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SOMATOGYRAL ILLUSION (THE 'LEANS')

Somatogyral illusion or the 'leans' is an illusion experienced by pilots during prolonged turns or rotation, where the inner ear’s balance mechanism can lead to a false sense of the aircraft’s orientation after returning to straight flight. The hair cells in the inner ear are responsible to sense the body's orientation. The 'leans' can occur under two common circumstances in flight:

1. Wings level after a gentle slow turn: A pilot begins a very gentle and slow turn where the movement of the inner ear's liquid is too slight to activate the hair cells. Consequently, although the aircraft is turning, the balance mechanism senses no change. When the pilot returns to straight and level flight afterward, the inner ear perceives this as a turn from straight and level flight. The pilot may feel as if the aircraft is turning in the opposite direction than before, even though it is actually straight and level.
2. Wings level after a prolonged turn: It happens during a prolonged turn where the aircraft remains in a banked position long enough for the hair cells in the inner ear canals to return to their neutral position. Upon rolling out of the turn, the movement of the hair cells gives a false sensation of turning in the opposite direction, despite the aircraft being level in reality.

The 'leans' illusion → The pilot may feel as if the aircraft is turning in the opposite direction than before, even though it is actually straight and level.