In article ,
(Robert M. Gary) wrote:
Because, a real world situation is that the AI dies very, very slowly.
There isn't that immediate "hmm, somethings wrong" like with a dead
vac (alarms going off etc). The TC has enough bounce in it that there
is really no way to detect small differences like 5 degrees when you
are in real IMC. Once you start bouncing around in the clouds the TC
starts bouncing back and forth. It will keep you upright but is far
from close enough to determine 5 or perhaps even 10 degrees off. The
DG you may or may not notice. In anycase two AIs being off is a pretty
quick and certain thing to notice.

Couple that with the fact that you have to decide which is right, the
TC or the AI. If you have 3 its easier to pick on that might be bad.


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.

Bottom line is that a scan should include all instruments. The more
data you incorporate into your scan, the more data you'll have to work
with if something looks amiss. The TC isn't the only practical backup
for the AI and, in fact, provides no pitch information anyway; for that,
you'll have to include airspeed and altimeter.

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.

Maybe it's just me. Wouldn't be the first time.



JKG