Pretty hard to imagine how a faulty HSI would cause loss of
control-even in inadvertent IMC....

You don't fly IFR do you? :-))
An AI, or HSI usually fails slowly. It just slowly rolls over to one
side or the other. If the pilot doesn't have a good scan technique
he just rolls with it.

Yes, but probably not as much as you.

Agreed about the AI, disagree about the HSI.

Seems to me there's a BIG difference in the implications and dangers
between AI failing, which can cause loss of airplane ATTITUDE control,
and and HSI or DG failing, which could cause some loss of NAVIGATIONAL
awareness.

If your Horizontal Situation Indicator gradually turns to an incorrect
heading, do you think you might gradually, unknowingly, increase your
bank angle trying to "follow" it? Or maybe the CDI going off-scale
would induce you into a 60-degree bank trying to "catch it"? I think
not.

--
Cheers,
John Clonts
Temple, Texas
N7NZ