In addition to the primary flight controls, there is, on most modern airplanes, a group termed "secondary controls." These include trim devices of various types, spoilers, and wing flaps.
 

Trim tabs are commonly used to relieve the pilot of maintaining continuous pressure on the primary controls when correcting for an unbalanced flight condition resulting from changes in aerodynamic forces or weight (Fig. 2-6). Some types also help to actuate the main control surfaces by exerting force on the main surface, thus reducing the amount of force the pilot must exert on the controls to maneuver the airplane. The trim tab is mounted on or attached to the primary control surfaces to provide easier movement or better aerodynamic balance of the surfaces. Most airplanes, except a few of the very oldest and lightest types, are equipped with trim tabs that can be controlled from the cockpit. On those other types, the tabs are manually adjustable only when the airplane is on the ground. Spoilers, though found only on certain airplane designs and most gliders, are mounted on the upper surface of each wing. Their purpose is to spoil or disrupt the smooth flow of air over the wing to reduce the lifting force of the wing. This provides the pilot with a means of increasing the rate of descent without increasing the airplane's speed.