Just a quick comment on the redundancy issue.  I read the comment that a vacuum system is not a good candidate for a back-up because of the known poor durability of most vacuum pumps.  But that doesn't by itself reduce reliability.  What happens if one has two electronic systems and both get disabled by a lightning strike?  Say there is a one in a million chance of getting struck by lightning and there is a 1 in 2 chance that both systems will be disabled.  That says that there is a 1 in 2 million chance that you will have a really bad rest-of-the-day.  Now put in a vacuum system that will absolutely not fail as a result of a lightning strike.  There is, say, a MBTF of 500 hours for the vacuum pump(I think it is more like 1,000 hours).  It will take maybe 30 minutes to get on the ground after the lightning strike takes out all the electronics.  What is the odds of the vacuum system failing in that 30 minutes?  Presumably 1 in 1,000.  So the odds of the electronic system failing AND the vacuum system failing in the next 30 minutes is 1 in a million times 1 in a thousand, or 1 in a billion.  You only need the vacuum system to keep working for the time it takes you to get to the ground.

So the all-electronic system will have a 1 in 2 million chances of killing you, while the combination of an electronic primary and the not-as-reliable vacuum system has a 1 in a billion chance of killing you.  500 times better than the all-electronic system.

That's why I have an engine-driven vacuum pump and a vacuum AI in mine, along with the electronic EFIS.

Gary Casey