An antiservo tab is a small, hinged surface set into the trailing edge of a light-aircraft stabilator. It moves in the same direction as the stabilator. Because the tab works against the main surface, it simultaneously relieves control pressure and decreases sensitivity, making the large elevator easier to hold at the desired deflection.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is a stabilator anti-servo tab?
A stabilator anti-servo tab is a control surface fitted along the trailing edge of a horizontal stabilator on a light aircraft. Control input causes the anti-servo tab to deflect in the same direction as, but further than the stabilator, and movement of the anti-servo tab is controlled by a trim wheel just like the elevator trim tab.
The tab on the trailing edge of the horizontal stabilator is called an anti-servo tab. The anti-servo tab is a secondary flight control surface that helps reduce control pressure and prevents over-controlling of the aircraft's pitch. The anti-servo tab is a trim device to maintain the stabilator in the desired position and relieve control pressure. It moves in the same direction as the control surface, making the movement of the control surface more difficult and requiring more force applied to the controls by the pilot. The primary function of the anti-servo tab is to increase travel resistance as the stabilator moves out of trail, thereby decreasing the sensitivity of the stabilator. Airplanes with an anti-servo tab have a horizontal stabilator instead of a horizontal stabilizer.
How does a stabilator trim tab work?
A stabilator trim tab works by reducing the amount of pilot input required. A stabilator trim tab is a small movable surface attached to the trailing edge of the stabilator, acting as a secondary flight control surface to relieve pilot workload by adjusting neutral position of the entire stabilator. When the pilot rotates the trim wheel in the cockpit, or actuates the electric switch on the yoke, the trim tab deflects up or down. Deflecting the tab down pushes the trailing edge of the stabilator up, causing the nose to rise, while deflecting the tab up pushes the trailing edge down and lowers the nose. This deflection creates an aerodynamic force that holds the stabilator in the new trimmed position without the pilot needing constant yoke pressure, reducing physical and mental fatigue. The same mechanism is used whether the command comes from the manual trim wheel, an electric trim system, or a fly-by-wire computer, and the tab always moves opposite to the desired stabilator movement to produce the necessary balancing force.
