The artificial horizon gives the pilot an indication of the aircraft’s attitude in pitch and roll. It uses an Earth gyro which rotation axis is maintained vertical by Earth’s gravity. Therefore, the rotating mass of the gyro spins in the horizontal plane giving stable lateral and longitudinal references.

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To maintain the rotor’s axis vertical in flight, the air driven artificial horizon will exhaust air through four slots, each of them being half covered by a pendulous vane. The four jets of air are equal in strength and will flow out the vanes in different directions when the airplane is in straight and level flight. This will keep the axis vertical and no precession will occur. However, if the rotor moves from its vertical position, one slot will completely obstruct the vane, while the opposite slot will leave the vane fully open. The resulting unbalanced airflow streaming out of the vanes will precess the gyro and return the gyro axis to the vertical.

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Additionally, the vanes will be affected by any acceleration, not just that due to gravity. For instance, when an aircraft accelerates in straight and level flight, or during the take-off run, a false nose-up indication will occur. The pitch error is due to the effect of acceleration on the lateral pendulous vanes.