OK, this might be a stupid idea, but what about a simple old spirit level, a fancy oil based one that will never freeze, mounted to the side of the cockpit to give the pilots a emergency true level backup when all else fails?
Gyroscopes work, which is why artificial horizons are a thing. But an artificial horizon doesn't show anything except where your nose is pointing. You need all sorts of other sensors to tell you which direction you are actually going.
You can not differentiate a force coming from plane acceleration and deceleration from gravity(which also comes from some kind of apparent deceleration). People in the space Station are being affected almost with the same gravity that you or me.
In fact, a spirit level or your ears provide you with wrong information all the time with no visibility.
With planes, the first thing you have to learn is not trusting your senses.