I have to mention this because it really touches a nerve as for why I chose so late in life a professional flying career. So I’m doing my type rating right now, and our TRI is a very nice elderly gentleman, one of only two TRIs left for the type in Europe.
Obviously he’s very knowledgeable, but what fascinates me the most are the stories he tells us about his earlier life as pilot. What an adventurous life this man must have had. Just yesterday he mentioned “Oh, yeah, I remember when I flew for Henry Kissinger…” Or casually telling us about flying the DH Vampire, the Draken and whatnot. Or being a crewmember in the Catalina…
These stories are a lesson on their own!
A Turn & Bank indicator can be a very valuable IFR instrument, but often gets torn out these days and replaced with backup AI’s. Howard Stark clarified his method on how to fly level using this and one more instrument alone in his 1931 book:
1. Center turn indicator with rudder only.
By using rudder you’ve now stopped the turn, but you don’t know if you’re level on the wings, which can then be derived from:
2. Center ball with ailerons only.
Now that you’ve eliminated any acceleration forces on the ball, it’s just a spirit leveler. You can now level wings using that.
3. Center climb indicator (VSI) with elevator only.
Self explanatory. Would work with altimeter as well, just not as instant.
Now, this does not work on Turn Coordinators as they blend roll and yaw rate, only works on the older style T&B’s. Some old school IFR pilots have sometimes exchanged the newer TC for T&B’s to counter this.
Here’s a link to Howard Starks 1931 book on Instrument Flying. It’s very interesting reading:
Blind or Instrument Flying by Howard Stark
Interestingly, this is also true in an engine out scenario in a twin (one of the last chapters in the above link). His method works equally well there. On top of this, the T&B indicator is also the only instrument that will tell you have to get out of a spin in IMC when an AI would just tumble.
Taking all this in, I nominate the T&B as the most important IFR instrument to have. Who’s with me? 
