Dave Butler wrote in
:

Brenor Brophy wrote:

liberal snippage

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

True. I have a non-slaved NSD 360. If I start the engine and have
vacuum, the AI works properly, but the HSI will show a "heading" warning
flag, and will not function as a DG (ie, I can taxi around, making turns,
but the compass card on the HSI will not show the turns) until I apply
power to the avionics bus. It doesn't make any sense to me; I cannot
understand why a vacuum powered gyro will not function as a DG without
electrical power, but that's the way it is.