Richard Kaplan wrote:

Very few piston general aviation airplanes have dual alternators and you
could not even begin to consider a non-vacuum-system airplane without both
dual alternators and dual electrical buses. Even dual electrical buses can
fail and that is why business jets often have an emergency battery-powered
gyro which completely self-contained; these backup gyros can easily cost
over $7,000.

It can be a fraction of that if you don't mind yet another portable device
cluttering your cockpit:

http://www.icarusinstruments.com/microEFIS.html

People have also reported some success under the hood flying a plane using
the display on the Garmin 196. Of course, until people are forced to used
it in actual IMC, we won't know how well it really works in an emergency.

At those prices, it seems a lot more practical or at least economical in a
GA airplane to simply have a conventional vacuum AI backed up by a
conventional electric AI.

That's just it -- even my cheap little Warrior has a lot of options for
keeping the wings level in IMC:

Vacuum-powered AI and HI
Electic-powered TC
Battery-powered GPS
Magnetic compass

Of course, these become less and less useful as you go down the list (I
wouldn't be much on my chances with just the magnetic compass), but in real
life, but how much redundancy do you need before you've overdesigned the
system? As I mentioned earlier, I have not yet managed to find a single
example of a fatal accident caused by a vacuum-pump failure in a fixed-gear
plane flying IFR. There must be one or two, but it does not appear to be a
significant risk.


All the best,


David