What is the definition of the "Leans" somatogyral illusion?

The "Leans" somatogyral illusion occurs when a pilot experiences a conflict between the vestibular system’s signals and the body’s perception of orientation. This particular illusion is typically caused by a slow or subtle roll that is below the threshold of detection, specifically in the range of 0.14°/sec² to 0.5°/sec². When there is a slow roll in one direction, the pilot may not initially perceive this motion. Upon inadvertently returning to a level position or correcting with a roll in the opposite direction, the pilot may feel as if there is an actual roll when, in fact, the aircraft is level. This can lead to significant disorientation, as the pilot's body becomes accustomed to the initial undetected roll and misidentifies the true orientation of the aircraft.

Understanding this phenomenon is crucial for pilots as it highlights the importance of relying on cockpit instruments rather than solely on sensory inputs, particularly in conditions where subtle movements might not be perceived accurately.