The Pitch-Up Illusion
OTOLITH ORGANS
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The Otolith Organs work like a caterpillar oriented from the front to the back of the head. When you move your head the multiple legs of the caterpillar (hairs) are stimulated by gravitational forces enabling you to have positional awareness. Due to inertia, linear acceleration cannot be distinguished from the force of gravity. If an adequate horizon is not visible, forward acceleration will result in backward displacement of the caterpillar, giving the forceful illusion of a pitchup in attitude. An adequate horizon may not be available outside of the aircraft when you are looking into the sun such as might exist at dawn or dusk.
Despite being in level flight or shallow climb, upon acceleration in such a situation a pilot might strongly perceive that the aircraft is pitching up and respond by pushing the nose down steeply.