On what principle does the attitude indicator operate?

The attitude indicator operates on the principle of rigidity in space, which also encompasses the concept of gyroscopic stability. This principle is based on the behavior of a spinning gyro. When a gyro is spinning, it tends to maintain its orientation regardless of the movement of the surrounding structure. As the aircraft changes its orientation, the gyro remains fixed relative to a stable frame of reference, thus providing the pilot with a consistent display of the aircraft's attitude.

When the aircraft banks, climbs, or descends, the gyroscope inside the attitude indicator remains aligned with the horizontal plane, allowing it to show the pilot the aircraft's attitude in relation to that plane. This reliable indication helps pilots maintain control during instrument flight, especially when visual cues are limited or absent.

The other choices, while related to the operation of flight instruments to some degree, do not specifically capture the fundamental mechanism behind how the attitude indicator provides accurate readings of an aircraft's orientation.