Abstract: A mechanical counterbalance assembly for an overhead, track mounted, aircraft door includes a frame, a shaft mounted for rotation in the frame, and a torsion spring interconnected between the shaft and the frame. As the shaft is turned in a first direction, the spring is wound so that a torque is applied to the shaft in the direction opposite to rotation. The spring rate of the torsional spring is selected so the spring torsional force slightly exceeds the component of the door weight acting along the path of motion of the door throughout the door travel. A cable drum is mounted for rotation on the shaft and is releasably affixed thereto. A cable is wrapped about the drum and has its free end connected to the overhead sliding door. A planetary gear train has its low speed input shaft coupled to the shaft and its high speed output shaft operatively coupled to a centrifugal brake and to a manually rotatable drive wheel.