Radio Navigator
Glossary

HSI, CDI and RMI

Direct answer

A CDI shows how far you are from a course you selected. An RMI shows a needle pointing directly at the station. An HSI combines a rotating heading card with a CDI, so course and heading are read together.

Also written as: HSI vs CDI, RMI vs CDI, course deviation indicator.

The distinction is about what question each instrument answers. A CDI answers "how far off my chosen course am I", and it needs you to have chosen a course for the answer to mean anything. An RMI answers "which way is the station from here", and needs nothing set at all: the needle simply points.

The HSI is the combination that makes an instrument scan quick. Because its card rotates with heading, the course pointer sits at its real orientation relative to the aircraft, so the deviation bar is always on the side you must turn toward. That removes the reverse-sensing trap that a fixed-card CDI leaves open, which is why the HSI is the instrument most IFR training is built around.

Where this comes up

Sources

  1. FAA Instrument Flying Handbook (FAA-H-8083-15B), Chapter 9 (Navigation Systems). CDI, RMI and HSI presentations and how each is interpreted.

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This article is for study. Follow the instructions from your qualified flight instructor. Use current charts, procedures, and the regulations from your aviation authority.

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