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Another factor that is critical to the longitudinal stability of a glider is its loading in relation to the CG. The CG of the glider is the point at which the total force of gravity is considered to act. When the glider is improperly loaded so it exceeds the aft CG limit, it loses longitudinal stability. As airspeed decreases, the nose of a glider rises. To recover, control inputs must be applied to force the nose down to return to a level flight attitude. It is possible that the glider could be loaded so far aft of the approved limits that control inputs are not sufficient to stop the nose from pitching up. If this were the case, the glider could enter a spin from which recovery would be impossible. Loading a glider with the CG too far forward is also hazardous. In extreme cases, the glider may not have enough pitch control to hold the nose up during an approach 

to a landing. For these reasons, it is important to ensure that the glider is within weight and balance limits prior to each flight. Proper loading of a glider and the importance of CG is discussed further in Chapter 5, Performance Limitations. 

## Flutter 

Another factor that can affect the ability to control the glider is flutter. Flutter occurs when rapid vibrations are induced through the control surfaces while the glider is traveling at high speeds. Looseness in the control surfaces can result in flutter while flying near maximum speed. Another factor that can reduce the airspeed at which flutter can occur is a disturbance to the balance of the control surfaces. If vibrations are felt in the control surfaces, reduce the airspeed. 

