneous display of navigation information from both receivers. Whichever configuration is installed, the RMI functions as a bearing-pointer instrument that gives pilots an instant, intuitive picture of position relative to navigation aids, simplifying en-route navigation and instrument approaches.
What are the parts of a radio magnetic indicator?
The parts of a radio magnetic indicator are detailed below.
- Compass Card: This displays the aircraft’s heading and rotates with the aircraft’s turns to maintain accurate directional information.
- Bearing Pointers: The RMI has two pointers: one is connected to a VOR receiver and the other to an ADF receiver. These pointers show the direction to the VOR or ADF station to the aircraft.
- Heading Indicator: This helps the pilot maintain the desired course by providing a clear visual indication of the aircraft’s current heading.
The compass card, actuated by the aircraft's compass system, displays the current magnetic heading opposite the upper-center index mark. Numerals and equally spaced marks on the card identify compass positions at 30-degree intervals, and the card turns as the airplane turns, giving a 360-degree image around the station. One needle, often called the green pointer, is driven by the automatic direction finder (ADF) and its head indicates the magnetic bearing to the NDB station, while its tail shows the bearing from the station. The second needle is driven by the VOR receiver and shows the compass bearing to the VOR station relative to the aircraft. The instrument therefore presents simultaneous magnetic-heading and relative-bearing data, eliminating the task of manually turning a compass card.
What is the difference between an ADF and an RMI in aviation?
The ADF, or Automatic Direction Finder, receives an omni-directional signal from an NDB and displays the relative bearing of the transmitter station. Its needle points to the NDB station, but the bearing indicator - whether fixed-card or adjustable - requires the pilot to perform mental math to determine the magnetic course. The ADF operates on low frequencies from 200 kHz to 1750 kHz, is not limited to line of sight, and can even use commercial AM station broadcasts. Because the ADF only shows the relative angle of the transmitter with respect to the aircraft, it does not tell whether the aircraft is going towards or away from the station. Another antenna is used by the ADF to tell if you are approaching or leaving the station. Traditional ADF installations are subject to quadrantal error, needle oscillation, night effect, terrain effect, mountain effect, and ore deposits, and the instrument is either a fixed-card or rotatable compass-card ADF.
The RMI is an alternate ADF display that provides more information than a standard ADF. It is a two-channel ADF with a compass card slaved to a magnetic compass. The azimuth is turned automatically by a remotely mounted slaved compass, so the needle always indicates magnetic bearing to the NDB. The RMI combines a magnetic compass card with one or two pointers: single-needle RMIs have a switch that allows the pilot to select either an ADF or VOR station, while dual-needle RMIs can display navigation information from either the ADF or the VOR receiver simultaneously. Because the compass card is actuated by the aircraft's compass system, the heading of the aircraft is displayed at the top of the azimuth, and the azimuth continually adjusts itself to reflect the aircraft's heading. This slaved compass eliminates the mental arithmetic required with a basic ADF, although the RMI lacks a course selection bug and requires slightly more mental effort to track a course than an HSI. In Flight Simulator, all RMIs are dual-needle and only provide information for the ADF, but in real aircraft the RMI can be used for both ADF and VOR navigation.
How does RMI work in aviation?
The Radio Magnetic Indicator works by combining a magnetic compass card with one or two pointers that are driven by remote sensors. Continuous heading is supplied by a fluxgate compass that updates the rotating compass card, whose upper-center index mark shows the current magnetic heading and compass card rotates to show pilots current magnetic heading, so the card always displays the exact heading toward the nose of the aircraft.
Navigation information comes from the VOR receiver. The unit passes bearing signals to the instrument, and the VOR needle (single or double bar) points to the VOR station. Because the same remote sensors provide needle guidance and heading to RMI, the pointer simultaneously shows where the aircraft is radially with respect to the VOR station while the compass card indicates heading opposite the upper center index mark.
In practice, RMI eliminates the need to add magnetic heading calculations into the IFR task load. Pilots simply read the number under the needle tail to obtain the inbound bearing. By combining bearing and heading in one glance, RMI simplifies navigation during enroute flying and approaches and eliminates the task of turning a compass card manually and therefore lightens pilot workload.
How to use a radio magnetic indicator?
To use a radio magnetic indicator, tune the desired VOR on the NAV receiver. No further knob is required - the needle simply points to the station, eliminating the need to calculate the bearing. Because VOR signals are not subject to errors and inaccuracies of LF transmissions, the pointer remains steady even in electrical storms, and since range equals 1.23 times the square root of altitude above the transmitter, you will safely track the 015 bearing to NAV 1 while climbing. To keep the heading reference accurate, the pilot must reset the heading indicator to the magnetic compass in level flight every ten minutes.
I deem a Radio Magnetic Indicator as the main instrument for direction. The smooth incorporation of dual guidance sources into one screen made monitoring easy: one needle points toward the ADF base and the other toward the selected VOR. I set the ADF receiver to the non-directional radio station, then watched the pointer. This offered direct definite evidence of my direction relative to the base facility. To fly to the facility I kept the needle on the 0-degree scale and to fly away I held it on the 180-degree spot. While en-route I utilized the indirect direction to cross-check my movement over each waypoint, always focusing on keeping the main line pointer centered. The tool's continuous real-time response was useful.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
How to read RMI aviation?
To read the Radio Magnetic Indicator (RMI) in aviation, begin by noting that the RMI compass card displays the current magnetic heading. For example, if the Aircraft heading is 011 it means the airplane is presently pointing 011 degrees. Next, observe the two needles: each pointer always points at the station it is tuned to. The head of each RMI needle shows magnetic bearing to ground station, instantly telling the pilot the magnetic bearing to the NDB or VOR. The tail always shows magnetic bearing from the station, revealing the radial on which the aircraft is positioned. Thus, when the head of the RMI needle points to 157 degrees, the airplane lies on the 157 magnetic bearing to the station, while the tail of the RMI needle will point to 337 degrees, indicating the outbound radial. Because the magnetic bearing indicator points at NDB, a single glance makes both current heading and relative position clear: if the tail of the RMI needle will point to 337 degrees while the aircraft heading is 011, the pilot knows the station is off the right rear quarter and that the outbound heading 337 is the magnetic bearing from the station to fly when flying away.
