Which statement about the Leans is true?

• A. Pilots will lean opposite of the correction or if straight and level the perceived vertical plane
• B. Angular acceleration greater than 10 degrees per second is not detected
• C. It is always obvious without visual cues
• D. It only occurs under instrument conditions

Answer: (B)

The Leans is a vestibular illusion caused by the brain not perceiving slow roll changes. Our inner ear’s semicircular canals detect angular velocity (rate of rotation), not a steady bank angle. When you roll slowly and then return to level, the canals may not signal a continuing turn, so you feel upright even though the aircraft is still banked. When you stop the roll, your sense of motion can lag, and you may tilt your body toward the outside of the turn or feel as if you’re turning the opposite way. This perceptual limitation—that small or slowly changing angular motion isn’t reliably detected—explains why this illusion can occur.

The statement about angular acceleration not being detected reflects the idea that the vestibular system isn’t sensitive to subtle changes in motion in the same way it is to certain rates of rotation; perception is dominated by angular velocity signals, and small or slow changes can go unnoticed, which is why the Leans can sneak in. This makes it the most relevant descriptor among the options.

The other ideas aren’t as accurate: the illusion isn’t always obvious without cues, it can occur even with some external cues, and it’s not limited to instrument conditions.