The Radio Magnetic Indicator (RMI) reduces mental math by slaving a compass card to an ADF, allowing magnetic bearings to and from a station to be read directly. On a single instrument the RMI shows combined information from a magnetic compass, VOR and ADF; the clock-like needle head points to the magnetic bearing to the station, while the tail depicts the magnetic radial outward.
Although the piloting curriculum expects every student to understand both RMI and Horizontal Situation Indicator navigation, modern avionics usually favour the HSI, which merges a CDI with a heading indicator and is often deemed the superior instrument. Designers routinely embed an RMI inside digital HSIs and glass-panel primary flight displays, keeping the classic bearing-reading feature available alongside the navigational accuracy of the full HSI.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is the difference between an HSI and RMI in aviation?
An HSI is a separate instrument that combines the OBI with the aircraft's gyroscopic direction indicator to provide a simplified moving map. It displays the aircraft's current heading, the relative direction of the selected VOR radial, course deviation, and glide slope all in one instrument. Because the course is shown directly on the heading indicator, the HSI eliminates reverse-sensing in most cases and behaves like an early form of the GPS moving-map.
The Radio-Magnetic Indicator displays relative and magnetic bearing to the station, but it lacks a course-selection bug. Consequently, the pilot must mentally super-impose the chosen radial on the magnetic heading, which calls for slightly more mental effort when tracking a course.
How does an RMI compare to a HSI?
Both instruments present VOR information, yet the Horizontal Situation Indicator is the superior steering aid. Because the HSI makes it much easier to avoid reverse sensing, the pilot can fly an assigned radial without mentally inverting the heading. When course is shown directly on the heading indicator, it is easier to avoid reverse sensing, so the head of the selectable course pointer arrow indicates the desired course while the tail shows current bearing from ground station. The Radio Magnetic Indicator only displays the magnetic bearing to the station; the needle points to the magnetic bearing to the station, but the pilot must still decide whether to fly toward or away from the pointer. Thus, the HSI integrates heading, course, and deviation cues on one face, whereas the RMI merely overlays an RBI on the heading indicator and leaves the selection and tracking of a specific VOR radial to separate mental calculations.
The Radio Magnetic Indicator gave immediate information of my heading to the chosen facility. Its motioning compass card, slaved to a removed compass, kept me continually aware of my relation to the base even while I was soaring over featureless tracts or gliding in borderline condition. When I switched to an aircraft outfitted with a Horizontal Situation Indicator, I saw the HSI unify the heading dial, direction pointer, and course-variance sign into a single screen. This integration made monitoring easy and reduced my workload.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
What is the difference between an RMI and a CDI in aviation?
An RMI normally consists of a rotating compass card and two bearing pointers that simply point to the station, while a CDI is a vertical needle that moves laterally along a row of dots to show aircraft lateral position relative to a selected radial track. The RMI is remotely coupled to a gyrocompass so the compass card indicates heading of aircraft opposite the upper center index mark, whereas the CDI operates independently of heading information and is used to determine aircraft lateral position in relation to course without reference to aircraft heading.
The CDI determines lateral position by displaying left/right deflection: needle deflects to right if aircraft is left of course, and deflects to left if aircraft is right of course. Full deflection of CDI on VOR equals 10, while full deflection of CDI on Localizer equals 2.5. A CDI is used for tracking to or from a station, course interception, and on instrument approach to track left/right deviation from runway. RMI requires more mental effort to track a course because the pilot must mentally compare aircraft heading shown on the compass card with the bearing pointer direction.
Both instruments serve different navigation functions: RMI combines heading indicator with bearing information for orientation and station location, while CDI is used for precise course tracking. CDI can be used with VOR, GPS, ILS, and localizer, and is part of HSI where CDI is overlayed on DG so HSI-CDI will not reverse read on back course. VOR receiver incorporates Course Deviation Indicator, and OBS rotation centers CDI with TO indication for course to VOR or FROM indication at present radial.
How does an RMI differ from an ADF in aviation?
The Radio-Magnetic Indicator is an alternate ADF display that automatically rotates the azimuth card to represent aircraft heading, whereas a movable-card ADF requires the pilot to reset the heading indicator manually. Because the compass card is actuated by the aircraft's compass system, the RMI allows the operator to read magnetic bearing without resorting to arithmetic, eliminating the mental math demanded by the older fixed-card or adjustable-card ADF.
An RMI is a two-channel ADF with a compass card slaved to the magnetic compass. It provides more information than a standard ADF by combining indications from the magnetic compass, VOR, and ADF on one instrument. Most single-needle RMIs have a switch that allows the pilot to select either an ADF or VOR station, while dual-needle RMIs only provide information for the ADF and VOR 2. In a Flight Simulator all of the RMIs are dual-needle, and the two needles can be used to indicate navigation information from either the ADF or the VOR receiver, letting the pilot triangulate from an ADF and a VOR, or two VORs, more or less at a glance.
The traditional ADF shows the relative bearing of the transmitter station relative to the longitudinal axis of the aircraft. Its needle head always points directly to the NDB and the tail always shows the magnetic bearing from the station. Because the fixed-card ADF card remains fixed with 360 degree bearing at the top, the pilot must add magnetic heading to relative bearing to compute the magnetic bearing to the station. The RMI overcomes these shortcomings: when a needle is driven by the ADF, the head of the needle indicates the MB TO the station tuned on the ADF receiver, and the slaved compass continually adjusts the azimuth card so the aircraft's current heading is displayed at the top of the azimuth.
An ADF needle deflected wildly, demanding continual work as I attempted its signal. The change to utilizing an RMI made work easier: the device's slaved navigational instrument board showed my magnetic direction steadily. With the RMI the transparent display permitted me to settle my charted path while I focused on aircraft command.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air