Landing gear drive system

A drive system for rotating a wheel of an aircraft landing gear, the drive system including a motor operable to rotate a drive pinion, and a driven gear adapted to be mounted to the wheel, wherein the drive system has a first configuration in which the drive pinion is capable of meshing with the driven gear to permit the motor to drive the driven gear and a second configuration in which the drive pinion is not capable of meshing with the driven gear, and the drive system further comprises a linear positioning actuator for moving the drive pinion relative to the driven gear, wherein the positioning actuator has a first end and a second end, the first end having a pivotal connection with a pivot axis spaced at a fixed distance from an axis of rotation of the driven gear, and the second end having a pivotal connection with a pivot axis spaced at a fixed distance from an axis of rotation of the drive pinion, wherein the positioning actuator is a self-locking actuator, wherein the drive system further comprises a biasing element for biasing the drive system to the second configuration, and wherein the biasing element is constrained by a guide to avoid buckling of the biasing element.