surfaces are signaled by electric input. In the case of hydraulic  system failure(s), manual linkage to a servo tab can be used  to deflect it. This, in turn, provides an aerodynamic force  that moves the primary control surface.  A control surface may require excessive force to move only  in the final stages of travel. When this is the case, a spring  tab can be used. This is essentially a servo tab that does not  activate until an effort is made to move the control surface  beyond a certain point. When reached, a spring in line of the  control linkage aids in moving the control surface through  the remainder of its travel.  [Figure 1-73] Figure 1-74  shows another way of assisting the movement of  an aileron on a large aircraft. It is called an aileron balance  panel. Not visible when approaching the aircraft, it is  positioned in the linkage that hinges the aileron to the wing.  Balance panels have been constructed typically of aluminum  skin-covered frame assemblies or aluminum honeycomb  structures. The trailing edge of the wing just forward of the  leading edge of the aileron is sealed to allow controlled airflow  in and out of the hinge area where the balance panel is located.  On heavy aircraft, large control surfaces require too much  force to be moved manually and are usually deflected out  of the neutral position by hydraulic actuators. These power  control units are signaled via a system of hydraulic valves  connected to the yoke and rudder pedals. On fly-by-wire  aircraft, the hydraulic actuators that move the flight control  Figure 1-72.  Servo tabs can be used to position flight control  surfaces in case of hydraulic failure. Figure 1-73.  Many tab linkages have a spring tab that kicks in as  the forces needed to deflect a control increase with speed and the  angle of desired deflection. Figure 1-71.  Balance tabs assist with forces needed to position  control surfaces.  Figure 1-74.  An aileron balance panel and linkage uses varying air pressure to assist in control surface positioning.  Hinge Vent gap Control tab Balance panel AILERON WING Vent gap Lower pressure Fixed surface Control Tab geared to deflect proportionally to the  control deflection, but in the opposite direction Tab Lift Free link Control surface hinge line Control stick Free link Spring Control stick 1-33