As another person noted, the turn coordinator in the SR20 and SR22 has a pair of 9V batteries to back them up just in case everything else goes dead.
And another person said, have a hand-held to back up your communications radios. I do this and once used my hand-held when the a/c’s radios jammed up. So I think this is a great idea.
Something that I haven’t seen posted yet is that one can use the HSI page on your GPS, which can be a battery-powered GPS, as backup for the heading indicator. I never fly IFR w/o my Garmin GPS (was a 195, now a 295). I fly club planes that don’t have any other GPS, or at most have an ancient non-map difficult-to-use, VFR, GPS. I have long thought that if I had a question about my heading indicator’s health, I’d look at the simulated HSI on the Garmin and compare, or just use the simulated HSI. They don’t show quite the same thing, one shows which way the A/C is pointed and the other shows which way it is going. The difference is wind and slip. Given that you’re not slipping in IMC, the difference is wind. And though this can be a significant difference, I don’t think that it would have a practical effect because you chose headings based on vectors, which controllers adjust based on observed course, or on what works to get to a fix, which the pilot adjusts as well.
Does this make sense, or am I all wet?
I am thrilled to be done with vacuum systems.
I totally agree with the comparison with ADF/NDB’s. I am curious to see the wiring diagram for the SR22 to see where the common points of failure are and how those are handled.
In the Duchess I fly, there are two busses, and each has about half the nav and have the comm’s. I’d guess that one of the '430’s in the SR22 is connected to one bus and the other '430 to the other bus. I’d further guess that there is a switch to connect or split the two busses, to allow for powering the whole a/c from one power source (switch on) or for isolating failures (switch off). But this is pure speculation. I wish Cirrus would put more information on their web site.