A spirit level does not work in an accelerating frame of reference. From the point of view of a spirit level, there is no difference between straight and level

A spirit level does not work in an accelerating frame of reference. From the point of view of a spirit level, there is no difference between straight and level flight (1g due to earth's gravity) or a spiral dive (>1g due to a spiraling turn downwards). 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.