I believe TC type instruments (tied gyros in general) are less susceptible to failure than AH instruments (earth gyros), and are designed to make it easy to detect a failure when it does occur. We've all flown VFR with slightly wobbly AH's, but hopefully less often just ignored a flagged TC or T&S.
Ted, I'm not sure you're considering the two main implications here. Disregard the odds of the instrument failing just as you enter IMC, and consider:
- It is cheaper to maintain a TC over the long term, than an AH, and
- A pilot is more likely to detect (and have repaired) a failed gyro straight away if it has a simple failure flag on it, as opposed to being a little wobbly at times.
One is important to the operator, and the other to the regulator. Current law covers both bases.
In terms of safety, once in IMC, an AH is safer than a TC. Difficult to argue otherwise I think Ixixly...