Harmon Technologies, Inc.

Compass slave controls

The compass slave controls indicate the status of how well the HSI indicator (in front of the pilot) is tracking the remotely mounted magnetic compass sensor.  Because of the normal characteristics of magnetic compasses, they tend to swing back and forth.  This makes them difficult to use for precise navigation.  The HSI display on N4907J handles this be gyro stabilizing the display.  The small meter on the slave control shows how well the magnetic compass sensor and gyro display agree.  Normally there is slight wavering back and forth but the needle indicates agreement.  When running normally, the slaving circuits will slowly adjust the gyro to keep in in sync with the average compass reading (eliminating the normal fluctuations).  Should the slaving system malfunction, the meter will indicate a permanent difference.  Under these conditions, the pilot can use the switches on the slave control to disconnect the HSI gyro and can then manually set the gyro periodically using against the aircraft magnetic compass at the top center of the panel.  Because all gyros creep slightly over time, this setting would then be checked every 20 minutes or so to make sure that it is staying accurate.

Such a failure would be very rare.  Most airplanes of this class don't have a slaving system and the pilot always has to manually set the gyro.  The slaving system reduces pilot workload and provides optimal accuracy of the compass display.  Slaving systems similar to this are found on almost all large aircraft (such as airliners).

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