Selects the Mode in which the HSI operates.

The orientation of the HSI and RMI compass cards is controlled by a remote Gyro located in the tail fin. A flux valve (also in the tail) detects the earth's magnetic north, and the Synchronizing System (of which the above is the control panel) continually, though quite slowly, aligns (or perhaps one should say re-aligns) the Gyro with Magnetic North as ascertained by the Flux Valve.

The process of synchronization as carried out in the Pre Flight Checks is effected using the Slave/Free Switch (which lifts "up" before its position can be changed.

To Synchronize the HSI, Set the top switch to Free, and using he bottom switch, motor the HSI card either Clockwise or anti clockwise so that the white arrow in the window aligns with the white mark beneath it. (The two are shown as aligned in the view above, but it is just possible to see that the needle is lying above the Mark).

When the two white marks are aligned, reset the top switch to Slave, and the HSI (which is gyroscopic and not of course magnetic) should thereafter give an accurate indication of Magnetic North unaffected by any precession that may occur.

Note that there are procedures for dealing with HSI Slaving failures. Refer to the Check List, for emergency procedures, and instructions on how to revert the HSI to a conventional DI.