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Is an RMI the same as a Remote Indicating Compass ?

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Is an RMI the same as a Remote Indicating Compass ?

Old 1st Mar 2021, 10:01
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Question Is an RMI the same as a Remote Indicating Compass ?

Sorry for asking a silly question, but as the saying goes, it’s better to ask a stupid question than make a stupid mistake. So..After flying the big jets for the last 30 years I’m trying to fly light aircraft again. I’ve been flying the big jets on my EASA ATPL and now I’m trying to get my Aussie Licences back into currency, including renewing my Aussie Instrument Rating. It’s no simple task.I’ve always known an RMI to be a directional gyro slaved to a flux valve so that it always accurately points North.But the Aussie Civil Aviation Orders don’t talk about RMIs.They talk about Remote Indicating Compass.Are they the same thing ? Cheers,

Last edited by Wings; 1st Mar 2021 at 10:03. Reason: Spelling errors and punctuation corrections.
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Old 1st Mar 2021, 10:25
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There is a decent explanation here: https://www.flight-mechanic.com/gyro...ating-compass/
Based on that I would say that you are mostly correct, but an RMI does not contain a directional gyro. The heading part of the RMI is derived from the flux valve. To break things down: a remote indicating compass is a magnetic compass that uses a sensor mounted outside of the instrument (the remote part) to obtain magnetic heading information and you can combine this with the ADF to get an RMI. This same remote indicating compass could also be combined with some other parts to form an HSI.
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Old 1st Mar 2021, 10:46
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What I understand an RMI to be is an indicator which has magnetic information fed to a compass card which is always aligned with North and on the face of which are needles which can display VOR or ADF bearing information. This means that you actually see the magnetic bearings of the stations, no mental gymnastics required. So easy to use.
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Old 1st Mar 2021, 11:02
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My understanding is: a Remote Indicating Compass is a system that provides magnetic heading information to one or more instruments, utilising a flux valve located remotely to minimise aircraft induced magnetic anomalies, whereas a Radio Magnetic Indicator is a specific instrument used to display ADF and/or VOR bearing information, utilising magnetic heading data supplied from a Remote Indicating Compass system.

This site explains it well, in particular the last paragraph:

https://aviationtheory.forumotion.ne...cating-compass
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Old 1st Mar 2021, 15:38
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It's pedantic really. How does an RMI work? The answer is very well, thankyou, but an HSI is better! Actually I'll happily take one of each over a sodding OBS and ADF any day!
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Old 1st Mar 2021, 15:38
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I think the RMI is just the instrument that displays compass heading and radio information (eg VOR and/or ADF) in the same place. The compass feed to the RMI can be from a remote indicating compass, a remote indicating gyro-compass, or a feed from INS/IRS/AHRS, whatever.
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Old 1st Mar 2021, 16:04
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Correct. Those of us who remember the relative bearing indicator for ADFs and adding the compass heading to get the magnetic bearing will always be thankful for the radio magnetic indicator RMI.
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Old 2nd Mar 2021, 03:57
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Yup. Fixed card ADF was no fun at all. It was a required recurrent training exercise at my carrier in the 1970s and was not anticipated with pleasure - especially if the examiner dialed in some significant wind.
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Old 2nd Mar 2021, 10:28
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From my recollection, RMI could optionally have the 'slaved gyro' (aka remote indicating compass).

The primary benefit was that the ADF bearing was no longer relative, but an actual bearing, as long as the gyro was in sync.
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Old 2nd Mar 2021, 12:17
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The RBI headache of 50+ years ago returns! I remember being taught (in the ground school at 5 FTS RAF Oakington) on the Vickers Varsity how to employ the "parallel needles" technique which involved setting the required bearing on the compass (G4B?) heading selector and turning (drag or push) until the ADF needle was parallel to the heading selector and then turning onto the heading plus or minus the drift - of which there was no indication. Sorry to ramble on about such a distant memory.
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Old 2nd Mar 2021, 13:48
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I too struggled with NDB tracking on very basic crude instruments.

When I got onto an aircraft on which you could overlay a bug or the course pointer and the ADF needle onto a moving magnetic compass card all on the same instrument - NDB tracking became so easy ! All one had to do was glance at the composite display to see if everything was still lined up, and if the ADF needle was even 3/4 of a needle's width out one side or the other, you simply moved the HDG bug one degree in the appropriate direction to bring it back.
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