You have three sources of bank information in a typical panel: the attitude
indicator, the turn coordinator, and the heading indicator. If two agree and
the third does not, it is faulty. Add a fourth source and it makes
elimination that much easier.

Bob Gardner

"Steve House" wrote in message
...
I've been reading with interest the several threads where a number of
people
have strongly pointed out the advantages of a backup electric AI to
supplant
a vacum driven main AI. But I'm reminded of the saying "A man with a good
watch always knows what time it is. A man with two watches is never
sure."
So I'm toodling along in IMC with no outside horizon reference and I see
my
two AIs don't agree with each other. How do I determine which to trust?
If
I had a third, I could go with a 2 of 3 voting strategy of course, but
with
only two, what do you do to decide which is operating properly and which
one
has faulted? Obviously I can look for consistency with other
instruments -
does my DG or Turn indicator show I'm turning, does the VSI show a climb
or
descent - but what would be the best strategy given the various ways
vacuum
or electric driven instruments can fail?