Andy
Thanks for the info on the HSI. You are quite correct in that the device is a slaved unit. On the dash panel there is a switch that I've seen the instructor use only once or twice. One of the settings is 'slave' - I don't know the other but it may be manual or master, the switch and labels are tiny and on the right hand side.
When this switch is flicked, when on the ground, the HSI will auto align itself. We do a quick check with the magnetic compass and the surroundings then put it back into slave mode. I don't recall having to do anything in flight.
They are a great help, I don't know if I'd want to have the additional load of adjusting the DI with the compass every 10 minutes
but it is something I will have to be aware of when switching to other a/c. I know the school has other PA28s and Cessnas, I might request a lesson in one of these when I'm happy with my nav skills in order to get to know and love the DI.
MyData