A spirit level does not tell you anything. The had existed in planes almost since planes exist.

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.