An actuator includes a cylinder and a piston disposed in the cylinder. The piston has a head and a rod. The actuator further includes a collapsible locking member disposed in the head of the piston, and a gas-filled capsule disposed in the cylinder. The gas-filled capsule is in an expanded position when fluid in the cylinder is below a threshold pressure, and the gas-filled capsule is in a collapsed position when fluid in the cylinder is above the threshold pressure. The actuator also includes a lock piston operably connected to the gas-filled capsule. The lock piston engages the collapsible locking member and biases the collapsible locking member radially outward to a locked position when the gas-filled capsule is in the expanded position.