The Leans is the most common type of spatial disorientation for aviators. Through stabilization of the fluid in the semicircular canals, a pilot may perceive straight and level flight when in actuality the plane will be banked.
The Leans is a type of vestibular illusion in flight which causes spatial disorientation. The process involves the semicircular canals of the vestibular system. The semicircular canals detect angular acceleration. In total, there are three semicircular canals: the anterior, posterior, and lateral canals. Each canal is filled with a fluid called endolymph and each canal arises from a small bag-like structure called a utricle. At the ends of each ducts, there is a saclike portion called the ampulla. Inside are hair cells and supporting cells known as the crista ampullaris.
Changing a person’s orientation will cause specific ducts to be stimulated due to these hair cells. When the head turns, the canals move but due to inertia, the endolymph fluid tends to lag and thereby stimulates the hair cells. This stimulation results in awareness of disorientation. After about 20 to 25 seconds the endolymph velocity matches that of the canal, which stops stimulation of the hair cells and causes obliviousness of the current state of disorientation. In addition, the canals cannot detect rotational acceleration of 2 degrees per second or lower. Therefore, a pilot may not notice a slow turn or a bank maintained long enough. After the pilot levels the wings, there is an illusion that the plane is banking too much. Thus, the pilot leans in the direction of the original turn in an attempt to allow straight and level flight.
Danger and risk
If a pilot does not notice the disorientation and continues to lean, the plane may over bank in the wrong direction and cause rolling. This is the most common spatial disorientation for pilot. 
- Saladin, K.S. (2011). Anatomy & Physiology. New York: McGraw-Hill. p. 605. ISBN 0-07-337825-9.
- Antunano, Melchor. "Spatial Disorientation". Medical Facts for Pilots. Atlas Aviation. Retrieved 8 December 2011.
- "Spatial Disorientation: Confusion that Kills". Safety Advisor for Air Safety. AOPA Air Safety Foundation. Retrieved 8 December 2011.