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What do you call an instrument that can help a pilot fly along a predetermined flight path (e.g. path thru GPS coordinates), showing them when they stray a little so they can correct? Not a control system, but just a visual feedback display. I want to say I've seen something like it before, that had two panels each with a plane symbol and "reticle", but I would like to know what it is that I'm describing. Thanks

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    $\begingroup$ If you talk about the display it may be the Navigation Display. The system calculating the legs between waypoints may be the FMS (Flight Manager System). All of that is independent of the Autopilot in a sense that Autopilot is using the information from FMS in some planes but both of them may live without the other. $\endgroup$ – Chris Lau Jan 7 at 21:16
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    $\begingroup$ Are you perhaps thinking of the flight director? That's a component of the visual display, but it works in connection with the FMS and other navigation systems to know what the predetermined flight path is. $\endgroup$ – Zach Lipton Jan 7 at 21:21
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What you are describing is a Course Deviation Indicator or CDI.

enter image description here

These are fairly standard across most aircraft however they may also be combined with a heading indicator resulting in an Horizontal Situation Indicator. There are also variants that have an aircraft symbol such as you describe.

Depending on how your aircraft is setup these units can be referenced off a radio beacon such as a VOR an ILS system for approaches or a programed GPS route.

This video covers some basic operation if you are curious.

A lot of GPS units with moving maps are generally able to display the info you describe as well.

enter image description here

There are even various apps that allow you to display such info on your tablet or smart phone

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