I presume you mean that your DG and AI are vacuum driven. Virtually all
TC'c are electric.

We have a backup electric AI in our Arrow, and it works very well. To
extend its life, we have an on/off switch for it, and only turn it on when
in IMC. It is mounted in an otherwise unused space on the right side of the
radio stack. But many owners are ditching their ADFs these days, and the
hole where the ADF head was makes a good home for an electric AI. Ours has
not required any maintenance in 1700 flight hours over 8 years. I think
installed cost for such an instrument today would be around $2300 or so.
One big advantage of the electric AI is that it provides backup for the
vacuum AI instrument as well as the vacuum system as a whole.

Another thing to remember for vacuum system backup is that most portable GPS
units have an HSI function that works quite well as a DG. Not perfect,
since it displays magnetic track rather than magnetic heading, but a whole
lot better than nothing.

Finally, vacuum failure in IMC is an EMERGENCY, and should be treated as
such. As soon as you can do so safely, let ATC know of your problem. Know
ahead of time whether your AP will be affected. If so, turn it off
IMMEDIATELY upon detecting vacuum failure. If not, use it to help keep
wings level.

--
-Elliott Drucker