Victor wrote:

The one which is better depends on which kind is made by the person
writing
the article.

Well, I was under the impression that the article I'd just read (in Aviation
Consumer) was written by a disinterested third party. I could be wrong, of
course laugh.

There are good and bad autopilots of both types. Vacuum or
electric AI for attitude based autopilots doesn't really matter.

I don't see that as being the case, for reasoning I explain below.

[...]
But to compare the sources of power for those, an electrical
bus with multiple sources of energy (alternator, battery) is statistically
more reliable than a vacuum pump.

That almost doesn't matter as I see it. The AP is electrically powered.
Lose electricity, and you've lost your AP. In that case, that the AI has
failed has no impact upon the AP.

On the other hand, for an AP with a vacuum driven AI as a sensor, loss of
either electrical power or vacuum disables the AP. So regardless of the
relative reliability of vacuum vs. electric, the AP on a vacuum AI is less
reliable than the AP on an electric AI (or electric TC if rate-based).

No?

[...]
Attitude based autopilots depend on electrical signals from an attitude
[...stuff I didn't know and am happy to learn snipped...]

[...]
... and you should ask Chelton about the AP-3C.
I think you'll find that in most or all cases, the autopilot will cease to
function if the attitude sensor has failed, including roll steering modes.
Let us know what Chelton says.

Will do.

[...]
If any autopilot experts out there think I've said something erroneous,
feel free to correct me so we can all learn.

Please.

- Andrew