A landing gear system for an aircraft comprises a truck beam (22); at least one steerable axle (42) mounted to the truck beam for pivotal steering movement; a steering actuator (46) connected to the steerable axle; and a locking mechanism (60) including a locking groove member (62) provided on the steerable axle for pivotal movement therewith, and a locking wedge member (64) engageable with the locking groove member to prevent pivotal movement of the steerable axle. The locking groove member includes a slot (63) having opposed sides converging towards one another, and the locking wedge member has correspondingly converging sides for mating engagement in the slot. The locking wedge member is resiliently biased (66, 69) into engagement with the locking groove member and the locking wedge member is mounted to the truck beam for pivotal movement (70).