If the weight on an aircraft is distributed unevenly around the center of gravity, it is said to be unbalanced.

Trim refers to balancing an aircraft in flight around its center of gravity. The center of gravity is the point at which an airplane would balance if it were suspended at that point. Balance is important to airplane stability and safety in flight.

The prime concern of airplane balancing is the fore and aft (front and rear) location of the center of gravity along the longitudinal axis. Location of the center of gravity with reference to the lateral axis is also important. For each item of weight existing to the left of the fuselage centerline, there is an equal weight existing at a corresponding location on the right. This may be upset, however, by unbalanced lateral loading and adverse effects will arise from a laterally unbalanced condition. The pilot can compensate for the resulting wing-heavy condition by adjusting the aileron trim tab or by holding a constant aileron control pressure. However, this places the airplane controls in an out-of-streamline condition, increases drag, and results in decreased operating efficiency. Trim tabs are small surfaces that mechanically or electronically manipulate the rudder, elevator, and ailerons to help stabilize the plane. Trim tabs free the pilot from constantly adjusting the controls.

The center of gravity depends on the distribution of weight in the airplane and changes as load items (such as fuel) are shifted or expended. If the center of gravity is displaced too far forward on the longitudinal axis, a nose-heavy condition will result. Conversely, if the center of gravity is displaced too far aft, a tail-heavy condition will result. An unfavorable location of the center of gravity could produce such an unstable condition that the pilot could not control the airplane. The airplane manufacturer sets the fore and aft limits for the location of the airplane's center gravity.