What is the main difference between an RMI and a CDI?

An RMI displays magnetic bearings to and from navigation stations, while a CDI shows lateral deviation from a selected course.

How does an RMI indicate the aircraft's position relative to a VOR?

The RMI needle point shows the magnetic bearing TO the VOR (QDM), and the tail indicates the radial FROM the station (QDR).

Does a CDI provide heading information?

No, a CDI does not display heading; it only shows deviation from a selected course.

Can an RMI display information from more than one navigation aid?

Yes, most RMIs can show bearings from two different navigation aids, such as VOR and NDB, simultaneously.

What happens if the compass system feeding the RMI is inaccurate?

Any compass error will affect the absolute bearings displayed by the RMI, but relative bearing remains correct.

Radio Magnetic Indicator (RMI) vs Course Deviation Indicator (CDI)

Medium4 min readRadio Navigation

Moderately Examined

Why this matters

Knowing when and how to use the RMI versus the CDI ensures accurate navigation, improves situational awareness, and supports safe, precise flying—especially during approaches, holds, and enroute navigation.

The Radio Magnetic Indicator (RMI) and Course Deviation Indicator (CDI) are essential navigation instruments, each serving a distinct role in radio navigation. The RMI continuously displays magnetic bearings to or from navigation aids, integrating heading information automatically. In contrast, the CDI shows lateral deviation from a selected course, indicating how far the aircraft is from the desired track. Understanding the difference between RMI and CDI is crucial for interpreting navigation information accurately and making effective course corrections.

Quick Check

What does the needle point of a Radio Magnetic Indicator (RMI) indicate when tuned to a VOR station?

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In depth

Explanation

RMI Explained

The Radio Magnetic Indicator (RMI) combines heading information with navigation aid bearings, presenting a real-time magnetic bearing to (QDM) and from (QDR) a selected station. The RMI uses a rotating compass card, always aligning the aircraft's current magnetic heading at the top. Its needles point directly to the station (tip = QDM) and indicate the radial from the station (tail = QDR). RMIs can display bearings from multiple sources (e.g., VOR, NDB) simultaneously, making them versatile for situational awareness.

CDI Explained

The Course Deviation Indicator (CDI) is part of VOR or ILS systems and shows the aircraft's lateral position relative to a selected course. The pilot selects a desired radial or localizer course, and the CDI needle moves left or right to display deviation from that course. Full-scale deflection on a VOR CDI typically represents 10° off course; for ILS, it’s about 2.5° for the localizer and 0.7° for the glideslope. The CDI also includes a TO/FROM indicator, clarifying whether the selected course leads toward or away from the station.

RMI vs CDI: Key Differences

The RMI always shows the magnetic bearing to/from a station, integrating heading automatically; it does not require course selection.
The CDI requires the pilot to select a course and shows only deviation from that specific line, not the bearing to the station.
RMIs are ideal for situational awareness and homing, while CDIs are used for precise tracking and intercepting of radials or localizer paths.
RMIs are less affected by heading errors for relative bearings, but compass errors affect the displayed QDM/QDR. CDIs are independent of heading; their accuracy depends on the selected course and signal quality.

Practical Use

Use the RMI for quick orientation, homing, and determining your position relative to multiple stations.
Use the CDI for intercepting and maintaining a specific course, such as flying a VOR radial or ILS approach.

The essentials

Key Points

The RMI displays magnetic bearings to (QDM) and from (QDR) navigation aids, integrating heading automatically.

The CDI shows lateral deviation from a pilot-selected course, with a needle indicating left/right displacement.

RMI is ideal for situational awareness and quickly determining position relative to stations.

CDI is essential for precise tracking of VOR radials or ILS localizer/glideslope paths.

RMI needles can show bearings from multiple sources (VOR, NDB) simultaneously.

CDI full-scale deflection represents 10° (VOR), 2.5° (ILS localizer), or 0.7° (ILS glideslope) off course.

Compass errors affect RMI absolute bearings but not relative bearings; CDI is independent of aircraft heading.

Watch out

Exam Traps & Typical Mistakes

Confusing the RMI's bearing indication (always magnetic) with the CDI's deviation (relative to a selected course).

Assuming the CDI shows the bearing to the station—it only shows deviation from the selected course.

Forgetting that RMI readings are affected by compass errors, while CDI is not.

Misinterpreting the TO/FROM indication on the CDI as a bearing, rather than a course relationship.

Believing the CDI requires heading input—it does not; it relies solely on the selected course and received signal.

Test yourself

Example Exam Questions

Question 2Medium

Which statement best describes the main function of a Course Deviation Indicator (CDI) when used with a VOR?

Question 3Medium

What is a key operational difference between an RMI and a CDI?

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