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. I also fly with the GPS on the HSI page to offer
yet another source of redundancy.

Personally, I think the instrument scan typically taught relies too heavily on
the AI given its relatively low reliability. Unfortunately, the alternative is
a scan that works a bit more like a partial panel scan using the AI as
supporting, not primary. Such a scan is much harder to master and requires
considerable finesse to keep from chasing the needles. It is not one I would
expect to be able to teach someone just learning to fly by instruments.


--
--Ray Andraka, P.E.
President, the Andraka Consulting Group, Inc.
401/884-7930 Fax 401/884-7950
email
http://www.andraka.com

"They that give up essential liberty to obtain a little
temporary safety deserve neither liberty nor safety."
-Benjamin Franklin, 1759