Illusions in aviation are caused when the brain cannot reconcile inputs from the vestibular system and visual system. For example the slope of a runway or surrounding terrain can deceive pilots, causing them to misjudge their altitude during approach. A downsloping runway may create the illusion of a lower altitude, leading to a higher approach, while an upsloping runway can make the aircraft appear higher, resulting in a lower approach. This phenomenon applies when flying around sloping terrain