A horizontal stabilizer is used to maintain the aircraft in longitudinal balance, or trim:[3] it exerts a vertical force at a distance so the summation of pitch moments about the center of gravity is zero.[4] The vertical force exerted by the stabilizer varies with flight conditions, in particular according to the aircraft lift coefficient and wing flaps deflection which both affect the position of the center of pressure, and with the position of the aircraft center of gravity (which changes with aircraft loading and fuel consumption). Transonic flight makes special demands on horizontal stabilizers; when the local speed of the air over the wing reaches the speed of sound there is a sudden move aft of the center of pressure.