Flight Control Tabs Type Activation Direction of Motion (in relation to control surface) Effect Trim Balance Servo Spring Opposite Opposite Opposite Same Opposite Set by pilot from cockpit. Uses independent linkage. Moves when pilot moves control surface.  Coupled to control surface linkage. Directly linked to flight control input device. Can be primary or back-up means of control. Directly linked to flight  control input device.  Located in line of direct linkage to servo tab. Spring assists when control forces  become too high in high-speed flight. Statically balances the aircraft in flight. Allows “hands off” maintenance of flight condition.  Aids pilot in overcoming the force  needed to move the control surface. Aerodynamically positions control surfaces that require too much  force to move manually. Increases force needed by pilot to change flight control position.  De-sensitizes flight controls. Enables moving control surface  when forces are high.  Inactive during slow flight. Anti-balance  or Anti-servo  Figure 1-69.  Various tabs and their uses. While in flight, it is desirable for the pilot to be able to take his  or her hands and feet off of the controls and have the aircraft  maintain its flight condition. Trims tabs are designed to allow  this. Most trim tabs are small movable surfaces located on  the trailing edge of a primary flight control surface. A small  movement of the tab in the direction opposite of the direction  the flight control surface is deflected, causing air to strike the  tab, in turn producing a force that aids in maintaining the flight  control surface in the desired position. Through linkage set  from the cockpit, the tab can be positioned so that it is actually  holding the control surface in position rather than the pilot.  Therefore, elevator tabs are used to maintain the speed of the  aircraft since they assist in maintaining the selected pitch.  Rudder tabs can be set to hold yaw in check and maintain  heading. Aileron tabs can help keep the wings level. Occasionally, a simple light aircraft may have a stationary  metal plate attached to the trailing edge of a primary flight  control, usually the rudder. This is also a trim tab as shown in  Figure 1-70.  It can be bent slightly on the ground to trim the  aircraft in flight to a hands-off condition when flying straight  and level. The correct amount of bend can be determined only  by flying the aircraft after an adjustment. Note that a small  amount of bending is usually sufficient. The aerodynamic phenomenon of moving a trim tab in one  direction to cause the control surface to experience a force  moving in the opposite direction is exactly what occurs with  the use of balance tabs.  [Figure 1-71]  Often, it is difficult to  move a primary control surface due to its surface area and  the speed of the air rushing over it. Deflecting a balance tab  hinged at the trailing edge of the control surface in the opposite  direction of the desired control surface movement causes  a force to position the surface in the proper direction with  reduced force to do so. Balance tabs are usually linked directly  to the control surface linkage so that they move automatically  when there is an input for control surface movement. They  also can double as trim tabs, if adjustable in the flight deck. A servo tab is similar to a balance tab in location and effect,  but it is designed to operate the primary flight control surface,  not just reduce the force needed to do so. It is usually used as  a means to back up the primary control of the flight control  surfaces.  [Figure 1-72] Ground adjustable rudder trim Figure 1-70.  Example of a trim tab.  1-32