Ray Andraka wrote:
There are several things you can add to help with the identification, In my
plane I have a low vacuum warning light (part of the precise flight backup)
mounted between the AI and DG. The AI is one of the sigmatec ones with a vacuum
flag, so that if vacuum is lost in the instrument but not in the system I still
know about it right away. These warnings cover identification of the more
common cause of loss of the AI. The other failure mode would be failure of the
gyro, in which case I don't believe you get the insidious gradual spin-down like
you do with loss of vacuum.

Ray,

I'll speak to the latter.

A failing horizon gyro may not "spin down". But it can still
be insidious. Example: our AI had a period where, in level flight,
it would jump up and indicate a rather nose-high attitude. Fail
to catch it and you'd be in a rather steep dive. Then it would
go back to normal. Then jump up again....finally it broke and
unmistakably started spinning in a nauseating fashion, but the
"breaking" process could easily have caused a loss of control for
a pilot w/out a good cross-check (our failure happened VMC)

Cheers,
Sydney