The Leans

There are several ways you can experience the “Leans”. Some may be perceptual, others may effect the vestibular apparatus which is part of your inner ear.

This is the most common illusion during flight, and is caused by a sudden return to level flight following a gradual and prolonged turn that went unnoticed by the pilot. The reason a pilot can be unaware of such a gradual turn is that after twenty seconds a standard rate turn of two degrees per second falls below the detection threshold level of the semicircular canals. 

When you level the wings of your plane after such a turn it may cause an illusion that the plane is banking in the opposite direction. In response, a pilot may lean in the direction of the original turn in a corrective attempt to regain the appearance of normal straight and level flight.

From a playground setting , after riding a merry-go-round slowly and then jump off, you attempt to lean in the opposite direction from the spin of the merry-go-round. Hey, whats the matter, you got a case “of the leans?”

In instrument conditions this may lead to the "death spiral" if the pilot does't have a instrument rating. It is extremely difficult to recover from this illusion if you don't believe your instruments or rely on flying by the "seat of your pants."

If you have regular instrument conditions for flying where you live, think about instruction leading to a instrument rating.