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About that Slaving Control and Compensator Unit for the HSI

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General Aviation

When do I ever need to switch from slave to free mode? I can't think of any other scenario except for the flux valve being inoperative or getting interference that will cause the compass card to read incorrectly. 

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1 Answers

  1. Skyfox on Jul 30, 2016

    I’m not terribly familiar with the remote indicating compass system but I’ll try to help out (I’ve flown with an HSI before but it was gyro only, not the RIC style). With that system, when it’s in slave mode, the flux gate thing is sending information to the DG, HSI, or RMI for automatic alignment with magnetic north. If there’s an error between the magnetic compass and the HSI or whatever, put it in free mode to use the clockwise and counterclockwise buttons to adjust it, then put it back in slave mode. If the thing is just plain not working right, keep it in free mode so it acts as a standard heading indicator. In that case, the clockwise/counterclockwise buttons can be used to correct for precession just as you would when using the compass card adjustment knob.

    It’s described in chapter 5 of the Instrument Flying Handbook (FAA-H-8083-15B) but it’s clear as mud and could use some revision for a better explanation of how it works.

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