What causes an airplane to pitch nose down when power is reduced and controls are not adjusted?

When power is reduced in an airplane's flight, the primary reason for the nose to pitch down is due to the reduction in downwash on the elevators. In level flight, the airflow over the wings creates a downwash that impacts the elevator's effectiveness. When power is decreased, the speed of the aircraft decreases, which in turn reduces the amount of airflow over the wings and the elevators.

The elevators rely on downwash to produce lift. As the power decreases, the airflow changes, and thus the effectiveness of the elevators diminishes, leading to a reduction in their ability to maintain the nose position. Consequently, without any control input to counteract this effect, the airplane naturally pitches nose down.

This phenomenon is crucial for pilots to understand, as failing to adjust controls when power is reduced could lead to unintentional descent or loss of altitude.