Jonathan Goodish wrote in message ...
In article ,
(Robert M. Gary) wrote:

Let's see, I have 4 instruments in a basic IFR panel that can indicate
the position of the wings. AI, DG, TC, and wet compass. If my AI
starts to roll over 5 or 10 degrees and I level it, that's not going to
put me in a death spiral, but I should notice heading changes regardless
of what the TC is doing. If the ride is so rough that you can't obtain
meaningful data from any of the other instruments, then you're not the
guy I'd send to buy my lottery ticket.

Its very common for me to be in actual conditions that are bouncy
enough that the TC isn't going to do anything other than keep me
upright. I'd be banking back and forth like a mad man as it flopped
around. The compass is pretty useless, it just spins back and forth.
That's the problem with most IFR, its usually very, very bouncy.

If I had to pick between having the TC, DG, altimeter, and airspeed as
my AI backup, or picking a second AI, I'd take the data from 4
instruments rather than one.

But its not one, its two. If you look at the two AIs and they disagree
you will say to yourself "Hmmm, something is wrong". The ability to
say that is HUGE. A real AI failure is so mild that you probably would
never notice. If just using the TC, DG and compass worked, you
wouldn't hear about people dieing after partial panel situations. You
wouldn't see big warnings on vac pumps. The airlines decided to get
extra AIs and dump the TC a while ago.