Why are pilots under the illusion of gaining altitude without doing so? We had a lecture about the balancing systems of the body (mainly the otolith organ / semilunar canals) in which a case was described where a pilot crashed into a ship. He was supposed to keep his altitude and had not been looking at his instruments, but was guiding the plane by feeling. However, he was under the illusion that the plane was gaining altitude when it wasn't and so steered it downwards, dropping and eventually crashing into a ship which happened to be at that particular spot that moment. Since the otolith organs detect de- and acceleration in all directions (that just comes from how the mechanism works), if the pilot was flying at steady altitude, why would they signal that he is rising? (Will add an article later if I find one.)

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.