Why are pilots under the illusion of gaining altitude without doing so?
See this link. It provides a great overview. The answer you want is in the otolith section. To paraphrase, it states that our otolith organs are designed to help us lean forward while running to maintain our center of gravity when in motion; however, in a plane this reflex can backfire by causing a pilot to believe he is going straight when in actuality he may be going downward.