It's true. Several years ago when we (the group that owned the Bonanza 36 I
was a member of) decided to replace our maintenance-hog Narco HSI, we chose
the King KNS-55 HSI because it worked on the electrical system only (no
vacuum). That meant that a vacuum failure left the HSI, turn coordinator,
and S-Tec autopilot working which would be pretty much a non-event.
Electrical failure would leave the backup DG (the DG that was replaced by
the original HSI was still in the panel) operating.

"Dave Butler" wrote in message
...
This quote is from the Avionics West
article at http://avionicswest.com/articles/kno..._autopilot.htm
This HSI, slaved or not, MUST have BOTH
vacuum and electrical inputs in order to operate. In other words, if you
lose vacuum or the electrical system, this HSI compass card will quit!

Can anyone confirm the statement above from Avionics West? I consider them
a
reliable source of information, but this seems counterintuitive to me. In
case
of electrical failure, why is the heading information not still available?
Is
the compass card not mechanically connected to the gyro?

Obviously, I can see that the navigation information would not be
available in
case of electric failure, but the quote specifically says the compass card
will
quit.

Thanks,

Dave