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 in a banked turn. This is caused by a quick return to level flight after a gradual, prolonged turn that the pilot failed to notice. The phenomenon consists of a false perception of angular displacement about the roll axis and therefore becomes an illusion of bank. This illusion is often associated with a vestibulospinal reflex that results in the pilot actually leaning in the direction of the falsely perceived vertical. Other common explanations of the leans are due to deficiencies of both otolith-organ and semicircular-duct sensory mechanisms.
Physiology
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 duct, 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 because of its inertia, the endolymph fluid tends to lag and thereby stimulates the hair cells. This stimulation results in awareness of angular acceleration in that plane. After about 10 seconds the endolymph velocity matches that of the canal, which stops stimulation of the hair cells and reduces the person's awareness of rotation. In addition, the canals cannot detect rotational acceleration of approximately 2 degreesper second or lower; this is the detection threshold of the semicircular canals. 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 in the opposite direction. As a response, the pilot often leans in the direction of the original turn to attempt to correct and regain his/her perception of the correct vertical position. The leans may also be caused by peripheral visual orientation cues that become misled.
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 pilots. In all cases of spatial orientation, pilots must be able to rely on their flight instruments when making control inputs to override false sensations.
