If the aircraft enters an unnoticed, prolonged turn gradually, then suddenly returns to level flight, the leans may result. The gradual turn sets the fluid into the semicircular canals into motion, and rotational acceleration of two degrees per second (or less) cannot be detected. Once the aircraft suddenly returns to level flight, the continued fluid motion gives the sensation the aircraft is banking in the opposite direction of the turn that just ended; the aviator may attempt to correct the misperception of the vertical by banking into the original turn. The leans is considered the most common form of spatial disorientation.